The Radiation Laboratory , commonly called the Rad Lab , was a microwave and radar research laboratory located at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts . It was first created in October 1940 and operated until 31 December 1945 when its functions were dispersed to industry, other departments within MIT, and in 1951, the newly formed MIT Lincoln Laboratory .
142-482: The use of microwaves for various radio and radar uses was highly desired before the war, but existing microwave devices like the klystron were far too low powered to be useful. Alfred Lee Loomis , a millionaire and physicist who headed his own private laboratory, organized the Microwave Committee to consider these devices and look for improvements. In early 1940, Winston Churchill organized what became
284-409: A feedback path from output to input by connecting the "catcher" and "buncher" cavities with a coaxial cable or waveguide . When the device is turned on, electronic noise in the cavity is amplified by the tube and fed back from the output catcher to the buncher cavity to be amplified again. Because of the high Q of the cavities, the signal quickly becomes a sine wave at the resonant frequency of
426-433: A marine radar mounted on a recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent advances in aviation weather-avoidance radar and in marine radar have successfully replaced
568-428: A sulfur lamp , a magnetron provides the microwave field that is passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g., sulfur , metal halides , etc.). Although efficient, these lamps are much more complex than other methods of lighting and therefore not commonly used. More modern variants use HEMTs or GaN-on-SiC power semiconductor devices instead of magnetrons to generate
710-501: A 10-cm gun-aiming system (called gun-laying or GL) for anti-aircraft batteries, and (3) a long-range airborne radio navigation system . To initiate the first two of these projects, the magnetron from Great Britain was used to build a 10-cm " breadboard " set; this was tested successfully from the rooftop of Building 4 in early January 1941. All members of the initial staff were involved in this endeavor. Under Project 1 led by Edwin M. McMillan , an "engineered" set with an antenna using
852-652: A 30-inch (76 cm) parabolic reflector followed. This, the first microwave radar built in America, was tested successfully in an aircraft on March 27, 1941. It was then taken to Great Britain by Taffy Bowen and tested in comparison with a 10-cm set being developed there. For the final system, the Rad Lab staff combined features from their own and the British set. It eventually became the SCR-720, used extensively by both
994-453: A cooling system. Some modern klystrons include depressed collectors, which recover energy from the beam before collecting the electrons, increasing efficiency. Multistage depressed collectors enhance the energy recovery by "sorting" the electrons in energy bins. The reflex klystron (also known as a Sutton tube after one of its inventors, Robert Sutton) was a low power klystron tube with a single cavity, which functioned as an oscillator . It
1136-454: A force at right angles to their direction of motion (the Lorentz force ). In this case, the electrons follow a curved path between the cathode and anode. The curvature of the path can be controlled by varying either the magnetic field using an electromagnet , or by changing the electrical potential between the electrodes. At very high magnetic field settings the electrons are forced back onto
1278-406: A high velocity stream of electrons. An external electromagnet winding creates a longitudinal magnetic field along the beam axis which prevents the beam from spreading. The beam first passes through the "buncher" cavity resonator, through grids attached to each side. The buncher grids have an oscillating AC potential across them, produced by standing wave oscillations within the cavity, excited by
1420-480: A higher incidence of cataracts in later life. There is also a considerable electrical hazard around magnetrons, as they require a high voltage power supply. Most magnetrons contain a small amount of beryllium oxide , and thorium mixed with tungsten in their filament . Exceptions to this are higher power magnetrons that operate above approximately 10,000 volts where positive ion bombardment becomes damaging to thorium metal, hence pure tungsten (potassium doped)
1562-494: A key advance, the use of two cathodes, was introduced by Habann in Germany in 1924. Further research was limited until Okabe's 1929 Japanese paper noting the production of centimeter-wavelength signals, which led to worldwide interest. The development of magnetrons with multiple cathodes was proposed by A. L. Samuel of Bell Telephone Laboratories in 1934, leading to designs by Postumus in 1934 and Hans Hollmann in 1935. Production
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#17327729676981704-417: A laser light beam causes bunching of the electrons. Then the beam passes through a second undulator, in which the electron bunches cause oscillation to create a second, more powerful light beam. The floating drift tube klystron has a single cylindrical chamber containing an electrically isolated central tube. Electrically, this is similar to the two cavity oscillator klystron with considerable feedback between
1846-505: A microwave signal from direct current electricity supplied to the vacuum tube. The use of magnetic fields as a means to control the flow of an electric current was spurred by the invention of the Audion by Lee de Forest in 1906. Albert Hull of General Electric Research Laboratory , USA, began development of magnetrons to avoid de Forest's patents, but these were never completely successful. Other experimenters picked up on Hull's work and
1988-424: A more difficult problem for a wider array of radar systems. Neither of these present a problem for continuous-wave radars , nor for microwave ovens. All cavity magnetrons consist of a heated cylindrical cathode at a high (continuous or pulsed) negative potential created by a high-voltage, direct-current power supply. The cathode is placed in the center of an evacuated , lobed, circular metal chamber. The walls of
2130-408: A negatively charged reflector electrode for another pass through the cavity, where they are then collected. The electron beam is velocity modulated when it first passes through the cavity. The formation of electron bunches takes place in the drift space between the reflector and the cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum as the electron beam re-enters
2272-623: A number of the Rad Lab physicists into Los Alamos and Lawrence's facility at Berkeley. This was made simpler by Lawrence and Loomis being involved in all of these projects. The Radiation Laboratory officially opened in November 1940, using 4,000 square feet (370 m) of space in MIT's Building 4, and under $ 500,000 initial funding from the NDRC. In addition to the Director, Lee DuBridge, I. I. Rabi
2414-520: A professor at Prague's Charles University , published first; however, he published in a journal with a small circulation and thus attracted little attention. Habann, a student at the University of Jena , investigated the magnetron for his doctoral dissertation of 1924. Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron. Most of these early magnetrons were glass vacuum tubes with multiple anodes. However,
2556-402: A resonant cavity magnetron to fill this need; it was quickly placed within the highest level of secrecy. Shortly after this breakthrough, Britain's Prime Minister Winston Churchill and President Roosevelt agreed that the two nations would pool their technical secrets and jointly develop many urgently needed warfare technologies. At the initiation of this exchange in the late summer of 1940,
2698-465: A rod-shaped cathode, placed in the middle of a magnet. The attempt to measure the electron mass failed because he was unable to achieve a good vacuum in the tube. However, as part of this work, Greinacher developed mathematical models of the motion of the electrons in the crossed magnetic and electric fields. In the US, Albert Hull put this work to use in an attempt to bypass Western Electric 's patents on
2840-409: A second cavity, called the "catcher", through a similar pair of grids on each side of the cavity. The function of the catcher grids is to absorb energy from the electron beam. The bunches of electrons passing through excite standing waves in the cavity, which has the same resonant frequency as the buncher cavity. Each bunch of electrons passes between the grids at a point in the cycle when the exit grid
2982-404: A slower and less faithful response to control current than electrostatic control using a control grid in a conventional triode (not to mention greater weight and complexity), so magnetrons saw limited use in conventional electronic designs. It was noticed that when the magnetron was operating at the critical value, it would emit energy in the radio frequency spectrum. This occurs because a few of
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#17327729676983124-452: A small book and transmitted from an antenna only centimeters long, reducing the size of practical radar systems by orders of magnitude. New radars appeared for night-fighters , anti-submarine aircraft and even the smallest escort ships, and from that point on the Allies of World War II held a lead in radar that their counterparts in Germany and Japan were never able to close. By the end of
3266-415: A staff of 800 persons in these efforts. A radically different type of antenna for X-band systems was invented by Luis W. Alvarez and used in three new systems: an airborne mapping radar called Eagle, a blind-landing Ground Control Approach (GCA) system, and a ground-based Microwave Early-Warning (MEW) system. The latter two were highly successful and carried over into post-war applications. Eagle eventually
3408-483: A submarine periscope, which allowed aircraft to attack and destroy submerged submarines which had previously been undetectable from the air. Centimetric contour mapping radars like H2S improved the accuracy of Allied bombers used in the strategic bombing campaign , despite the existence of the German FuG 350 Naxos device to specifically detect it. Centimetric gun-laying radars were likewise far more accurate than
3550-635: A target, could be used just as well to decelerate electrons (i.e., transfer their kinetic energy to RF energy in a resonator). During the Second World War, Hansen lectured at the MIT Radiation labs two days a week, commuting to Boston from Sperry Gyroscope Company on Long Island. His resonator was called a "rhumbatron" by the Varian brothers. Hansen died of beryllium disease in 1949 as a result of exposure to beryllium oxide (BeO). During
3692-440: Is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output. Where there are an even number of cavities, two concentric rings can connect alternate cavity walls to prevent inefficient modes of oscillation. This
3834-436: Is a high-power vacuum tube used in early radar systems and subsequently in microwave ovens and in linear particle accelerators . A cavity magnetron generates microwaves using the interaction of a stream of electrons with a magnetic field , while moving past a series of cavity resonators , which are small, open cavities in a metal block. Electrons pass by the cavities and cause microwaves to oscillate within, similar to
3976-458: Is an obsolete type in which the electron beam was reflected back along its path by a high potential electrode, used as an oscillator. The name klystron comes from the Greek verb κλύζω ( klyzo ) referring to the action of waves breaking against a shore, and the suffix -τρον ("tron") meaning the place where the action happens. The name "klystron" was suggested by Hermann Fränkel , a professor in
4118-678: Is called pi-strapping because the two straps lock the phase difference between adjacent cavities at π radians (180°). The modern magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1-kilowatt input will generally create about 700 watts of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Large S band magnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75 kW. Some large magnetrons are water cooled. The magnetron remains in widespread use in roles which require high power, but where precise control over frequency and phase
4260-428: Is captured by a collector electrode. To make an oscillator , the output cavity can be coupled to the input cavity(s) with a coaxial cable or waveguide . Positive feedback excites spontaneous oscillations at the resonant frequency of the cavities. The simplest klystron tube is the two-cavity klystron. In this tube there are two microwave cavity resonators, the "catcher" and the "buncher". When used as an amplifier,
4402-568: Is electrically insulated from the cavity walls, and DC bias is applied separately. The DC bias on the drift tube may be adjusted to alter the transit time through it, thus allowing some electronic tuning of the oscillating frequency. The amount of tuning in this manner is not large and is normally used for frequency modulation when transmitting. Klystrons can produce far higher microwave power outputs than solid state microwave devices such as Gunn diodes . In modern systems, they are used from UHF (hundreds of megahertz) up to hundreds of gigahertz (as in
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4544-416: Is negative with respect to the entrance grid, so the electric field in the cavity between the grids opposes the electrons motion. The electrons thus do work on the electric field, and are decelerated, their kinetic energy is converted to electric potential energy , increasing the amplitude of the oscillating electric field in the cavity. Thus the oscillating field in the catcher cavity is an amplified copy of
4686-514: Is no longer in print, but the series was re-released as a two- CD-ROM set in 1999 ( ISBN 1-58053-078-8 ) by publisher Artech House. More recently, it has become available online. Postwar declassification of the work at the MIT Rad Lab made available, via the Series, a quite large body of knowledge about advanced electronics. A reference (identity long forgotten) credited the Series with
4828-423: Is small enough that the power output essentially remains constant. At regions far from the optimum voltage, no oscillations are obtained at all. There are often several regions of reflector voltage where the reflex klystron will oscillate; these are referred to as modes. The electronic tuning range of the reflex klystron is usually referred to as the variation in frequency between half power points—the points in
4970-423: Is unimportant. In a radar set, the magnetron's waveguide is connected to an antenna . The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high-power microwave energy being radiated. As in all primary radar systems, the radiation reflected from a target is analyzed to produce a radar map on a screen. Several characteristics of the magnetron's output make radar use of
5112-794: The General Electric Company Research Laboratories in Wembley , London , was taken on the Tizard Mission in September 1940. As the discussion turned to radar, the US Navy representatives began to detail the problems with their short-wavelength systems, complaining that their klystrons could only produce 10 W. With a flourish, "Taffy" Bowen pulled out a magnetron and explained it produced 1000 times that. Bell Telephone Laboratories took
5254-750: The Harvard University campus (just a mile from MIT) and became the Radio Research Laboratory (RRL). Organizationally separate from the Rad Lab, but also under the OSRD, the two operations had much in common throughout their existences. When the Radiation Laboratory closed, the OSRD agreed to continue funding for the Basic Research Division, which officially became part of MIT on July 1, 1946, as
5396-518: The Nobel Prize for Physics in 1905. In the USA it was later patented by Lee de Forest , resulting in considerable research into alternate tube designs that would avoid his patents. One concept used a magnetic field instead of an electrical charge to control current flow, leading to the development of the magnetron tube. In this design, the tube was made with two electrodes, typically with the cathode in
5538-499: The Tizard Mission brought to America one of the first of the new magnetrons. On October 6, Edward George Bowen , a key developer of RDF at the Telecommunications Research Establishment (TRE) and a member of the mission, demonstrated the magnetron, producing some 15,000 watts (15 kW ) of power at 3 GHz, i.e. a wavelength of 10 cm. American researchers and officials were amazed at
5680-505: The Tizard Mission to introduce U.S. researchers to several new technologies the UK had been developing. Among these was the cavity magnetron , a leap forward in the creation of microwaves that made them practical for use in aircraft for the first time. GEC made 12 prototype cavity magnetrons at Wembley in August 1940, and No 12 was sent to America with Bowen via the Tizard Mission , where it
5822-617: The Tizard Mission , where it was shown on 19 September 1940 in Alfred Loomis’ apartment. The American NDRC Microwave Committee was stunned at the power level produced. However Bell Labs' director was upset when it was X-rayed and had eight holes rather than the six holes shown on the GEC plans. After contacting (via the transatlantic cable) Dr Eric Megaw, GEC’s vacuum tube expert Megaw recalled that when he had asked for 12 prototypes he said make 10 with 6 holes, one with 7 and one with 8; there
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5964-566: The U.S. Army Air Corps and the British Royal Air Force . For Project 2, a 4-foot- and later 6-foot-wide (1.2 then 1.8 m) parabolic reflector on a pivoting mount was selected. Also, this set would use an electro-mechanical computer (called a predictor-correlator) to keep the antenna aimed at an acquired target. Ivan A. Getting served as the project leader. Being much more complicated than aircraft interception and required to be very rugged for field use, an engineered GL
6106-552: The USSR , and Japan . These usually operated at Very High Frequency (VHF) wavelengths in the electromagnetic spectrum and carried several cover names, such as Ranging and Direction Finding (RDF) in Great Britain. In 1941, the U.S. Navy coined the acronym 'RADAR' (RAdio Detection And Ranging) for such systems; this soon led to the name ' radar ' and spread to other countries. The potential advantages of operating such systems in
6248-504: The Ultra High Frequency (UHF or microwave ) region were well known and vigorously pursued. One of these advantages was smaller antennas , a critical need for detection systems on aircraft. The primary technical barrier to developing UHF systems was the lack of a usable source for generating high-power microwaves . In February 1940, researchers John Randall and Harry Boot at Birmingham University in Great Britain built
6390-463: The University of Birmingham in the UK, John Randall and Harry Boot produced a working prototype of a cavity magnetron that produced about 400 W. Within a week this had improved to 1 kW, and within the next few months, with the addition of water cooling and many detail changes, this had improved to 10 and then 25 kW. To deal with its drifting frequency, they sampled the output signal and synchronized their receiver to whatever frequency
6532-404: The University of Birmingham , England in 1940. Their first working example produced hundreds of watts at 10 cm wavelength, an unprecedented achievement. Within weeks, engineers at GEC had improved this to well over a kilowatt (kW), and within months 25 kW, over 100 kW by 1941 and pushing towards a megawatt by 1943. The high power pulses were generated from a device the size of
6674-493: The anode . The components are normally arranged concentrically, placed within a tubular-shaped container from which all air has been evacuated, so that the electrons can move freely (hence the name "vacuum" tubes, called "valves" in British English). If a third electrode (called a control grid ) is inserted between the cathode and the anode, the flow of electrons between the cathode and anode can be regulated by varying
6816-494: The low-UHF band to start with for front-line aircraft, were not a match for their British counterparts. Likewise, in the UK, Albert Beaumont Wood proposed in 1937 a system with "six or eight small holes" drilled in a metal block, differing from the later production designs only in the aspects of vacuum sealing. However, his idea was rejected by the Navy, who said their valve department was far too busy to consider it. In 1940, at
6958-592: The British H2S bombing radar that operated at shorter wavelengths in the X band . The Rad Lab also developed Loran-A , the first worldwide radio navigation system, which originally was known as "LRN" for Loomis Radio Navigation. During the mid- and late-1930s, radio systems for the detection and location of distant targets had been developed under great secrecy in the United States and Great Britain , as well as in several other nations, notably Germany ,
7100-562: The Extended Interaction Klystrons in the CloudSat satellite). Klystrons can be found at work in radar , satellite and wideband high-power communication (very common in television broadcasting and EHF satellite terminals), medicine ( radiation oncology ), and high-energy physics ( particle accelerators and experimental reactors). At SLAC , for example, klystrons are routinely employed which have outputs in
7242-642: The Japanese Attack on Pearl Harbor and the entry of the U.S. into World War II, work at the Rad Lab greatly expanded. At the height of its activities, the Rad Lab employed nearly 4,000 people working in several countries. The Rad Lab had constructed, and was the initial occupant of, MIT's famous Building 20 . Costing just over $ 1 million, this was one of the longest-surviving World War II temporary structures. Activities eventually encompassed physical electronics, electromagnetic properties of matter, microwave physics, and microwave communication principles, and
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#17327729676987384-527: The MIT campus, was intentionally deceptive, albeit obliquely correct in that radar uses radiation in a portion of the electromagnetic spectrum . It was chosen to imply the laboratory's mission was similar to that of the Ernest O. Lawrence 's Radiation Laboratory at UC Berkeley ; i.e., that it employed scientists to work on nuclear physics research. At the time, nuclear physics was regarded as relatively theoretical and inapplicable to military equipment, as this
7526-406: The Rad Lab director. The lab rapidly expanded, and within months was larger than the UK's efforts which had been running for several years by this point. By 1943 the lab began to deliver a stream of ever-improved devices, which could be produced in huge numbers by the U.S.'s industrial base. At its peak, the Rad Lab employed 4,000 at MIT and several other labs around the world, and designed half of all
7668-686: The Rad Lab made fundamental advances in all of these fields. Half of the radars deployed by the U.S. military during World War II were designed at the Rad Lab, including over 100 different microwave systems costing $ 1.5 billion . All of these sets improved considerably on pre-microwave, VHF systems from the Naval Research Laboratory and the Army's Signal Corps Laboratories , as well as British radars such as Robert Watson-Watt 's Chain Home and Taffy Bowen's early airborne RDF sets. Although
7810-457: The Rad Lab staff. A prototype was flown in August 1944, and the system became operational early the next year. Although too late to affect the final war effort, the project laid the foundation for significant developments in the following years. As the Rad Lab started, a laboratory was set up to develop electronic countermeasures (ECM), technologies to block enemy radars and communications. With Frederick E. Terman as director, this soon moved to
7952-568: The Rad Lab was initiated as a joint Anglo-American operation and many of its products were adopted by the British military, researchers in Great Britain* continued with the development of microwave radar and, particularly with cooperation from Canada, produced many types of new systems. For the exchange of information, the Rad Lab established a branch operation in England, and a number of British scientists and engineers worked on assignments at
8094-466: The Rad Lab. *At the T. R. E., Telecommunications Research Establishment The resonant cavity magnetron continued to evolve at the Rad Lab. A team led by I.I. Rabi first extended the operation of the magnetron from 10-cm (called S-band), to 6-cm (C-band), then to 3-cm (X-band), and eventually to 1-cm (K-band). To keep pace, all of the other radar sub-systems also were evolving continuously. The Transmitter Division, under Albert G. Hill , eventually involved
8236-686: The Research Laboratory of Electronics at MIT (RLE). Other wartime research was taken up by the MIT Laboratory for Nuclear Science, which was founded at the same time. Both laboratories principally occupied Building 20 until 1957. Most of the important research results of the Rad Lab were documented in a 28-volume compilation entitled the MIT Radiation Laboratory Series , edited by Louis N. Ridenour and published by McGraw-Hill between 1947 and 1953. This
8378-573: The Second World War, the Axis powers relied mostly on (then low-powered and long wavelength) klystron technology for their radar system microwave generation, while the Allies used the far more powerful but frequency-drifting technology of the cavity magnetron for much shorter-wavelength centimetric microwave generation. Klystron tube technologies for very high-power applications, such as synchrotrons and radar systems, have since been developed. Right after
8520-490: The United Kingdom used the magnetron to develop a revolutionary airborne, ground-mapping radar codenamed H2S. The H2S radar was in part developed by Alan Blumlein and Bernard Lovell . The cavity magnetron was widely used during World War II in microwave radar equipment and is often credited with giving Allied radar a considerable performance advantage over German and Japanese radars, thus directly influencing
8662-427: The amplifier. No two klystrons are exactly identical (even when comparing like part/model number klystrons). Each unit has manufacturer-supplied calibration values for its specific performance characteristics. Without this information the klystron would not be properly tunable, and hence not perform well, if at all. Tuning a klystron is delicate work which, if not done properly, can cause damage to equipment or injury to
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#17327729676988804-421: The anode, rather than external circuits or fields. Mechanically, the cavity magnetron consists of a large, solid cylinder of metal with a hole drilled through the centre of the circular face. A wire acting as the cathode is run down the center of this hole, and the metal block itself forms the anode. Around this hole, known as the "interaction space", are a number of similar holes ("resonators") drilled parallel to
8946-474: The areas around them. The anode is constructed of a highly conductive material, almost always copper, so these differences in voltage cause currents to appear to even them out. Since the current has to flow around the outside of the cavity, this process takes time. During that time additional electrons will avoid the hot spots and be deposited further along the anode, as the additional current flowing around it arrives too. This causes an oscillating current to form as
9088-417: The cathode, depositing their energy on it and causing it to heat up. As this normally causes more electrons to be released, it could sometimes lead to a runaway effect, damaging the device. The great advance in magnetron design was the resonant cavity magnetron or electron-resonance magnetron , which works on entirely different principles. In this design the oscillation is created by the physical shape of
9230-453: The cathode, preventing current flow. At the opposite extreme, with no field, the electrons are free to flow straight from the cathode to the anode. There is a point between the two extremes, the critical value or Hull cut-off magnetic field (and cut-off voltage), where the electrons just reach the anode. At fields around this point, the device operates similar to a triode. However, magnetic control, due to hysteresis and other effects, results in
9372-406: The cavities. In all modern klystrons, the number of cavities exceeds two. Additional "buncher" cavities added between the first "buncher" and the "catcher" may be used to increase the gain of the klystron or to increase the bandwidth. The residual kinetic energy in the electron beam when it hits the collector electrode represents wasted energy, which is dissipated as heat, which must be removed by
9514-419: The cavities. In some systems the tap wire is replaced by an open hole, which allows the microwaves to flow into a waveguide . As the oscillation takes some time to set up, and is inherently random at the start, subsequent startups will have different output parameters. Phase is almost never preserved, which makes the magnetron difficult to use in phased array systems. Frequency also drifts from pulse to pulse,
9656-448: The chamber are the anode of the tube. A magnetic field parallel to the axis of the cavity is imposed by a permanent magnet . The electrons initially move radially outward from the cathode attracted by the electric field of the anode walls. The magnetic field causes the electrons to spiral outward in a circular path, a consequence of the Lorentz force . Spaced around the rim of the chamber are cylindrical cavities. Slots are cut along
9798-484: The classics department at Stanford University when the klystron was under development. The klystron was the first significantly powerful source of radio waves in the microwave range; before its invention the only sources were the Barkhausen–Kurz tube and split-anode magnetron , which were limited to very low power. It was invented by the brothers Russell and Sigurd Varian at Stanford University . Their prototype
9940-456: The control of the ratio of the magnetic and electric field strengths. He released several papers and patents on the concept in 1921. Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves. However, in 1924, Czech physicist August Žáček (1886–1961) and German physicist Erich Habann (1892–1968) independently discovered that the magnetron could generate waves of 100 megahertz to 1 gigahertz. Žáček,
10082-440: The current tries to equalize one spot, then another. The oscillating currents flowing around the cavities, and their effect on the electron flow within the tube, cause large amounts of microwave radiofrequency energy to be generated in the cavities. The cavities are open on one end, so the entire mechanism forms a single, larger, microwave oscillator. A "tap", normally a wire formed into a loop, extracts microwave energy from one of
10224-631: The development of the post-World War II electronics industry. With the cryptology and cryptographic efforts centered at Bletchley Park and Arlington Hall and the Manhattan Project , the development of microwave radar at the Radiation Laboratory represents one of the most significant, secret, and outstandingly successful technological efforts spawned by the Anglo-American relations in World War II. The Radiation Laboratory
10366-408: The device somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter frequency. This instability results not only in frequency shifts from one pulse to the next, but also a frequency shift within an individual transmitted pulse. The second factor is that the energy of the transmitted pulse is spread over a relatively wide frequency spectrum, which requires
10508-405: The drive power for modern particle accelerators . In a klystron, an electron beam interacts with radio waves as it passes through resonant cavities , metal boxes along the length of a tube. The electron beam first passes through a cavity to which the input signal is applied. The energy of the electron beam amplifies the signal, and the amplified signal is taken from a cavity at the other end of
10650-399: The electric field is in the same direction are accelerated, causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities. The beam then passes through a "drift" tube, in which the faster electrons catch up to the slower ones, creating the "bunches", then through a "catcher" cavity. In
10792-530: The electron beam, such a tool can be pulled into the unit by the intense magnetic force, smashing fingers, injuring the technician, or damaging the unit. Special lightweight nonmagnetic (or rather very weakly diamagnetic ) tools made of beryllium alloy have been used for tuning U.S. Air Force klystrons. Precautions are routinely taken when transporting klystron devices in aircraft, as the intense magnetic field can interfere with magnetic navigation equipment. Special overpacks are designed to help limit this field "in
10934-404: The electrons in the flow experienced this looping motion, the amount of RF energy being radiated was greatly improved. And as the motion occurred at any field level beyond the critical value, it was no longer necessary to carefully tune the fields and voltages, and the overall stability of the device was greatly improved. Unfortunately, the higher field also meant that electrons often circled back to
11076-405: The electrons, instead of reaching the anode, continue to circle in the space between the cathode and the anode. Due to an effect now known as cyclotron radiation , these electrons radiate radio frequency energy. The effect is not very efficient. Eventually the electrons hit one of the electrodes, so the number in the circulating state at any given time is a small percentage of the overall current. It
11218-603: The example and quickly began making copies, and before the end of 1940, the Radiation Laboratory had been set up on the campus of the Massachusetts Institute of Technology to develop various types of radar using the magnetron. By early 1941, portable centimetric airborne radars were being tested in American and British aircraft. In late 1941, the Telecommunications Research Establishment in
11360-412: The extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar. The size of the cavities determine the resonant frequency, and thereby the frequency of the emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance , with changes in the supply current, and with
11502-413: The few devices known to create microwaves, interest in the device and potential improvements was widespread. The first major improvement was the split-anode magnetron , also known as a negative-resistance magnetron . As the name implies, this design used an anode that was split in two—one at each end of the tube—creating two half-cylinders. When both were charged to the same voltage the system worked like
11644-413: The field," and thus allow such devices to be transported safely. The technique of amplification used in the klystron is also being applied experimentally at optical frequencies in a type of laser called the free-electron laser (FEL); these devices are called optical klystrons . Instead of microwave cavities, these use devices called undulators . The electron beam passes through an undulator, in which
11786-427: The form of a metal rod in the center, and the anode as a cylinder around it. The tube was placed between the poles of a horseshoe magnet arranged such that the magnetic field was aligned parallel to the axis of the electrodes. With no magnetic field present, the tube operates as a diode, with electrons flowing directly from the cathode to the anode. In the presence of the magnetic field, the electrons will experience
11928-455: The functioning of a whistle producing a tone when excited by an air stream blown past its opening. The resonant frequency of the arrangement is determined by the cavities' physical dimensions. Unlike other vacuum tubes, such as a klystron or a traveling-wave tube (TWT), the magnetron cannot function as an amplifier for increasing the intensity of an applied microwave signal; the magnetron serves solely as an electronic oscillator generating
12070-515: The heart of your microwave oven today. The cavity magnetron's invention changed the world. Because France had just fallen to the Nazis and Britain had no money to develop the magnetron on a massive scale, Winston Churchill agreed that Sir Henry Tizard should offer the magnetron to the Americans in exchange for their financial and industrial help. An early 10 kW version, built in England by
12212-413: The input signal at the cavity's resonant frequency applied by a coaxial cable or waveguide. The direction of the field between the grids changes twice per cycle of the input signal. Electrons entering when the entrance grid is negative and the exit grid is positive encounter an electric field in the same direction as their motion, and are accelerated by the field. Electrons entering a half-cycle later, when
12354-433: The interaction depends on the resonance condition, larger cavity dimensions than a conventional klystron can be used. This allows the gyroklystron to deliver high power at very high frequencies which is challenging using conventional klystrons. Some klystrons have cavities that are tunable. By adjusting the frequency of individual cavities, the technician can change the operating frequency, gain, output power, or bandwidth of
12496-400: The interaction space, connected to the interaction space by a short channel. The resulting block looks something like the cylinder on a revolver , with a somewhat larger central hole. Early models were cut using Colt pistol jigs. Remembering that in an AC circuit the electrons travel along the surface , not the core, of the conductor, the parallel sides of the slot act as a capacitor while
12638-425: The length of the cavities that open into the central, common cavity space. As electrons sweep past these slots, they induce a high-frequency radio field in each resonant cavity, which in turn causes the electrons to bunch into groups. A portion of the radio frequency energy is extracted by a short coupling loop that is connected to a waveguide (a metal tube, usually of rectangular cross section). The waveguide directs
12780-440: The magnetron with microwave semiconductor oscillators , which have a narrower output frequency range. These allow a narrower receiver bandwidth to be used, and the higher signal-to-noise ratio in turn allows a lower transmitter power, reducing exposure to EMR. In microwave ovens , the waveguide leads to a radio-frequency-transparent port into the cooking chamber. As the fixed dimensions of the chamber and its physical closeness to
12922-458: The magnetron would normally create standing wave patterns in the chamber, the pattern is randomized by a motorized fan-like mode stirrer in the waveguide (more often in commercial ovens), or by a turntable that rotates the food (most common in consumer ovens). An early example of this application was when British scientists in 1954 used a microwave oven to resurrect cryogenically frozen hamsters . In microwave-excited lighting systems, such as
13064-495: The magnetron, and the NDRC immediately started plans for manufacturing and incorporating the devices. Alfred Lee Loomis , who headed the NDRC Microwave Committee, led in establishing the Radiation Laboratory at MIT as a joint Anglo - American effort for microwave research and system development using the new magnetron. The name 'Radiation Laboratory', selected by Loomis when he selected the building for it on
13206-470: The microwaves, which are substantially less complex and can be adjusted to maximize light output using a PID controller . In 1910, Hans Gerdien (1877–1951) of the Siemens Corporation invented a magnetron. In 1912, Swiss physicist Heinrich Greinacher was looking for new ways to calculate the electron mass . He settled on a system consisting of a diode with a cylindrical anode surrounding
13348-403: The modulation forces alter the cyclotron frequency and hence the azimuthal component of motion, resulting in phase bunches. In the output cavity, electrons which arrive at the correct decelerating phase transfer their energy to the cavity field and the amplified signal can be coupled out. The gyroklystron has cylindrical or coaxial cavities and operates with transverse electric field modes. Since
13490-484: The most important invention that came out of the Second World War", while professor of military history at the University of Victoria in British Columbia, David Zimmerman, states: The magnetron remains the essential radio tube for shortwave radio signals of all types. It not only changed the course of the war by allowing us to develop airborne radar systems, it remains the key piece of technology that lies at
13632-536: The older technology. They made the big-gunned Allied battleships more deadly and, along with the newly developed proximity fuze , made anti-aircraft guns much more dangerous to attacking aircraft. The two coupled together and used by anti-aircraft batteries, placed along the flight path of German V-1 flying bombs on their way to London , are credited with destroying many of the flying bombs before they reached their target. Since then, many millions of cavity magnetrons have been manufactured; while some have been for radar
13774-404: The original model. But by slightly altering the voltage of the two plates , the electrons' trajectory could be modified so that they would naturally travel towards the lower voltage side. The plates were connected to an oscillator that reversed the relative voltage of the two plates at a given frequency. At any given instant, the electron will naturally be pushed towards the lower-voltage side of
13916-415: The oscillating mode where the power output is half the maximum output in the mode. Modern semiconductor technology has effectively replaced the reflex klystron in most applications. The gyroklystron is a microwave amplifier with operation dependent on the cyclotron resonance condition. Similarly to the klystron, its operation depends on the modulation of the electron beam, but instead of axial bunching
14058-524: The outcome of the war. It was later described by American historian James Phinney Baxter III as "[t]he most valuable cargo ever brought to our shores". Centimetric radar, made possible by the cavity magnetron, allowed for the detection of much smaller objects and the use of much smaller antennas. The combination of small-cavity magnetrons, small antennas, and high resolution allowed small, high quality radars to be installed in aircraft. They could be used by maritime patrol aircraft to detect objects as small as
14200-445: The output "catcher" cavity, each bunch enters the cavity at the time in the cycle when the electric field opposes the electrons' motion, decelerating them. Thus the kinetic energy of the electrons is converted to potential energy of the field, increasing the amplitude of the oscillations . The oscillations excited in the catcher cavity are coupled out through a coaxial cable or waveguide . The spent electron beam, with reduced energy,
14342-484: The polarity is opposite, encounter an electric field which opposes their motion, and are decelerated. Beyond the buncher grids is a space called the drift space . This space is long enough so that the accelerated electrons catch up with electrons that were decelerated at an earlier time, forming "bunches" longitudinally along the beam axis. Its length is chosen to allow maximum bunching at the resonant frequency, and may be several feet long. The electrons then pass through
14484-542: The project and headed by Donald G. Fink . Operating in the Low Frequency ( LF ) portion of the radio spectrum, LORAN was the only non-microwave project of the Rad Lab. Incorporating major elements of GEE, LORAN was highly successful and beneficial to the war effort. By the end of hostilities, about 30 percent of the Earth's surface was covered by LORAN stations and used by 75,000 aircraft and surface vessels. Following
14626-406: The radar display. The magnetron remains in use in some radar systems, but has become much more common as a low-cost source for microwave ovens. In this form, over one billion magnetrons are in use today. In a conventional electron tube ( vacuum tube ), electrons are emitted from a negatively charged, heated component called the cathode and are attracted to a positively charged component called
14768-563: The radar systems used during the war. By the end of the war, the U.S. held a leadership position in a number of microwave-related fields. Among their notable products were the SCR-584 , the finest gun-laying radar of the war, and the SCR-720 , an aircraft interception radar that became the standard late-war system for both U.S. and UK night fighters . They also developed the H2X , a version of
14910-550: The range of 50 MW (pulse) and 50 kW (time-averaged) at 2856 MHz. The Arecibo Planetary Radar used two klystrons that provided a total power output of 1 MW (continuous) at 2380 MHz. Popular Science ' s "Best of What's New 2007" described a company, Global Resource Corporation, currently defunct, using a klystron to convert the hydrocarbons in everyday materials, automotive waste, coal , oil shale , and oil sands into natural gas and diesel fuel . Cavity magnetron The cavity magnetron
15052-429: The receiver to have a correspondingly wide bandwidth. This wide bandwidth allows ambient electrical noise to be accepted into the receiver, thus obscuring somewhat the weak radar echoes, thereby reducing overall receiver signal-to-noise ratio and thus performance. The third factor, depending on application, is the radiation hazard caused by the use of high-power electromagnetic radiation. In some applications, for example,
15194-511: The resonant cavity, thus ensuring a maximum of energy is transferred from the electron beam to the RF oscillations in the cavity. The reflector voltage may be varied slightly from the optimum value, which results in some loss of output power, but also in a variation in frequency. This effect is used to good advantage for automatic frequency control in receivers, and in frequency modulation for transmitters. The level of modulation applied for transmission
15336-420: The round holes form an inductor : an LC circuit made of solid copper, with the resonant frequency defined entirely by its dimensions. The magnetic field is set to a value well below the critical, so the electrons follow curved paths towards the anode. When they strike the anode, they cause it to become negatively charged in that region. As this process is random, some areas will become more or less charged than
15478-404: The signal applied to the buncher cavity. The amplified signal is extracted from the catcher cavity through a coaxial cable or waveguide. After passing through the catcher and giving up its energy, the lower energy electron beam is absorbed by a "collector" electrode, a second anode which is kept at a small positive voltage. An electronic oscillator can be made from a klystron tube, by providing
15620-433: The technician due to the very high voltages that could be produced. The technician must be careful not to exceed the limits of the graduations, or damage to the klystron can result. Other precautions taken when tuning a klystron include using nonferrous tools. Some klystrons employ permanent magnets . If a technician uses ferrous tools (which are ferromagnetic ) and comes too close to the intense magnetic fields that contain
15762-474: The temperature of the tube. This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices, such as the klystron are used. The magnetron is a self-oscillating device requiring no external elements other than a power supply. A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage
15904-439: The tens of kilovolts). This beam passes through an input cavity resonator . RF energy has been fed into the input cavity at, or near, its resonant frequency , creating standing waves , which produce an oscillating voltage, which acts on the electron beam. The electric field causes the electrons to "bunch": electrons that pass through when the electric field opposes their motion are slowed, while electrons which pass through when
16046-482: The transmitters and receivers. In some applications Klystrons have been replaced by solid state transistors. High efficiency Klystrons have been developed with have 10% more effiency than conventional Klystrons. Klystrons amplify RF signals by converting the kinetic energy in a DC electron beam into radio frequency power. In a vacuum, a beam of electrons is emitted by an electron gun or thermionic cathode and accelerated by high-voltage electrodes (typically in
16188-506: The triode. Western Electric had gained control of this design by buying Lee De Forest 's patents on the control of current flow using electric fields via the "grid". Hull intended to use a variable magnetic field, instead of an electrostatic one, to control the flow of the electrons from the cathode to the anode. Working at General Electric 's Research Laboratories in Schenectady, New York , Hull built tubes that provided switching through
16330-401: The tube. The electron will then oscillate back and forth as the voltage changes. At the same time, a strong magnetic field is applied, stronger than the critical value in the original design. This would normally cause the electron to circle back to the cathode, but due to the oscillating electrical field, the electron instead follows a looping path that continues toward the anodes. Since all of
16472-413: The tube. The output signal can be coupled back into the input cavity to make an electronic oscillator to generate radio waves. The power gain of klystrons can be high, up to 60 dB (an increase in signal power of a factor of one million), with output power up to tens of megawatts , but the bandwidth is narrow, usually a few percent although it can be up to 10% in some devices. A reflex klystron
16614-426: The two cavities. Electrons exiting the source cavity are velocity modulated by the electric field as they travel through the drift tube and emerge at the destination chamber in bunches, delivering power to the oscillation in the cavity. This type of oscillator klystron has an advantage over the two-cavity klystron on which it is based, in that it needs only one tuning element to effect changes in frequency. The drift tube
16756-399: The two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency. While radar was being developed during World War II , there arose an urgent need for a high-power microwave generator that worked at shorter wavelengths , around 10 cm (3 GHz), rather than the 50 to 150 cm (200 MHz) that was available from tube-based generators of the time. It
16898-476: The vast majority have been for microwave ovens . The use in radar itself has dwindled to some extent, as more accurate signals have generally been needed and developers have moved to klystron and traveling-wave tube systems for these needs. At least one hazard in particular is well known and documented. As the lens of the eye has no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation. This heating can in turn lead to
17040-442: The voltage on this third electrode. This allows the resulting electron tube (called a " triode " because it now has three electrodes) to function as an amplifier because small variations in the electric charge applied to the control grid will result in identical variations in the much larger current of electrons flowing between the cathode and anode. The idea of using a grid for control was invented by Philipp Lenard , who received
17182-460: The war, AT&T used 4-watt klystrons in its brand new network of microwave relay links that covered the contiguous United States . The network provided long-distance telephone service and also carried television signals for the major TV networks. Western Union Telegraph Company also built point-to-point microwave communication links using intermediate repeater stations at about 40 mile intervals at that time, using 2K25 reflex klystrons in both
17324-485: The war, practically every Allied radar was based on the magnetron. The magnetron continued to be used in radar in the post-war period but fell from favour in the 1960s as high-power klystrons and traveling-wave tubes emerged. A key characteristic of the magnetron is that its output signal changes from pulse to pulse, both in frequency and phase. This renders it less suitable for pulse-to-pulse comparisons for performing moving target indication and removing " clutter " from
17466-429: The weak microwave signal to be amplified is applied to the buncher cavity through a coaxial cable or waveguide, and the amplified signal is extracted from the catcher cavity. At one end of the tube is the hot cathode which produces electrons when heated by a filament. The electrons are attracted to and pass through an anode cylinder at a high positive potential; the cathode and anode act as an electron gun to produce
17608-424: Was actually being generated. In 1941, the problem of frequency instability was solved by James Sayers coupling ("strapping") alternate cavities within the magnetron, which reduced the instability by a factor of 5–6. (For an overview of early magnetron designs, including that of Boot and Randall, see .) GEC at Wembley made 12 prototype cavity magnetrons in August 1940, and No 12 was sent to America with Bowen on
17750-445: Was also noticed that the frequency of the radiation depends on the size of the tube, and even early examples were built that produced signals in the microwave regime. Early conventional tube systems were limited to the high frequency bands, and although very high frequency systems became widely available in the late 1930s, the ultra high frequency and microwave bands were well beyond the ability of conventional circuits. The magnetron
17892-475: Was an airborne early warning and control system, providing the U.S. Navy with a surveillance capability to detect low-flying enemy aircraft at a range in excess of 100 miles (161 km). The project was initiated at a low level in mid-1942, but with the later advent of Japanese Kamikaze threats in the Pacific Theater of Operations , the work was greatly accelerated, eventually involving 20 percent of
18034-469: Was before atomic bomb development had begun. Ernest Lawrence was an active participant in forming the Rad Lab and personally recruited many key members of the initial staff. Most of the senior staff were Ph.D. physicists who came from university positions. They usually had no more than an academic knowledge of microwaves, and almost no background involving electronic hardware development. Their capability, however, to tackle complex problems of almost any type
18176-401: Was completed and demonstrated successfully on August 30, 1937. Upon publication in 1939, news of the klystron immediately influenced the work of US and UK researchers working on radar equipment. The Varians went on to found Varian Associates to commercialize the technology (for example, to make small linear accelerators to generate photons for external beam radiation therapy ). Their work
18318-512: Was converted to a very effective mapping radar called H2X or Mickey and used by the U.S. Army Air Force and U.S. Navy as well as the British Royal Air Force. The most ambitious Rad Lab effort with long-term significance was Project Cadillac. Led by Jerome B. Wiesner , the project involved a high-power radar carried in a pod under a TBM Avenger aircraft and a Combat Information Center aboard an aircraft carrier. The objective
18460-530: Was inadequate, in both range and accuracy, to support aircraft during bombing runs on distant targets in Europe. When briefed by the Tizard Mission about GEE, Alfred Loomis personally conceptualized a new type of system that would overcome the deficiencies of GEE, and the development of his LORAN (an acronym for Long Range Navigation) was adopted as an initial project. The LORAN Division was established for
18602-570: Was known that a multi-cavity resonant magnetron had been developed and patented in 1935 by Hans Hollmann in Berlin . However, the German military considered the frequency drift of Hollman's device to be undesirable, and based their radar systems on the klystron instead. But klystrons could not at that time achieve the high power output that magnetrons eventually reached. This was one reason that German night fighter radars, which never strayed beyond
18744-749: Was named an IEEE Milestone in 1990. Nine members of the Radiation Laboratory went on to win Nobel Prizes later in life: Klystron A klystron is a specialized linear-beam vacuum tube , invented in 1937 by American electrical engineers Russell and Sigurd Varian , which is used as an amplifier for high radio frequencies , from UHF up into the microwave range. Low-power klystrons are used as oscillators in terrestrial microwave relay communications links, while high-power klystrons are used as output tubes in UHF television transmitters , satellite communication , radar transmitters , and to generate
18886-485: Was no time to amend the drawings. And No 12 with 8 holes was chosen for the Tizard Mission. So Bell Labs chose to copy the sample; and while early British magnetrons had six cavities the American ones had eight cavities. According to Andy Manning from the RAF Air Defence Radar Museum , Randall and Boot's discovery was "a massive, massive breakthrough" and "deemed by many, even now [2007], to be
19028-521: Was no time to amend the drawings. No 12 with 8 holes was chosen for the Tizard Mission. So Bell Labs chose to copy the sample; and while early British magnetrons had six cavities American ones had eight cavities. Loomis arranged for funding under the National Defense Research Committee (NDRC) and reorganized the Microwave Committee at MIT to study the magnetron and radar technology in general. Lee A. DuBridge served as
19170-410: Was not completed until December 1941. This eventually was fielded as the ubiquitous SCR-584 , first gaining attention by directing the anti-aircraft fire that downed about 85 percent of German V-1 flying bombs ("buzz bombs") attacking London. Project 3, a long-range navigation system, was of particular interest to Great Britain. They had an existing hyperbolic navigation system, called GEE , but it
19312-409: Was one of the few devices able to generate signals in the microwave band and it was the only one that was able to produce high power at centimeter wavelengths. The original magnetron was very difficult to keep operating at the critical value, and even then the number of electrons in the circling state at any time was fairly low. This meant that it produced very low-power signals. Nevertheless, as one of
19454-404: Was outstanding. In June 1941, the NDRC became part of the new Office of Scientific Research and Development (OSRD), also administered by Vannevar Bush , who reported directly to President Roosevelt. The OSRD was given almost unlimited access to funding and resources, with the Rad Lab receiving a large share for radar research and development. Starting in 1942, the Manhattan Project absorbed
19596-524: Was preceded by the description of velocity modulation by A. Arsenjewa-Heil and Oskar Heil (wife and husband) in 1935, though the Varians were probably unaware of the Heils' work. The work of physicist W. W. Hansen was instrumental in the development of the klystron and was cited by the Varian brothers in their 1939 paper. His resonator analysis, which dealt with the problem of accelerating electrons toward
19738-543: Was shown on 19 September 1940 in Alfred Loomis’ apartment. The American NDRC Microwave Committee was stunned at the power level produced. However Bell Labs director Mervin Kelly was upset when it was X-rayed and had eight holes rather than the six holes shown on the GEC plans. After contacting (via the transatlantic cable) Dr Eric Megaw, GEC’s vacuum tube expert, Megaw recalled that when he had asked for 12 prototypes he said make 10 with 6 holes, one with 7 and one with 8; and there
19880-472: Was taken up by Philips , General Electric Company (GEC), Telefunken and others, limited to perhaps 10 W output. By this time the klystron was producing more power and the magnetron was not widely used, although a 300 W device was built by Aleksereff and Malearoff in the USSR in 1936 (published in 1940). The cavity magnetron was a radical improvement introduced by John Randall and Harry Boot at
20022-444: Was the deputy director for scientific matters, and F. Wheeler Loomis (no relation to Alfred Loomis) was deputy director for administration. E. G. ("Taffy") Bowen was assigned as a representative of Great Britain. Even before opening, the founders identified the first three projects for the Rad Lab. In the order of priority, these were (1) a 10-cm detection system (called aircraft interception radar , or AI) for fighter aircraft , (2)
20164-409: Was used as a local oscillator in some radar receivers and a modulator in microwave transmitters in the 1950s and 1960s, but is now obsolete, replaced by semiconductor microwave devices. In the reflex klystron the electron beam passes through a single resonant cavity. The electrons are fired into one end of the tube by an electron gun . After passing through the resonant cavity they are reflected by
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