Misplaced Pages

#189810

66-482: NCSX was one of a number of new stellarator designs from the 1990s that arose after studies illustrated new geometries that offered better performance than the simpler machines of the 1950s and 1960s. Compared to the more common tokamak , these were much more difficult to design and build, but produced far more stable plasma, the main problem with successful fusion. The design proved to be too difficult to build, repeatedly running over its budget and timelines. The project

132-417: A T c of 18 K. When operating at 4.2 K they are able to withstand a much higher magnetic field intensity , up to 25 T to 30 T. Unfortunately, it is far more difficult to make the required filaments from this material. This is why sometimes a combination of Nb 3 Sn for the high-field sections and NbTi for the lower-field sections is used. Vanadium–gallium is another material used for

198-420: A plasma inside a machine - the plasma would drift across the fields and eventually strike the vessel. His solution was simple; by bending the machine through a 180 degree twist, forming a figure-eight instead of a donut, the plasma would alternately find itself on the inside or outside of the vessel, drifting in opposite directions. The cancellation of net drift would not be perfect, but on paper, it appeared that

264-403: A stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix . Devices like the z-pinch and stellarator had attempted this, but demonstrated serious instabilities. It was the development of the concept now known as the safety factor (labelled q in mathematical notation) that guided tokamak development; by arranging the reactor so this critical factor q

330-488: A cryocooler operates using a piston type displacer and heat exchanger. Alternatively, 1999 marked the first commercial application using a pulse tube cryocooler . This design of cryocooler has become increasingly common due to low vibration and long service interval as pulse tube designs use an acoustic process in lieu of mechanical displacement. In a typical two-stage refrigerator, the first stage will offer higher cooling capacity but at higher temperature (≈ 77 K) with

396-500: A day, as protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting bending magnets is increased from 0.54 T to 8.3 T. The central solenoid and toroidal field superconducting magnets designed for the ITER fusion reactor use niobium–tin (Nb 3 Sn) as a superconductor. The central solenoid coil carries a current of 46 kA and produce a magnetic field of 13.5 T. The 18 toroidal field coils at

462-417: A goal of creating a 30-tesla superconducting magnet. Globally in 2014, almost six billion US dollars worth of economic activity resulted from which superconductivity is indispensable. MRI systems, most of which employ niobium–titanium, accounted for about 80% of that total. In 2016, Yoon et al. reported a 26 T no-insulation superconducting magnet that they built out of GdBa 2 Cu 3 O 7– x , using

528-420: A lower level than their design current. CERN states that this is due to electromagnetic forces causing tiny movements in the magnets, which in turn cause superconductivity to be lost when operating at the high precision needed for their planned current. By repeatedly running the magnets at a lower current and then slightly increasing the current until they quench under control, the magnet will gradually both gain

594-487: A machine where the maximum variation from the perfect placement was no more than 1.5 millimetres (0.059 in) across the entire device. The vacuum vessel surrounding all of this was likewise very complex, with the added complication of carrying all of the wiring to feed power to the magnets. The assembly tolerances were very tight and required state of the art use of metrology systems including Laser Tracker and photogrammetry equipment. $ 50 million of additional funding

660-399: A magnet quench is a "fairly routine event" during the operation of a particle accelerator. In certain cases, superconducting magnets designed for very high currents require extensive bedding in, to enable the magnets to function at their full planned currents and fields. This is known as "training" the magnet, and involves a type of material memory effect. One situation this is required in is

726-476: A new series of machines were designed that would run on a fusion fuel of deuterium and tritium . These machines, notably the Joint European Torus (JET) and Tokamak Fusion Test Reactor (TFTR), had the explicit goal of reaching breakeven. Instead, these machines demonstrated new problems that limited their performance. Solving these would require a much larger and more expensive machine, beyond

SECTION 10

#1732776363190

792-405: A practical superconducting electromagnet had to await the discovery of superconducting materials that could support large critical supercurrent densities in high magnetic fields. The first successful superconducting magnet was built by G.B. Yntema in 1955 using niobium wire and achieved a field of 0.7 T at 4.2 K. Then, in 1961, J.E. Kunzler , E. Buehler, F.S.L. Hsu, and J.H. Wernick made

858-411: A superconducting magnet are made of wires or tapes of Type II superconductors (e.g. niobium–titanium or niobium–tin ). The wire or tape itself may be made of tiny filaments (about 20 micrometres thick) of superconductor in a copper matrix. The copper is needed to add mechanical stability, and to provide a low resistance path for the large currents in case the temperature rises above T c or

924-502: A technique which was previously reported in 2013. In 2017, a YBCO magnet created by the National High Magnetic Field Laboratory (NHMFL) broke the previous world record with a strength of 32 T. This is an all superconducting user magnet, designed to last for many decades. They hold the current record as of March 2018. In 2019, a new world-record of 32.35 T with all-superconducting magnet

990-535: A thermal shield made of conductive material and maintained in 40 K – 60 K temperature range, cooled by conductive connections to the cryocooler cold head, is placed around the helium-filled vessel to keep the heat input to the latter at acceptable level. One of the goals of the search for high temperature superconductors is to build magnets that can be cooled by liquid nitrogen alone. At temperatures above about 20 K cooling can be achieved without boiling off cryogenic liquids. Because of increasing cost and

1056-406: Is a small residual resistance in the superconducting windings due to joints or a phenomenon called flux motion resistance. Nearly all commercial superconducting magnets are equipped with persistent switches. A quench is an abnormal termination of magnet operation that occurs when part of the superconducting coil enters the normal ( resistive ) state. This can occur because the field inside the magnet

1122-524: Is achieved by Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS). No-insulation technique for the HTS insert magnet is also used. In 2019, the NHMFL also developed a non-insulated YBCO test coil combined with a resistive magnet and broke the lab's own world record for highest continuous magnetic field for any configuration of magnet at 45.5 T. A 1.2 GHz (28.2 T) NMR magnet

1188-517: Is an electromagnet made from coils of superconducting wire . They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire has no electrical resistance and therefore can conduct much larger electric currents than ordinary wire, creating intense magnetic fields. Superconducting magnets can produce stronger magnetic fields than all but the strongest non-superconducting electromagnets , and large superconducting magnets can be cheaper to operate because no energy

1254-543: Is dissipated as heat in the windings. They are used in MRI instruments in hospitals, and in scientific equipment such as NMR spectrometers, mass spectrometers , fusion reactors and particle accelerators . They are also used for levitation, guidance and propulsion in a magnetic levitation (maglev) railway system being constructed in Japan . During operation, the magnet windings must be cooled below their critical temperature ,

1320-459: Is generally more stable, resulting in less noisy measurements. They can be smaller, and the area at the center of the magnet where the field is created is empty rather than being occupied by an iron core. Large magnets can consume much less power. In the persistent state (above), the only power the magnet consumes is that needed for refrigeration equipment. Higher fields can be achieved with cooled resistive electromagnets, as superconducting coils enter

1386-501: Is its upper critical field . Another limiting factor is the "critical current", I c , at which the winding material also ceases to be superconducting. Advances in magnets have focused on creating better winding materials. The superconducting portions of most current magnets are composed of niobium–titanium . This material has critical temperature of 10  K and can superconduct at up to about 15  T . More expensive magnets can be made of niobium–tin (Nb 3 Sn). These have

SECTION 20

#1732776363190

1452-406: Is made by a 'persistent switch', a piece of superconductor inside the magnet connected across the winding ends, attached to a small heater. When the magnet is first turned on, the switch wire is heated above its transition temperature, so it is resistive. Since the winding itself has no resistance, no current flows through the switch wire. To go to persistent mode, the supply current is adjusted until

1518-411: Is too large, the rate of change of field is too large (causing eddy currents and resultant heating in the copper support matrix), or a combination of the two. More rarely a defect in the magnet can cause a quench. When this happens, that particular spot is subject to rapid Joule heating from the enormous current, which raises the temperature of the surrounding regions. This pushes those regions into

1584-439: Is used as a coolant for many superconductive windings. It has a boiling point of 4.2 K, far below the critical temperature of most winding materials. The magnet and coolant are contained in a thermally insulated container ( dewar ) called a cryostat . To keep the helium from boiling away, the cryostat is usually constructed with an outer jacket containing (significantly cheaper) liquid nitrogen at 77 K. Alternatively,

1650-715: The United States Department of Energy . Its fate is reminiscent of other Department of Energy projects, such as the Mirror Fusion Test Facility , which was constructed but never used, and the Superconducting Super Collider , which cost $ 2 billion prior to its cancellation. Tokamak A tokamak ( / ˈ t oʊ k ə m æ k / ; Russian : токамáк ) is a device which uses a powerful magnetic field generated by external magnets to confine plasma in

1716-427: The 1980s it was noted that one way to improve tokamak performance was to use non-circular cross-sections for the plasma confinement area; ions moving in these non-uniform areas would mix and break up the formation of large-scale instabilities. Applying the same logic to the stellarator appeared to offer the same advantages. Yet, as the stellarator lacked, or lowered, the plasma current, the plasma would be more stable from

1782-581: The Japanese government gave permission to JR Central to build the Chūō Shinkansen , linking Tokyo to Nagoya and later to Osaka. One of the most challenging uses of superconducting magnets is in the LHC particle accelerator. Its niobium–titanium (Nb–Ti) magnets operate at 1.9 K to allow them to run safely at 8.3 T. Each magnet stores 7 MJ. In total the magnets store 10.4 GJ . Once or twice

1848-614: The abilities of any one country. After an initial agreement between Ronald Reagan and Mikhail Gorbachev in November 1985, the International Thermonuclear Experimental Reactor (ITER) effort emerged and remains the primary international effort to develop practical fusion power. Many smaller designs, and offshoots like the spherical tokamak , continue to be used to investigate performance parameters and other issues. As of 2024 , JET remains

1914-512: The case of particle colliders such as CERN 's Large Hadron Collider . The magnets of the LHC were planned to run at 8 TeV (2 × 4 TeV) on its first run and 14 TeV (2 × 7 TeV) on its second run, but were initially operated at a lower energy of 3.5 TeV and 6.5 TeV per beam respectively. Because of initial crystallographic defects in the material, they will initially lose their superconducting ability ("quench") at

1980-513: The coil windings is provided by a high current, very low voltage DC power supply , since in steady state the only voltage across the magnet is due to the resistance of the feeder wires. Any change to the current through the magnet must be done very slowly, first because electrically the magnet is a large inductor and an abrupt current change will result in a large voltage spike across the windings, and more importantly because fast changes in current can cause eddy currents and mechanical stresses in

2046-515: The coils would need 15 minutes to re-cool between high It plasma runs. Baseline total project cost of $ 102M for completion date of July 2009. First contracts placed in 2004. With the design largely complete, the PPPL began the process of building such a machine, the NCSX, which would test all of these concepts. The design used eighteen complicated hand-wound magnets, which then had to be assembled into

National Compact Stellarator Experiment - Misplaced Pages Continue

2112-535: The current rises above I c and superconductivity is lost. These filaments need to be this small because in this type of superconductor the current only flows in a surface layer whose thickness is limited to the London penetration depth (see Skin effect ). The coil must be carefully designed to withstand (or counteract) magnetic pressure and Lorentz forces that could otherwise cause wire fracture or crushing of insulation between adjacent turns. The current to

2178-436: The delay in drift rates was more than enough to allow the plasma to reach fusion conditions. In practice, this proved not to be. A problem seen in all fusion reactor designs of the era was that the plasma ions were drifting much faster than classical theory predicted, hundreds to thousands of times faster. Designs that suggested stability on the order of seconds turned into machines that were stable for microseconds at best. By

2244-446: The design, but each one further delayed the completion and required more funding. (The 2008 cost estimate was $ 170M with an August 2013 scheduled completion.) Eventually a go/no-go condition was imposed, and when the goal was not met on budget, the project was cancelled. Due to its cancellation in 2008, the project has been cited as a case study of the hypothetical demon of Bureaucratic Chaos, which "blocks good things from happening" at

2310-411: The desired magnetic field is obtained, then the heater is turned off. The persistent switch cools to its superconducting temperature, short-circuiting the windings. Then the power supply can be turned off. The winding current, and the magnetic field, will not actually persist forever, but will decay slowly according to a normal inductive time constant ( L / R ): where R {\displaystyle R}

2376-410: The dipole bending magnets are connected in series, each power circuit includes 154 individual magnets, and should a quench event occur, the entire combined stored energy of these magnets must be dumped at once. This energy is transferred into dumps that are massive blocks of metal which heat up to several hundreds of degrees Celsius due to the resistive heating in a matter of seconds. Although undesirable,

2442-460: The discovery that a compound of niobium and tin could support critical-supercurrent densities greater than 100,000 amperes per square centimetre in magnetic fields of 8.8 teslas. Despite its brittle nature, niobium–tin has since proved extremely useful in supermagnets generating magnetic fields up to 20 T. The persistent switch was invented in 1960 by Dwight Adams while a postdoctoral associate at Stanford University. The second persistent switch

2508-428: The dwindling availability of liquid helium, many superconducting systems are cooled using two stage mechanical refrigeration. In general two types of mechanical cryocoolers are employed which have sufficient cooling power to maintain magnets below their critical temperature. The Gifford–McMahon cryocooler has been commercially available since the 1960s and has found widespread application. The G-M regenerator cycle in

2574-476: The electronic plasma temperature of 1 keV was reached on the tokamak T-3, built at the I. V. Kurchatov Institute of Atomic Energy under the leadership of academician L. A. Artsimovich. By the mid-1960s, the tokamak designs began to show greatly improved performance. The initial results were released in 1965, but were ignored; Lyman Spitzer dismissed them out of hand after noting potential problems in their system for measuring temperatures. A second set of results

2640-415: The high-field inserts. High-temperature superconductors (e.g. BSCCO or YBCO ) may be used for high-field inserts when required magnetic fields are higher than Nb 3 Sn can manage. BSCCO, YBCO or magnesium diboride may also be used for current leads, conducting high currents from room temperature into the cold magnet without an accompanying large heat leak from resistive leads. The coil windings of

2706-518: The latest stellarator designs; the Model C had only recently started operations, and was rapidly converted into the Symmetric Tokamak. By the late 1980s it was clear that while the tokamak was a great step forward, it also introduced new problems. In particular, the plasma current the tokamak used for stabilization and heating was itself a source of instabilities as the current grew. Much of

National Compact Stellarator Experiment - Misplaced Pages Continue

2772-400: The magnet has been energized. The windings become a closed superconducting loop, the power supply can be turned off, and persistent currents will flow for months, preserving the magnetic field. The advantage of this persistent mode is that stability of the magnetic field is better than is achievable with the best power supplies, and no energy is needed to power the windings. The short circuit

2838-444: The magnet is rare, but components can be damaged by localized heating, high voltages, or large mechanical forces. In practice, magnets usually have safety devices to stop or limit the current when the beginning of a quench is detected. If a large magnet undergoes a quench, the inert vapor formed by the evaporating cryogenic fluid can present a significant asphyxiation hazard to operators by displacing breathable air. A large section of

2904-434: The mid-1960s the entire fusion energy field appeared stalled. It was only the 1968 introduction of the tokamak design that rescued the field; Soviet machines were performing at least an order of magnitude better than western designs, although still far short of practical values. The improvement was so dramatic that work on other designs largely ended as teams around the world began to study the tokamak approach. This included

2970-424: The most widely used supermagnet materials. In 1986, the discovery of high temperature superconductors by Georg Bednorz and Karl Müller energized the field, raising the possibility of magnets that could be cooled by liquid nitrogen instead of the more difficult-to-work-with helium. In 2007, a magnet with windings of YBCO achieved a world record field of 26.8  T . The US National Research Council has

3036-519: The non-superconducting state at high fields. Steady fields of over 40 T can be achieved, usually by combining a Bitter electromagnet with a superconducting magnet (often as an insert). Superconducting magnets are widely used in MRI scanners, NMR equipment, mass spectrometers , magnetic separation processes, and particle accelerators . In Japan, after decades of research and development into superconducting maglev by Japanese National Railways and later Central Japan Railway Company (JR Central),

3102-448: The normal state as well, which leads to more heating in a chain reaction. The entire magnet rapidly becomes normal (this can take several seconds, depending on the size of the superconducting coil). This is accompanied by a loud bang as the energy in the magnetic field is converted to heat, and rapid boil-off of the cryogenic fluid. The abrupt decrease of current can result in kilovolt inductive voltage spikes and arcing. Permanent damage to

3168-1057: The record holder for fusion output, with 69 MJ of energy output over a 5-second period. The word tokamak is a transliteration of the Russian word токамак , an acronym of either: то роидальная to roidal'naya to roidal ка мера ka mera cha mber с s with ма гнитными ma gnitnymi ma gnetic к атушками k atushkami c oils то роидальная ка мера с ма гнитными к атушками to roidal'naya ka mera s ma gnitnymi k atushkami to roidal cha mber with ma gnetic c oils or: то роидальная to roidal'naya to roidal кам ера kam era cham ber с s with ак сиальным ak sial'nym ax ial магнитным magnitnym magnetic полем polem field то роидальная кам ера с ак сиальным магнитным полем to roidal'naya kam era s ak sial'nym magnitnym polem to roidal cham ber with ax ial magnetic field Superconducting magnet A superconducting magnet

3234-436: The required ability to withstand the higher currents of its design specification without quenches occurring, and have any such issues "shaken" out of them, until they are eventually able to operate reliably at their full planned current without experiencing quenches. Although the idea of making electromagnets with superconducting wire was proposed by Heike Kamerlingh Onnes shortly after he discovered superconductivity in 1911,

3300-450: The second stage reaching ≈ 4.2 K and <  2.0  W of cooling power. In use, the first stage is used primarily for ancillary cooling of the cryostat with the second stage used primarily for cooling the magnet. The maximal magnetic field achievable in a superconducting magnet is limited by the field at which the winding material ceases to be superconducting, its "critical field", H c , which for type-II superconductors

3366-456: The shape of an axially symmetrical torus . The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power . The tokamak concept is currently one of the leading candidates for a practical fusion reactor . The proposal to use controlled thermonuclear fusion for industrial purposes and a specific scheme using thermal insulation of high-temperature plasma by an electric field

SECTION 50

#1732776363190

3432-437: The start. When one considers the magnet layout needed to achieve both goals, a twisted path around the circumference of the device as well as many smaller twists and mixes along the way, the design becomes extremely complex, well beyond the abilities of conventional design tools. It was only through the use of massively parallel computers that the designs could be studied in depth, and suitable magnet designs created. The result

3498-404: The stellarator in particular appeared to have a number of potential changes that would greatly improve its performance. The basic idea of the stellarator was to use the layout of the magnets to cancel out ion drift, but the simple designs of the 1950s did not do this to the degree needed. A greater problem were the instabilities and collisional effects that greatly increased the diffusion rates. In

3564-404: The stellarators of the 1960s, the new machines could use superconducting magnets for much higher field strengths, be only slightly larger than the Model C yet have far larger plasma volume, and have a plasma area inside that varied from circular to planar and back while twisting several times. The 18 modular coils have a complicated 3D shape, ~ 9 different curves in different planes. Some of

3630-412: The subsequent 30 years of tokamak development has focused on ways to increase this current to the levels required to sustain useful fusion while ensuring that same current does not cause the plasma to break up. As the magnitude of the problem with the tokamak became evident, fusion teams around the world began to take a fresh look at other design concepts. Among a number of ideas noted during this process,

3696-471: The superconducting magnets in CERN 's Large Hadron Collider unexpectedly quenched during start-up operations in 2008, necessitating the replacement of a number of magnets. In order to mitigate against potentially destructive quenches, the superconducting magnets that form the LHC are equipped with fast-ramping heaters that are activated once a quench event is detected by the complex quench protection system. As

3762-618: The temperature at which the winding material changes from the normal resistive state and becomes a superconductor , which is in the cryogenic range far below room temperature. The windings are typically cooled to temperatures significantly below their critical temperature, because the lower the temperature, the better superconductive windings work—the higher the currents and magnetic fields they can stand without returning to their non-superconductive state. Two types of cooling systems are commonly used to maintain magnet windings at temperatures sufficient to maintain superconductivity: Liquid helium

3828-407: The windings that can precipitate a quench (see below). So the power supply is usually microprocessor-controlled, programmed to accomplish current changes gradually, in gentle ramps. It usually takes several minutes to energize or de-energize a laboratory-sized magnet. An alternate operating mode used by most superconducting magnets is to short-circuit the windings with a piece of superconductor once

3894-440: Was a very compact device, significantly smaller outside than a classical design for any given volume of plasma, with a low aspect ratio . Lower aspect ratios are highly desirable, because they allow a machine of any given power to be smaller, which lowers construction costs. By the late 1990s the studies into new stellarator designs had reached a suitable point for the construction of a machine using these concepts. In comparison to

3960-650: Was achieved in 2020 using an HTS magnet. In 2022, the Hefei Institutes of Physical Science, Chinese Academy of Sciences (HFIPS, CAS) claims new world record for strongest steady magnetic field of 45.22 T reached, while the previous NHMFL 45.5 T record in 2019 was actually reached when the magnet failed immediately in a quench . Superconducting magnets have a number of advantages over resistive electromagnets. They can generate much stronger magnetic fields than ferromagnetic-core electromagnets , which are limited to fields of around 2 T. The field

4026-417: Was always greater than 1, the tokamaks strongly suppressed the instabilities which plagued earlier designs. By the mid-1970s, dozens of tokamaks were in use around the world. By the late 1970s, these machines had reached all of the conditions needed for practical fusion , although not at the same time nor in a single reactor . With the goal of breakeven (a fusion energy gain factor equal to 1) now in sight,

SECTION 60

#1732776363190

4092-696: Was constructed at the University of Florida by M.S. student R.D. Lichti in 1963. It has been preserved in a showcase in the UF Physics Building. In 1962, T.G. Berlincourt and R.R. Hake discovered the high-critical-magnetic-field, high-critical-supercurrent-density properties of niobium–titanium alloys. Although niobium–titanium alloys possess less spectacular superconducting properties than niobium–tin, they are highly ductile, easily fabricated, and economical. Useful in supermagnets generating magnetic fields up to 10 teslas, niobium–titanium alloys are

4158-442: Was eventually cancelled on 22 May 2008, having spent over $ 70 M. Wendelstein 7-X explores many of the same concepts that NCSX intended to. Stellarators are one of the first fusion power concepts, originally designed by Princeton astrophysicist Lyman Spitzer in 1952 while riding the chairlifts at Aspen . Spitzer, considering the motion of plasmas in the stars, realized that any simple arrangements of magnets would not confine

4224-508: Was first formulated by the Soviet physicist Oleg Lavrentiev in a mid-1950 paper. In 1951, Andrei Sakharov and Igor Tamm modified the scheme by proposing a theoretical basis for a thermonuclear reactor, where the plasma would have the shape of a torus and be held by a magnetic field. The first tokamak was built in 1954, and for over a decade this technology existed only in the USSR. In 1968

4290-530: Was needed, spread over the next 3 years, to complete the assembly within tolerance requirements. Components for the Stellarator were measured with 3d laser scanning, and inspected to design models at multiple stages in the manufacturing process. The required tolerances could not be achieved; As the modules were assembled, parts were found to be in contact, would sag once installed, and other unexpected effects made alignment very difficult. Fixes were worked into

4356-582: Was published in 1968, this time claiming performance far in advance of any other machine. When these were also met skeptically, the Soviets invited British scientists from the laboratory in Culham Centre for Fusion Energy (Nicol Peacock et al.) to the USSR with their equipment. Measurements on the T-3 confirmed the results, spurring a worldwide stampede of tokamak construction. It had been demonstrated that

#189810