The neutron is a subatomic particle, symbol n or n0, with no net electric charge and a mass larger than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave within the nucleus, each has a mass of one atomic mass unit, they are both referred to as nucleons, their properties and interactions are described by nuclear physics. The chemical and nuclear properties of the nucleus are determined by the number of protons, called the atomic number, the number of neutrons, called the neutron number; the atomic mass number is the total number of nucleons. For example, carbon has atomic number 6, its abundant carbon-12 isotope has 6 neutrons, whereas its rare carbon-13 isotope has 7 neutrons; some elements occur in nature with only one stable isotope, such as fluorine. Other elements occur with many stable isotopes, such as tin with ten stable isotopes. Within the nucleus and neutrons are bound together through the nuclear force. Neutrons are required for the stability of nuclei, with the exception of the single-proton hydrogen atom.
Neutrons are produced copiously in nuclear fusion. They are a primary contributor to the nucleosynthesis of chemical elements within stars through fission and neutron capture processes; the neutron is essential to the production of nuclear power. In the decade after the neutron was discovered by James Chadwick in 1932, neutrons were used to induce many different types of nuclear transmutations. With the discovery of nuclear fission in 1938, it was realized that, if a fission event produced neutrons, each of these neutrons might cause further fission events, etc. in a cascade known as a nuclear chain reaction. These events and findings led to the first self-sustaining nuclear reactor and the first nuclear weapon. Free neutrons, while not directly ionizing atoms, cause ionizing radiation; as such they can be a biological hazard, depending upon dose. A small natural "neutron background" flux of free neutrons exists on Earth, caused by cosmic ray showers, by the natural radioactivity of spontaneously fissionable elements in the Earth's crust.
Dedicated neutron sources like neutron generators, research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. An atomic nucleus is formed by a number of protons, Z, a number of neutrons, N, bound together by the nuclear force; the atomic number defines the chemical properties of the atom, the neutron number determines the isotope or nuclide. The terms isotope and nuclide are used synonymously, but they refer to chemical and nuclear properties, respectively. Speaking, isotopes are two or more nuclides with the same number of protons; the atomic mass number, symbol A, equals Z+N. Nuclides with the same atomic mass number are called isobars; the nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atomic nuclei are composed of two or more protons and various numbers of neutrons.
The most common nuclide of the common chemical element lead, 208Pb, has 82 protons and 126 neutrons, for example. The table of nuclides comprises all the known nuclides. Though it is not a chemical element, the neutron is included in this table; the free neutron has 1.674927471 × 10 − 27 kg, or 1.00866491588 u. The neutron has a mean square radius of about 0.8×10−15 m, or 0.8 fm, it is a spin-½ fermion. The neutron has no measurable electric charge. With its positive electric charge, the proton is directly influenced by electric fields, whereas the neutron is unaffected by electric fields; the neutron has a magnetic moment, however. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin. A free neutron is unstable, decaying to a proton and antineutrino with a mean lifetime of just under 15 minutes; this radioactive decay, known as beta decay, is possible because the mass of the neutron is greater than the proton. The free proton is stable. Neutrons or protons bound in a nucleus can be stable or unstable, depending on the nuclide.
Beta decay, in which neutrons decay to protons, or vice versa, is governed by the weak force, it requires the emission or absorption of electrons and neutrinos, or their antiparticles. Protons and neutrons behave identically under the influence of the nuclear force within the nucleus; the concept of isospin, in which the proton and neutron are viewed as two quantum states of the same particle, is used to model the interactions of nucleons by the nuclear or weak forces. Because of the strength of the nuclear force at short distances, the binding energy of nucleons is more than seven orders of magnitude larger than the electromagnetic energy binding electrons in atoms. Nuclear reactions therefore have an energy density, more than ten million times that of chemical reactions; because of the mass–energy equivalence, nuclear binding energies reduce the mass of nuclei. The ability of the nuclear force to store energy arising from the electromagnetic repulsion of nuclear components is the basis for most of the energy that makes nuclear reactors or bombs possible.
In nuclear fission, the absorption of a neutron by a heavy nuclide causes the nuclide to become unstable and break into light nuclides and additional neu
A stellarator is a plasma device that relies on external magnets to confine a plasma. In the future, scientists researching magnetic confinement fusion aim to use stellarator devices as a vessel for nuclear fusion reactions; the name refers to the possibility of harnessing the power source of the stars, including the sun.stellar object. It is one of the earliest fusion power devices, along with the magnetic mirror; the stellarator was invented by Lyman Spitzer of Princeton University in 1951, much of its early development was carried out by his team at what became the Princeton Plasma Physics Laboratory. Lyman's Model A demonstrated that stellarators could confine plasmas. Larger models followed, but these demonstrated poor performance, suffering from a problem known as pump-out that caused them to lose plasma at rates far worse than theoretical predictions. By the early 1960s, any hope of producing a commercial machine faded, attention turned to studying the fundamental theory of high-energy plasmas.
By the mid-1960s, Spitzer was convinced that the stellarator was matching the Bohm diffusion rate, which suggested it would never be a practical fusion device. The release of information on the USSR's tokamak design in 1968 indicated a leap in performance; this led to the Model C stellarator being converted to the Symmetrical Tokamak as a way to confirm or deny these results. ST confirmed them, large-scale work on the stellarator concept ended as the tokamak got most of the attention; the tokamak proved to have similar problems to the stellarators, but for different reasons. Since the 1990s, this has led to renewed interest in the stellarator design. New methods of construction have increased the quality and power of the magnetic fields, improving performance. A number of new devices have been built to test these concepts. Major examples include Wendelstein 7-X in Germany, the Helically Symmetric Experiment in the USA, the Large Helical Device in Japan. In 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford were the first to achieve fusion on Earth, using a particle accelerator to shoot deuterium nuclei into a metal foil containing deuterium, lithium or other elements.
This system allowed them to measure the nuclear cross section of various fusion reactions, determined that the tritium-deuterium reaction occurred at a lower energy than any other fuel, peaking at about 100,000 electronvolts.100 keV corresponds to a temperature of about a billion kelvins. Due to the Maxwell–Boltzmann statistics, a bulk gas at a much lower temperature will still contain some particles at these much higher energies; because the fusion reactions release so much energy a small number of these reactions can release enough energy to keep the gas at the required temperature. In 1944, Enrico Fermi demonstrated that this would occur at a bulk temperature of about 50 million Celsius, still hot but within the range of existing experimental systems; the key problem was confining such a plasma. But because plasmas are electrically conductive, they are subject to electric and magnetic fields which provide a number of solutions. In a magnetic field, the electrons and nuclei of the plasma circle the magnetic lines of force.
One way to provide some confinement would be to place a tube of fuel inside the open core of a solenoid. A solenoid creates magnetic lines running down its center, fuel would be held away from the walls by orbiting these lines of force, but such an arrangement does not confine the plasma along the length of the tube. The obvious solution is to bend the tube around into a torus shape, so that any one line forms a circle, the particles can circle forever. However, this solution does not work. For purely geometric reasons, the magnets ringing the torus are closer together on the inside curve, inside the "donut hole". Fermi noted this would cause the electrons to drift away from the nuclei causing them to separate and cause large voltages to develop; the resulting electric field would cause the plasma ring inside the torus to expand until it hit the walls of the reactor. In the post-war era, a number of researchers began considering different ways to confine a plasma. George Paget Thomson of Imperial College London proposed a system now known as z-pinch, which runs a current through the plasma.
Due to the Lorentz force, this current creates a magnetic field that pulls the plasma in on itself, keeping it away from the walls of the reactor. This eliminates the need for magnets on the outside. Various teams in the UK had built a number of small experimental devices using this technique by the late 1940s. Another person working on controlled fusion reactors was Ronald Richter, a former German scientist who moved to Argentina after the war, his thermotron used a system of electrical arcs and mechanical compression for heating and confinement. He convinced Juan Perón to fund development of an experimental reactor on an isolated island near the Chilean border. Known as the Huemul Project, this was completed in 1951. Richter soon convinced himself fusion had been achieved in spite of other people working on the project disagreeing; the "success" was announced by Perón on 24 March 1951, becoming the topic of newspaper stories around the world. While preparing for a ski trip to Aspen, Lyman Spitzer received a telephone call from his father, who mentioned an article on Huemul in the New York Times.
Looking over the description in the article, Spitzer concluded it could not work. But the idea stuck with him, he began considering systems that would wor
A nova or classical nova is a transient astronomical event that causes the sudden appearance of a bright "new" star, that fades over several weeks or many months. Novae involve an interaction between two stars that cause the flareup, perceived as a new entity, much brighter than the stars involved. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve located binary stars, either a pair of red dwarfs in the process of merging, or a white dwarf and another star; the main sub-classes of novae are classical novae, recurrent novae, dwarf novae. They are all considered to be cataclysmic variable stars. Luminous red novae share the name and are cataclysmic variables, but are a different type of event caused by a stellar merger. With similar names are the much more energetic supernovae and kilonovae. Classical nova eruptions are the most common type of nova, they are created in a close binary star system consisting of a white dwarf and either a main sequence, sub-giant, or red giant star.
When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere is hydrogen and is thermally heated by the hot white dwarf, which reaches a critical temperature causing rapid runaway ignition by fusion. From the dramatic and sudden energies created, the now hydrogen-burnt atmosphere is dramatically expelled into interstellar space, its brightened envelope is seen as the visible light created from the nova event, was mistaken as a "new" star. A few novae produce short-lived nova remnants, lasting for several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf. Novae most occur in the sky along the path of the Milky Way near the observed galactic centre in Sagittarius.
They occur far more than galactic supernovae, averaging about ten per year. Most are found telescopically only one every year to eighteen months reaching naked-eye visibility. Novae reaching first or second magnitude occur only several times per century; the last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013. During the sixteenth century, astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, he described it in his book De nova stella. In this work he argued that a nearby object should be seen to move relative to the fixed stars, that the nova had to be far away. Although this event was a supernova and not a nova, the terms were considered interchangeable until the 1930s. After this, novae were classified as classical novae to distinguish them from supernovae, as their causes and energies were thought to be different, based in the observational evidence. Despite the term "stella nova" meaning "new star", novae most take place as a result of white dwarfs: remnants of old stars.
Evolution of potential novae begins with two main sequence stars in a binary system. One of the two evolves into a red giant, leaving its remnant white dwarf core in orbit with the remaining star; the second star—which may be either a main sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe. As a result, the white dwarf captures matter from the companion's outer atmosphere in an accretion disk, in turn, the accreted matter falls into the atmosphere; as the white dwarf consists of degenerate matter, the accreted hydrogen does not inflate, but its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K, initiating nuclear burning, via the CNO cycle. Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally and converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction, liberating an enormous amount of energy.
This blows the remaining gases away from the surface of the white dwarf surface and produces an bright outburst of light. The rise to peak brightness may be rapid, or gradual; this is related to the speed class of the nova. The time taken for a nova to decay by around 2 or 3 magnitudes from maximum optical brightness is used for classification, via its speed class. Fast novae will take fewer than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days. In spite of their violence the amount of material ejected in novae is only about 1⁄10,000 of a solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused during the power outburst. Nonetheless, this is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second—higher for fast novae than slow ones—with a concurrent rise in luminosity from a few times solar to 50,000–100,000 times solar. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova can emit gamma-rays.
A white dwarf can generate multiple novae over t
Reversed field pinch
A reversed-field pinch is a device used to produce and contain near-thermonuclear plasmas. It is a toroidal pinch which uses a unique magnetic field configuration as a scheme to magnetically confine a plasma to study magnetic fusion energy, its magnetic geometry is somewhat different from that of the more common tokamak. As one moves out radially, the portion of the magnetic field pointing toroidally reverses its direction, giving rise to the term "reversed field"; this configuration can be sustained with comparatively lower fields than that of a tokamak of similar power density. One of the disadvantages of this configuration is that it tends to be more susceptible to non-linear effects and turbulence; this makes it a perfect laboratory for non-ideal magnetohydrodynamics. RFPs are used in the study of astrophysical plasmas as they share many features; the largest Reversed Field Pinch device presently in operation is the RFX in Italy. Others include the MST in the United States, EXTRAP T2R in Sweden, TPE-RX in Japan, KTX in China.
Unlike the Tokamak, which has a much larger magnetic field in the toroidal direction than the poloidal direction, an RFP has a comparable field strength in both directions. Moreover, a typical RFP has a field strength one half to one tenth that of a comparable Tokamak; the RFP relies on driving current in the plasma to reinforce the field from the magnets through the dynamo effect. The reversed-field pinch works towards a state of minimum energy; the magnetic field lines coil loosely around a center torus. They coil outwards. Near the plasma edge, the toroidal magnetic field reverses and the field lines coil in the reverse direction. Internal fields are bigger than the fields at the magnets; the RFP has many features. Due to the lower overall fields, an RFP reactor might not need superconducting magnets; this is a large advantage over tokamaks since superconducting magnets are delicate and expensive and so must be shielded from the neutron rich fusion environment. RFPs are susceptible to so require a close fitting shell.
Some experiments use their close fitting shell as a magnetic coil by driving current through the shell itself. This is attractive from a reactor standpoint since a solid copper shell would be robust against high energy neutrons, compared with superconducting magnets. There is no established beta limit for RFPs. There exists a possibility that a reversed field pinch could achieve ignition with ohmic power, which would be much simpler than tokamak designs, though it could not be operated in steady state. RFPs require a large amount of current to be driven, although promising experiments are underway, there is no established method of replacing ohmically driven current, fundamentally limited by the machine parameters. RFPs are prone to tearing modes which lead to overlapping magnetic islands and therefore rapid transport from the core of the plasma to the edge; these problems are areas of active research in the RFP community. The plasma confinement in the best RFP's is only about 1% as good as in the best tokamaks.
One reason for this is that all existing RFP's are small. MST was larger than any previous RFP device, thus it tested this important size issue.. The RFP is believed to require a shell with high electrical conductivity close to the boundary of the plasma; this requirement is an unfortunate complication in a reactor. The Madison Symmetric Torus was designed to test this assumption and to learn how good the conductor must be and how close to the plasma it must be placed. In RFX, the thick shell was replaced with an active system of 192 coils, which cover the entire torus with their saddle shape, response to the magnetic push of the plasma. Active control of plasma modes is possible with this system; the Reversed Field Pinch is interesting from a physics standpoint. RFP dynamics are turbulent. RFPs exhibit a strong plasma dynamo, similar to many astrophysical bodies. Basic plasma science is another important aspect of Reversed Field Pinch research. Self-organized plasmas RFX: Reversed-Field eXperiment Measurement of superthermal electron flow and temperature in a reversed-field pinch experiment by an electrostatic electron energy analyser
A levitated dipole is a type of nuclear fusion reactor design using a superconducting torus, magnetically levitated inside the reactor chamber. The name refers to the magnetic dipole that forms within the reaction chamber, similar to Earth's or Jupiter's magnetospheres, it is believed that such an apparatus could contain plasma more efficiently than other fusion reactor designs. The Levitated Dipole Experiment was funded by the US Department of Energy's Office of Fusion Energy; the machine was run in a collaboration between Columbia University. Funding for the LDX was ended in November 2011 to concentrate resources on tokamak designs; the Earth's magnetic field is generated by the circulation of charges in the Earth's molten core. The resulting magnetic dipole field forms a shape with magnetic field lines passing through the Earth's center, reaching the surface near the poles and extending far into space above the equator. Charged particles entering the field will tend to follow the lines of force, moving south.
As they reach the polar regions, the magnetic lines begin to cluster together, this increasing field can cause particles below a certain energy threshold to reflect, begin travelling in the opposite direction. Such particles bounce forth between the poles until the collide with other particles. Particles with greater energy continue towards the Earth, impacting the atmosphere and causing the aurora; this basic concept is used in the magnetic mirror approach to fusion energy. The mirror uses a solenoid to confine the plasma in the center of a cylinder, two magnets at either end to force the magnetic lines closer together to create reflecting areas. One of the most promising of the early approaches to fusion, the mirror proved to be "leaky", with the fuel refusing to properly reflect from the ends as the density and energy were increased. Annoyingly, it was the particles with the most energy, those most to undergo fusion, that preferentially escaped. Research into large mirror machines ended in the 1980s as it became clear they would not reach fusion breakeven in a sized device.
The levitated dipole can be thought of, in some ways, as a toroidal mirror, much more similar to the Earth's field than the linear system in a traditional mirror. In this case, the confinement area is not the linear area between the mirrors, but the toroidal area around the outside of the central magnet, similar to the area around the Earth's equator. Particles in this area that move up or down see increasing magnetic density and tend to move back towards the equator area again; this gives the system some level of natural stability. Particles with higher energy, the ones that would escape a traditional mirror, instead follow the field lines through the hollow center of the magnet, recirculating back into the equatorial area again; this makes the Levitated Dipole unique. In those experiments, small fluctuations can cause significant energy loss. By contrast, in a dipolar magnetic field, fluctuations tend to compress the plasma, without energy loss; this compression effect was first noticed by Akira Hasegawa after participating in the Voyager 2 encounter with Uranus.
Adapting this concept to a fusion experiment was first proposed by Dr. Jay Kesner and Dr. Michael Mauel in the mid to late nineties; the pair raised money to build the machine. They achieved first plasma on Friday, August 13, 2004 at 12:53 PM. First plasma was done by levitating the dipole magnet and RF heating the plasma; the LDX team has since conducted several levitation tests, including a 40-minute suspension of the superconducting coil on February 9, 2007. Shortly after, the coil was damaged in a control test in February 2007 and replaced in May 2007; the replacement coil was inferior, a copper wound electromagnet, water cooled. Scientific results, including the observation of an inward turbulent pinch, were reported in Nature Physics; this experiment needed a special free-floating electromagnet, which created the unique "toilet-bowl" magnetic field. The magnetic field was made of two counter-wound rings of currents; each ring contained a 19-strand niobium-tin Rutherford cable. These looped around inside a Inconel magnet.
The donut was charged using induction. Once charged, it generated a magnetic field for an 8-hour period. Overall, the ring levitated 1.6 meters above a superconducting ring. The ring produced a 5-tesla field; this superconductor was encased inside a liquid helium, which kept the electromagnet below 10 kelvins. This design is similar to the D20 dipole experiment at Berkeley and the RT-1 experiment at the University of Tokyo; the dipole was suspended inside a mushroom-shaped vacuum chamber, about 5 meters in diameter and ~3 meters high. At the base of the chamber was a charging coil; this coil is used using induction. The coil exposing the dipole to a varying magnetic field. Next, the dipole is raised into the center of the chamber; this could be using the field itself. Around the outside of this chamber were Helmholtz coils, which were used to produce a uniform surrounding magnetic field; this external field would interact with the dipole field. It was in this surrounding field; the plasma forms inside the chamber.
The plasma is formed by heating a low pressure gas. The gas is heated using a radio frequency microwaving the plasma in a 17-kilowatt field; the machine was monitored
In fusion power research, the Z-pinch known as zeta pinch, is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it. These systems were referred to as pinch or Bennett pinch, but the introduction of the θ-pinch concept led to the need for increased clarity; the name refers to the direction of the current in the devices, the Z-axis on a normal three-dimensional graph. Any machine that causes a pinch effect due to current running in that direction is referred to as a Z-pinch system, this encompasses a wide variety of devices used for an wide variety of purposes. Early uses focused on fusion research in donut-shaped tubes with the Z-axis running down the inside the tube, while modern devices are cylindrical and used to generate high-intensity x-ray sources for the study of nuclear weapons and other roles; the Z-pinch is an application of the Lorentz force, in which a current-carrying conductor in a magnetic field experiences a force.
One example of the Lorentz force is that, if two parallel wires are carrying current in the same direction, the wires will be pulled toward each other. In a Z-pinch machine the wires are replaced by a plasma, which can be thought of as many current-carrying wires; when a current is run through the plasma, the particles in plasma are pulled toward each other by the Lorentz force, thus the plasma contracts. The contraction is counteracted by the increasing gas pressure of the plasma; as the plasma is electrically conductive, a magnetic field nearby will induce a current in it. This provides a way to run a current into the plasma without physical contact, important as a plasma can erode mechanical electrodes. In practical devices this was arranged by placing the plasma vessel inside the core of a transformer, arranged so the plasma itself would be the secondary; when current was sent into the primary side of the transformer, the magnetic field induced a current into the plasma. As induction requires a changing magnetic field, the induced current is supposed to run in a single direction in most reactor designs, the current in the transformer has to be increased over time to produce the varying magnetic field.
This places a limit on the product of confinement time and magnetic field, for any given source of power. In Z-pinch machines the current is provided from a large bank of capacitors and triggered by a spark gap, known as a Marx Bank or Marx generator; as the conductivity of plasma is good, about that of copper, the energy stored in the power source is depleted by running through the plasma. Z-pinch devices are inherently pulsed in nature. Pinch devices were among the earliest efforts in fusion power. Research began in the UK in the immediate post-war era, but a lack of interest led to little development until the 1950s; the announcement of the Huemul Project in early 1951 led to fusion efforts around the world, notably in the UK and in the US. Small experiments were built at labs as various practical issues were addressed, but all of these machines demonstrated unexpected instabilities of the plasma that would cause it to hit the walls of the container vessel; the problem became known as the "kink instability".
By 1953 the "stabilized pinch" seemed to solve the problems encountered on earlier devices. Stabilized pinch machines added external magnets that created a toroidal magnetic field inside the chamber; when the device was fired, this field added to the one created by the current in the plasma. The result was that the straight magnetic field was twisted into a helix, which the particles followed as they traveled around the tube driven by the current. A particle near the outside of the tube that wanted to kink outward would travel along these lines until it returned to the inside of the tube, where its outward-directed motion would bring it back into the centre of the plasma. Researchers in the UK started construction of ZETA in 1954. ZETA was by far the largest fusion device of its era. At the time all fusion research was classified, so progress on ZETA was unknown outside the labs working on it; however US researchers realized that they were about to be outpaced. Teams on both sides of the Atlantic rushed to be the first to complete stabilized pinch machines.
ZETA won the race, by the summer of 1957 it was producing bursts of neutrons on every run. Despite the researchers' reservations, their results were released with great fanfare as the first successful step on the path to commercial fusion energy. However, further study soon demonstrated that the measurements were misleading, none of the machines were near fusion levels. Interest in pinch devices faded, although ZETA and its cousin Sceptre served for many years as experimental devices. A concept of Z-pinch fusion propulsion system was developed through collaboration between NASA and private companies; the energy released by the Z-pinch effect accelerates the lithium propellant to a high speed, resulting in a specific impulse value of 19400 s and thrust of 38 kN. A magnetic nozzle is required to convert the released energy into a useful impulse; this propulsion method could reduce interplanetary travel times. For example, a mission to Mars would take about 35 days one-way with a total burn time of 20 days and a burned propellant mass of 350 tonnes.
Although it remained unknown for years, Soviet scientists used the pinch concept to develop the tokamak device. Unlike the stabilized pinch devices in the US and UK, the tokamak used more energy in the stabilizing magnets, much less in the plasma current; this reduced the instabilities due to the large currents in the plasma, led to g