# Fusion power

(Redirected from Fusion generator)
The Sun is a natural fusion reactor, using stellar nucleosynthesis to transform lighter elements into heavier elements plus energy.

Fusion power is energy generated by nuclear fusion, or more broadly, the use of that power as an energy source. Fusion has a number of advantages over fission as a source of power, including reduced radioactivity, ample fuel supplies, and increased safety. However, controlled fusion has proven to be extremely difficult to produce in a practical manner. Research into fusion reactors began in the 1940s, but as of 2017 no design has produced positive net energy.

Fusion reactions fuse two lighter atomic nuclei to form a heavier nucleus. It is the process used in stars to produce energy and heavier elements. The reaction normally takes place in a plasma of deuterium and tritium heated to millions of degrees. At such temperatures the only way to confine the plasma while the reactions take place is to use electric or magnetic fields. Designing a system that can confine the plasma long enough at high enough temperature and density is the major challenge in the development of fusion power.

Many confinement concepts have been investigated. In the early days the three main systems were the z-pinch, stellarator and magnetic mirror. Today, the current leading designs are the tokamak and inertial confinement (ICF) by laser. Both of these designs are being built at very large scales, most notably the ITER tokamak in France, and the National Ignition Facility laser in the USA. Many other designs are also being studied as they may offer lower-cost approaches, among these the magnetized target fusion and new inertial electrostatic confinement designs are seeing increased interest. As of 2017, these technologies cannot produce more energy than are required to initiate and sustain a fusion reaction.[1]

## Background

Binding energy for different atoms. Iron-56 has the highest, making it the most stable. Atoms to the left are likely to fuse; atoms to the right are likely to split.

### Mechanism

Fusion reactions occur when two or more atomic nuclei come close enough for long enough that the strong nuclear force pulling them together exceeds the electrostatic force pushing them apart, fusing them into heavier nuclei. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy. For nuclei heavier than iron-56, the reaction is endothermic, requiring an external source of energy.[2] Hence, nuclei smaller than iron-56 are more likely to fuse while those heavier than iron-56 are more likely to break apart.

The strong force acts only over short distances. The repulsive electrostatic force acts over longer distances, so kinetic energy is needed to overcome this "Coulomb barrier" before fusion can take place. Ways of doing this include speeding up atoms in a particle accelerator, or heating them to high temperatures.

Once an atom is heated above its ionization energy, its electrons are stripped away (it is ionized), leaving just the bare nucleus (the ion). The result is a hot cloud of ions and the electrons formerly attached to them. This cloud is known as a plasma. Because the charges are separated, plasmas are electrically conductive and magnetically controllable. Many fusion devices take advantage of this to control the particles as they are heated.

### Cross Section

The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The deuterium-tritium fusion rate peaks at a lower temperature (about 70 keV, or 800 million kelvin) and at a higher value than other reactions commonly considered for fusion energy.

A reaction's cross section, denoted σ, is the measure of the probability that a fusion reaction will happen. This depends on the relative velocity of the two nuclei. Higher relative velocities increase the probability. Cross sections for many fusion reactions were measured (mainly in the 1970s) using particle beams.[3]

In a plasma, particle velocity can be characterized using a probability distribution. If the plasma is thermalized, the distribution looks like a bell curve, or maxwellian distribution. In this case, it is useful to use the average particle cross section over the velocity distribution. This is entered into the volumetric fusion rate:[4]

${\displaystyle P_{\text{fusion}}=n_{A}n_{B}\langle \sigma v_{A,B}\rangle E_{\text{fusion}}}$

where:

• ${\displaystyle P_{\text{fusion}}}$ is the energy made by fusion, per time and volume
• n is the number density of species A or B, of the particles in the volume
• ${\displaystyle \langle \sigma v_{A,B}\rangle }$ is the cross section of that reaction, average over all the velocities of the two species v
• ${\displaystyle E_{\text{fusion}}}$ is the energy released by that fusion reaction.

### Lawson Criterion

The Lawson Criterion shows how energy output varies with temperature, density, speed of collision and fuel. This equation was central to John Lawson's analysis of fusion working with a hot plasma. Lawson assumed an energy balance, shown below.[4]

${\displaystyle P_{\text{out}}=\eta _{\text{capture}}\left(P_{\text{fusion}}-P_{\text{conduction}}-P_{\text{radiation}}\right)}$
• η, efficiency
• ${\displaystyle P_{\text{conduction}}}$, conduction losses as energy laden mass leaves
• ${\displaystyle P_{\text{radiation}}}$, radiation losses as energy leaves as light
• ${\displaystyle P_{\text{out}}}$, net power from fusion
• ${\displaystyle P_{\text{fusion}}}$, is rate of energy generated by the fusion reactions.

Plasma clouds lose energy through conduction and radiation.[4] Conduction occurs when ions, electrons or neutrals impact a surface and transfer a portion of their kinetic energy to the atoms of the surface. Radiation is energy that leaves the cloud as light in the visible, UV, IR, or X-ray spectra. Radiation increases with temperature. Fusion power technologies must overcome these losses.

### Triple product: density, temperature, time

The Lawson criterion argues that a machine holding a thermalized and quasi-neutral plasma has to meet basic criteria to overcome radiation losses, conduction losses and reach efficiency of 30 percent.[4][5] This became known as the "triple product": the plasma density, temperature and confinement time.[6] Attempts to increase the triple product led to targeting larger plants. Larger plants move structural materials further away from the centre of the plasma, which reduces conduction and radiation losses since more of the radiation is internally reflected. This emphasis on ${\displaystyle (nT\tau )}$ as a metric of success has impacted other considerations such as cost, size, complexity and efficiency.[dubious ] This has led to larger, more complicated and more expensive machines such as ITER and NIF.[citation needed]

### Plasma behavior

Plasma is an ionized gas that conducts electricity.[7] In bulk, it is modeled using magnetohydrodynamics, which is a combination of the Navier-Stokes equations governing fluids and Maxwell's equations governing how magnetic and electric fields behave.[8] Fusion exploits several plasma properties, including:

Self-organizing plasma conducts electric and magnetic fields. Its motions can generate fields that can in turn contain it.[9]

Diamagnetic plasma can generate its own internal magnetic field. This can reject an externally applied magnetic field, making it diamagnetic.[10]

Magnetic mirrors can reflect plasma when it moves from a low to high density field.[11]

### Energy capture

Multiple approaches have been proposed for energy capture. The simplest is to heat a fluid. The neutrons generated by fusion can re-generate a spent fission fuel.[12] Direct energy conversion was developed (at LLNL in the 1980s) as a method to maintain a voltage using the fusion reaction products. This has demonstrated energy capture efficiency of 48 percent.[13]

## Approaches

### Magnetic confinement

Tokamak: the most well-developed and well-funded approach to fusion energy. This method races hot plasma around in a magnetically confined, donut-shaped ring, with an internal current. When completed, ITER will be the world's largest tokamak. As of April 2012 an estimated 215 experimental tokamaks were either planned, decommissioned or currently operating (35) worldwide.[14]

Spherical tokamak: also known as spherical torus A variation on the tokamak with a spherical shape.

Stellarator: Twisted rings of hot plasma. The stellarator attempts to create a natural twist plasma path, using external magnets, while tokamaks create those magnetic fields using an internal current. Stellarators were developed by Lyman Spitzer in 1950 and have four designs: Torsatron, Heliotron, Heliac and Helias. One example is Wendelstein 7-X, a German fusion device that produced its first plasma on December 10, 2015. It is the world's largest stellarator,[15] designed to investigate the suitability of this type of device for a power station.

Levitated Dipole Experiment (LDX): These use a solid superconducting torus. This is magnetically levitated inside the reactor chamber. The superconductor forms an axisymmetric magnetic field that contains the plasma. The LDX was developed by MIT and Columbia University after 2000 by Jay Kesner and Michael E. Mauel.[16]

Magnetic mirror: Developed by Richard F. Post and teams at LLNL in the 1960s.[17] Magnetic mirrors reflected hot plasma back and forth in a line. Variations included the magnetic bottle and the biconic cusp.[18] A series of well-funded, large, mirror machines were built by the US government in the 1970s and 1980s.[19]

Field-reversed configuration: This device traps plasma in a self-organized quasi-stable structure; where the particle motion makes an internal magnetic field which then traps itself.[20]

Spheromak Very similar to a field reversed configuration, a semi-stable plasma structure made by using the plasmas' own self-generated magnetic field. A spheromak has both a toroidal and poloidal fields, while a Field Reversed Configuration only has no toroidal field[21].

Reversed field pinch: Here the plasma moves inside a ring. It has an internal magnetic field. Moving out from the center of this ring, the magnetic field reverses direction.

### Inertial confinement

Direct drive: In this technique, lasers directly blast a pellet of fuel. The goal is to ignite a fusion chain reaction. Ignition was first suggested by John Nuckolls, in 1972.[22] Notable direct drive experiments have been conducted at the Laboratory for Laser Energetics, Laser Mégajoule and the GEKKO XII facilities. Good implosions require fuel pellets with close to a perfect shape in order to generate a symmetrical inward shock wave that produces the high-density plasma.

Fast ignition: This method uses two laser blasts. The first blast compresses the fusion fuel, while the second high energy pulse ignites it. Experiments have been conducted at the Laboratory for Laser Energetics using the Omega and Omega EP systems and at the GEKKO XII laser at the Institute for Laser Engineering in Osaka Japan.

Indirect drive: In this technique, lasers blasts a structure around the pellet of fuel. This structure is known as a Hohlraum. As it disintegrates the pellet is bathed in a more uniform x-ray light, creating better compression. The largest system using this method is the National Ignition Facility.

Magneto-inertial fusion or Magnetized Liner Inertial Fusion: This combines a laser pulse with a magnetic pinch. The pinch community refers to it as magnetized liner Inertial fusion while the ICF community refers to it as magneto-inertial fusion.[23]

Heavy Ion Beams There are also proposals to do inertial confinement fusion with ion beams instead of laser beams.[24] The main difference is the mass of the beam has momentum, whereas lasers do not.

### Magnetic or electric pinches

Z-Pinch: This method sends a strong current (in the z-direction) through the plasma. The current generates a magnetic field that squeezes the plasma to fusion conditions. Pinches were the first method for man-made controlled fusion.[25][26] Some examples include the Dense plasma focus and the Z machine at Sandia National Laboratories.

Theta-Pinch: This method sends a current inside a plasma, in the theta direction.

Screw Pinch: This method combines a theta and z-pinch for improved stabilization.[27]

### Inertial electrostatic confinement

Main article: Inertial Electrostatic Confinement

Fusor: This method uses an electric field to heat ions to fusion conditions. The machine typically uses two spherical cages, a cathode inside the anode, inside a vacuum. These machines are not considered a viable approach to net power because of their high conduction and radiation[28] losses. They are simple enough to build that amateurs have fused atoms using them.[29]

Polywell: This design attempts to combine magnetic confinement with electrostatic fields, to avoid the conduction losses generated by the cage.[30]

### Other

Magnetized target fusion: This method confines hot plasma using a magnetic field and squeezes it using inertia. Examples include LANL FRX-L machine,[31] General Fusion and the plasma liner experiment.[32]

Uncontrolled: Fusion has been initiated by man, using uncontrolled fission explosions to ignite so-called Hydrogen Bombs. Early proposals for fusion power included using bombs to initiate reactions.

Beam fusion: A beam of high energy particles can be fired at another beam or target and fusion will occur. This was used in the 1970s and 1980s to study the cross sections of high energy fusion reactions.[3]

Bubble fusion: This was a fusion reaction that was supposed to occur inside extraordinarily large collapsing gas bubbles, created during acoustic liquid cavitation.[33] This approach was discredited.

Cold fusion: This is a hypothetical type of nuclear reaction that would occur at, or near, room temperature. Cold fusion is discredited and gained a reputation as pathological science.[34][35]

Muon-catalyzed fusion: Muons allow atoms to get much closer and thus reduce the kinetic energy required to initiate fusion. Muons require more energy to produce than can be obtained from muon-catalysed fusion, making this approach impractical for power generation.[36]

Gravitational-confinement fusion (GCF) Direct Photo-Electric Conversion: Also known as Space-Based Solar Power argues that a majority of available fusion fuels exists within the sphere of the Sun where it is gravitationally confined, and that a tractable way to accomplish large-scale fusion power is to build very large space-borne platforms that capture energy via photons rather than via a carnot cycle. The theoretical limit of producing power by such means is a type-2 civilization using a Dyson Sphere.

## Common tools

### Heating

Gas is heated to form a plasma hot enough to start fusion reactions. A number of heating schemes have been explored:[37]

Radiofrequency Heating A radio wave is applied to the plasma, causing it to oscillate. This is basically the same concept as a microwave oven. This is also known as electron cyclotron resonance heating or Dielectric heating[citation needed].

Electrostatic Heating An electric field can do work on charged ions or electrons, heating them.[citation needed].

Neutral Beam Injection An external source of hydrogen is ionized and accelerated by an electric field to form a charged beam which is shone through a source of neutral hydrogen gas towards the plasma which itself is ionized and contained in the reactor by a magnetic field. Some of the intermediate hydrogen gas is accelerated towards the plasma by collisions with the charged beam while remaining neutral: this neutral beam is thus unaffected by the magnetic field and so shines through it into the plasma. Once inside the plasma the neutral beam transmits energy to the plasma by collisions as a result of which it becomes ionized and thus contained by the magnetic field thereby both heating and refuelling the reactor in one operation. The remainder of the charged beam is diverted by magnetic fields onto cooled beam dumps.

Magnetic Oscillations[38]

### Measurement

Thomson Scattering Light scatters from plasma. This light can be detected and used to reconstruct the plasmas' behavior. This technique can be used to find its density and temperature. It is common in Inertial confinement fusion,[39] Tokamaks[40] and fusors. In ICF systems, this can be done by firing a second beam into a gold foil adjacent to the target. This makes x-rays that scatter or traverse the plasma. In Tokamaks, this can be done using mirrors and detectors to reflect light across a plane (two dimensions) or in a line (one dimension).

Langmuir probe This is a metal object placed in a plasma. A potential is applied to it, giving it a positive or negative voltage against the surrounding plasma. The metal collects charged particles, drawing a current. As the voltage changes, the current changes. This makes a IV Curve. The IV-curve can be used to determine the local plasma density, potential and temperature.[41]

Neutron detectors Deuterium or tritium fusion produces neutrons. Neutrons interact with surrounding matter in ways that can be detected. Several types of neutron detectors exist which can record the rate at which neutrons are produced during fusion reactions. They are an essential tool for demonstrating success.

Flux loop A loop of wire is inserted into the magnetic field. As the field passes through the loop, a current is made. The current is measured and used to find the total magnetic flux through that loop. This has been used on the National Compact Stellarator Experiment,[42] the polywell[43] and the LDX machines.

X-ray detector All plasma loses energy by emitting light. This covers the whole spectrum: visible, IR, UV, and X-rays. This occurs anytime a particle changes speed, for any reason.[44] If the reason is deflection by a magnetic field, the radiation is Cyclotron radiation at low speeds and Synchrotron radiation at high speeds. If the reason is deflection by another particle, plasma radiates X-rays, known as Bremsstrahlung radiation. X-rays are termed in both hard and soft, based on their energy.

### Power production

Steam turbines It has been proposed [45] that steam turbines be used to convert the heat from the fusion chamber into electricity. The heat is transferred into a working fluid that turns into steam, driving electric generators.

Neutron blankets Deuterium and tritium fusion generates neutrons. This varies by technique (NIF has a record of 3E14 neutrons per second[46] while a typical fusor produces 1E5–1E9 neutrons per second). It has been proposed to use these neutrons as a way to regenerate spent fission fuel [47] or as a way to breed tritium using a breeder blanket consisting of liquid lithium or, as in more recent reactor designs, a helium cooled pebble bed consisting of lithium bearing ceramic pebbles fabricated from materials such as Lithium titanate, lithium orthosilicate or mixtures of these phases.[48]

Direct conversion This is a method where the kinetic energy of a particle is converted into voltage.[49] It was first suggested by Richard F. Post in conjunction with magnetic mirrors, in the late sixties. It has also been suggested for Field-Reversed Configurations. The process takes the plasma, expands it, and converts a large fraction of the random energy of the fusion products into directed motion. The particles are then collected on electrodes at various large electrical potentials. This method has demonstrated an experimental efficiency of 48 percent.[50]

## Confinement

Parameter space occupied by inertial fusion energy and magnetic fusion energy devices as of the mid 1990s. The regime allowing thermonuclear ignition with high gain lies near the upper right corner of the plot.

Confinement refers to all the conditions necessary to keep a plasma dense and hot long enough to undergo fusion. Here are some general principles.

• Equilibrium: The forces acting on the plasma must be balanced for containment. One exception is inertial confinement, where the relevant physics must occur faster than the disassembly time.
• Stability: The plasma must be so constructed so that disturbances will not lead to the plasma disassembling.
• Transport or conduction: The loss of material must be sufficiently slow.[4] The plasma carries off energy with it, so rapid loss of material will disrupt any machines power balance. Material can be lost by transport into different regions or conduction through a solid or liquid.

To produce self-sustaining fusion, the energy released by the reaction (or at least a fraction of it) must be used to heat new reactant nuclei and keep them hot long enough that they also undergo fusion reactions.

### Unconfined

The first human-made, large-scale fusion reaction was the test of the hydrogen bomb, Ivy Mike, in 1952. As part of the PACER project, it was once proposed to use hydrogen bombs as a source of power by detonating them in underground caverns and then generating electricity from the heat produced, but such a power station is unlikely ever to be constructed.

### Magnetic confinement

At the temperatures required for fusion, the fuel is heated to a plasma state. In this state it has a very good electrical conductivity. This opens the possibility of confining the plasma with magnetic fields. This is the case of magnetized plasma, where the magnetic fields and plasma intermix. This is generally known as magnetic confinement. The field lines put a Lorentz force on the plasma. The force works perpendicular to the magnetic fields, so one problem in magnetic confinement is preventing the plasma from leaking out the ends of the field lines. A general measure of magnetic trapping in fusion is the beta ratio:

${\displaystyle \beta ={\frac {p}{p_{mag}}}={\frac {nk_{B}T}{(B^{2}/2\mu _{0})}}}$ [51]

This is the ratio of the externally applied field to the internal pressure of the plasma. A value of 1 is ideal trapping. Some examples of beta values include:

1. The START machine: 0.32
2. The Levitated dipole experiment:[52] 0.26
3. Spheromaks: ≈ 0.1,[53] Maximum 0.2 based on Mercier limit.[54]
4. The DIII-D machine: 0.126[citation needed]
5. The Gas Dynamic Trap a magnetic mirror: 0.6 [55] for 5E-3 seconds.[56]
6. The Sustained Spheromak Plasma Experiment at Los Alamos National labs < 0.05 for 4E-6 seconds. [57]

Magnetic Mirror One example of magnetic confinement is with the magnetic mirror effect. If a particle follows the field line and enters a region of higher field strength, the particles can be reflected. There are several devices that try to use this effect. The most famous was the magnetic mirror machines, which was a series of large, expensive devices built at the Lawrence Livermore National Laboratory from the 1960s to mid 1980s.[58] Some other examples include the magnetic bottles and Biconic cusp.[59] Because the mirror machines were straight, they had some advantages over a ring shape. First, mirrors were easier to construct and maintain and second direct conversion energy capture, was easier to implement.[13] As the confinement achieved in experiments was poor, this approach was abandoned.[citation needed]

Magnetic Loops Another example of magnetic confinement is to bend the field lines back on themselves, either in circles or more commonly in nested toroidal surfaces. The most highly developed system of this type is the tokamak, with the stellarator being next most advanced, followed by the Reversed field pinch. Compact toroids, especially the Field-Reversed Configuration and the spheromak, attempt to combine the advantages of toroidal magnetic surfaces with those of a simply connected (non-toroidal) machine, resulting in a mechanically simpler and smaller confinement area.

### Inertial confinement

Inertial confinement is the use of rapidly imploding shell to heat and confine plasma. The shell is imploded using a direct laser blast (direct drive) or a secondary x-ray blast (indirect drive) or heavy ion beams. Theoretically, fusion using lasers would be done using tiny pellets of fuel that explode several times a second. To induce the explosion, the pellet must be compressed to about 30 times solid density with energetic beams. If direct drive is used—the beams are focused directly on the pellet—it can in principle be very efficient, but in practice is difficult to obtain the needed uniformity.[60] The alternative approach, indirect drive, uses beams to heat a shell, and then the shell radiates x-rays, which then implode the pellet. The beams are commonly laser beams, but heavy and light ion beams and electron beams have all been investigated.[61]

### Electrostatic confinement

There are also electrostatic confinement fusion devices. These devices confine ions using electrostatic fields. The best known is the Fusor. This device has a cathode inside an anode wire cage. Positive ions fly towards the negative inner cage, and are heated by the electric field in the process. If they miss the inner cage they can collide and fuse. Ions typically hit the cathode, however, creating prohibitory high conduction losses. Also, fusion rates in fusors are very low because of competing physical effects, such as energy loss in the form of light radiation.[62] Designs have been proposed to avoid the problems associated with the cage, by generating the field using a non-neutral cloud. These include a plasma oscillating device,[63] a magnetically-shielded-grid a penning trap and the polywell.[64] The technology is relatively immature, however, and many scientific and engineering questions remain.

## History of research

### 1920s

Research into nuclear fusion started in the early part of the 20th century. In 1920 the British physicist Francis William Aston discovered that the total mass equivalent of four hydrogen atoms (two protons and two neutrons) are heavier than the total mass of one helium atom (He-4), which implied that net energy can be released by combining hydrogen atoms together to form helium, and provided the first hints of a mechanism by which stars could produce energy in the quantities being measured. Through the 1920s, Arthur Stanley Eddington became a major proponent of the proton–proton chain reaction (PP reaction) as the primary system running the Sun.

### 1930s

Neutrons from fusion was first detected by staff members at Ernest Rutherfords' at the University of Cambridge, in 1933 [65]. The experiment was developed by Mark Oliphant and involved the acceleration of an protons towards a target [66] at energies of up to 600,000 electron volts. In 1933, the Cavendish Laboratory received a gift from the American physical chemist Gilbert N. Lewis of a few drops of heavy water. The accelerator was used to fire heavy hydrogen nuclei deuterons at various targets. Working with Rutherford and others, Oliphant discovered the nuclei of Helium-3 (helions) and tritium (tritons).[67][68][69][70]

A theory was verified by Hans Bethe in 1939 showing that beta decay and quantum tunneling in the Sun's core might convert one of the protons into a neutron and thereby producing deuterium rather than a diproton. The deuterium would then fuse through other reactions to further increase the energy output. For this work, Bethe won the Nobel Prize in Physics.

### 1940s

In 1942, nuclear fusion research was subsumed into the Manhattan Project when the secrecy surrounding the field obscured by the science. The first patent related to a fusion reactor was registered in 1946[71] by the United Kingdom Atomic Energy Authority. The inventors were Sir George Paget Thomson and Moses Blackman. This was the first detailed examination of the Z-pinch concept.

Z-pinch is based on the fact that plasmas are electrically conducting. Running a current through the plasma, will generate a magnetic field around the plasma. This field will, according to Lenz's law, create an inward directed force that causes the plasma to collapse inward, raising its density. Denser plasmas generate denser magnetic fields, increasing the inward force, leading to a chain reaction. If the conditions are correct, this can lead to the densities and temperatures needed for fusion. The difficulty is getting the current into the plasma, which would normally melt any sort of mechanical electrode. A solution emerges again because of the conducting nature of the plasma; by placing the plasma in the middle of an electromagnet, induction can be used to generate the current.

Starting in 1947, two UK teams carried out small experiments and began building a series of ever-larger experiments. When the Huemul results hit the news (see below), James L. Tuck, a UK physicist working at Los Alamos, introduced the pinch concept in the US and produced a series of machines known as the Perhapsatron. The Soviet Union, unbeknownst to the West, was also building a series of similar machines. All of these devices quickly demonstrated a series of instabilities when the pinch was applied. This broke up the plasma column long before it reached the densities and temperatures required for fusion.

### 1950s

The first man-made device to achieve ignition was the detonation of this fusion device, codenamed Ivy Mike.
Early photo of plasma inside a pinch machine (imperial college 1950/1951)

The first successful man-made fusion device was the boosted fission weapon tested in 1951 in the Greenhouse Item test. This was followed by true fusion weapons in 1952's Ivy Mike, and the first practical examples in 1954's Castle Bravo. This was uncontrolled fusion. In these devices, the energy released by the fission explosion is used to compress and heat fusion fuel, starting a fusion reaction. Fusion releases neutrons. These neutrons hit the surrounding fission fuel, causing the atoms to split apart much faster than normal fission processes—almost instantly by comparison. This increases the effectiveness of bombs: normal fission weapons blow themselves apart before all their fuel is used; fusion/fission weapons do not have this practical upper limit.

In 1949 an expatriate German, Ronald Richter, proposed the Huemul Project in Argentina, announcing positive results in 1951. These turned out to be fake, but it prompted considerable interest in the concept as a whole. In particular, it prompted Lyman Spitzer to begin considering ways to solve some of the more obvious problems involved in confining a hot plasma, and, unaware of the z-pinch efforts, he developed a new solution to the problem known as the stellarator. Spitzer applied to the US Atomic Energy Commission for funding to build a test device. During this period, Jim Tuck who had worked with the UK teams had been introducing the z-pinch concept to his coworkers at his new job at Los Alamos National Laboratory (LANL). When he heard of Spitzer's pitch for funding, he applied to build a machine of his own, the Perhapsatron.

Spitzer's idea won funding and he began work on the stellarator under the code name Project Matterhorn. His work led to the creation of the Princeton Plasma Physics Laboratory. Tuck returned to LANL and arranged local funding to build his machine. By this time, however, it was clear that all of the pinch machines were suffering from the same issues involving stability, and progress stalled. In 1953, Tuck and others suggested a number of solutions to the stability problems. This led to the design of a second series of pinch machines, led by the UK ZETA and Sceptre devices.

Spitzer had planned an aggressive development project of four machines, A, B, C, and D. A and B were small research devices, C would be the prototype of a power-producing machine, and D would be the prototype of a commercial device. A worked without issue, but even by the time B was being used it was clear the stellarator was also suffering from instabilities and plasma leakage. Progress on C slowed as attempts were made to correct for these problems.

By the mid-1950s it was clear that the simple theoretical tools being used to calculate the performance of all fusion machines were simply not predicting their actual behavior. Machines invariably leaked their plasma from their confinement area at rates far higher than predicted. In 1954, Edward Teller held a gathering of fusion researchers at the Princeton Gun Club, near the Project Matterhorn (now known as Project Sherwood) grounds. Teller started by pointing out the problems that everyone was having, and suggested that any system where the plasma was confined within concave fields was doomed to fail. Attendees remember him saying something to the effect that the fields were like rubber bands, and they would attempt to snap back to a straight configuration whenever the power was increased, ejecting the plasma. He went on to say that it appeared the only way to confine the plasma in a stable configuration would be to use convex fields, a "cusp" configuration.[72]

When the meeting concluded, most of the researchers quickly turned out papers saying why Teller's concerns did not apply to their particular device. The pinch machines did not use magnetic fields in this way at all, while the mirror and stellarator seemed to have various ways out. This was soon followed by a paper by Martin David Kruskal and Martin Schwarzschild discussing pinch machines, however, which demonstrated instabilities in those devices were inherent to the design.

The largest "classic" pinch device was the ZETA, including all of these suggested upgrades, starting operations in the UK in 1957. In early 1958, John Cockcroft announced that fusion had been achieved in the ZETA, an announcement that made headlines around the world. When physicists in the US expressed concerns about the claims they were initially dismissed. US experiments soon demonstrated the same neutrons, although temperature measurements suggested these could not be from fusion reactions. The neutrons seen in the UK were later demonstrated to be from different versions of the same instability processes that plagued earlier machines. Cockcroft was forced to retract the fusion claims, and the entire field was tainted for years. ZETA ended its experiments in 1968.

The first controlled fusion experiment was accomplished using Scylla I at the Los Alamos National Laboratory in 1958. This was a pinch machine, with a cylinder full of deuterium. Electric current shot down the sides of the cylinder. The current made magnetic fields that compressed the plasma to 15 million degrees Celsius, squeezed the gas, fused it and produced neutrons.[25][26]

In 1950–1951 I.E. Tamm and A.D. Sakharov in the Soviet Union, first discussed a tokamak-like approach. Experimental research on those designs began in 1956 at the Kurchatov Institute in Moscow by a group of Soviet scientists led by Lev Artsimovich. The tokamak essentially combined a low-power pinch device with a low-power simple stellarator. The key was to combine the fields in such a way that the particles orbited within the reactor a particular number of times, today known as the "safety factor". The combination of these fields dramatically improved confinement times and densities, resulting in huge improvements over existing devices.

### 1960s

A key plasma physics text was published by Lyman Spitzer at Princeton in 1963.[73] Spitzer took the ideal gas laws and adopted them to an ionized plasma, developing many of the fundamental equations used to model a plasma.

Laser fusion was suggested in 1962 by scientists at Lawrence Livermore National Laboratory, shortly after the invention of the laser itself in 1960. At the time, Lasers were low power machines, but low-level research began as early as 1965. Laser fusion, formally known as inertial confinement fusion, involves imploding a target by using laser beams. There are two ways to do this: indirect drive and direct drive. In direct drive, the laser blasts a pellet of fuel. In indirect drive, the lasers blast a structure around the fuel. This makes x-rays that squeeze the fuel. Both methods compress the fuel so that fusion can take place.

At the 1964 World's Fair, the public was given its first demonstration of nuclear fusion.[74] The device was a θ-pinch from General Electric. This was similar to the Scylla machine developed earlier at Los Alamos.

The magnetic mirror was first published in 1967 by Richard F. Post and many others at the Lawrence Livermore National Laboratory.[17] The mirror consisted of two large magnets arranged so they had strong fields within them, and a weaker, but connected, field between them. Plasma introduced in the area between the two magnets would "bounce back" from the stronger fields in the middle.

The A.D. Sakharov group constructed the first tokamaks, the most successful being the T-3 and its larger version T-4. T-4 was tested in 1968 in Novosibirsk, producing the world's first quasistationary fusion reaction.[75] When this were first announced, the international community was highly skeptical. A British team was invited to see T-3, however, and after measuring it in depth they released their results that confirmed the Soviet claims. A burst of activity followed as many planned devices were abandoned and new tokamaks were introduced in their place — the C model stellarator, then under construction after many redesigns, was quickly converted to the Symmetrical Tokamak.

In his work with vacuum tubes, Philo Farnsworth observed that electric charge would accumulate in regions of the tube. Today, this effect is known as the Multipactor effect.[76] Farnsworth reasoned that if ions were concentrated high enough they could collide and fuse. In 1962, he filed a patent on a design using a positive inner cage to concentrate plasma, in order to achieve nuclear fusion.[77] During this time, Robert L. Hirsch joined the Farnsworth Television labs and began work on what became the fusor. Hirsch patented the design in 1966[78] and published the design in 1967.[79]

### 1970s

Shiva laser, 1977, the largest ICF laser system built in the seventies
The Tandem Mirror Experiment (TMX) in 1979

In 1972, John Nuckolls outlined the idea of ignition.[22] This is a fusion chain reaction. Hot helium made during fusion reheats the fuel and starts more reactions. John argued that ignition would require lasers of about 1 kJ. This turned out to be wrong. Nuckolls's paper started a major development effort. Several laser systems were built at LLNL. These included the argus, the Cyclops, the Janus, the long path, the Shiva laser and the Nova in 1984. This prompted the UK to build the Central Laser Facility in 1976.[80]

During this time, great strides in understanding the tokamak system were made. A number of improvements to the design are now part of the "advanced tokamak" concept, which includes non-circular plasma, internal diverters and limiters, often superconducting magnets, and operate in the so-called "H-mode" island of increased stability. Two other designs have also become fairly well studied; the compact tokamak is wired with the magnets on the inside of the vacuum chamber, while the spherical tokamak reduces its cross section as much as possible.

In 1974 a study of the ZETA results demonstrated an interesting side-effect; after an experimental run ended, the plasma would enter a short period of stability. This led to the reversed field pinch concept, which has seen some level of development since. On May 1, 1974, the KMS fusion company (founded by Kip Siegel) achieves the world's first laser induced fusion in a deuterium-tritium pellet.[81]

In the mid-1970s, Project PACER, carried out at Los Alamos National Laboratory (LANL) explored the possibility of a fusion power system that would involve exploding small hydrogen bombs (fusion bombs) inside an underground cavity.[82] As an energy source, the system is the only fusion power system that could be demonstrated to work using existing technology. It would also require a large, continuous supply of nuclear bombs, however, making the economics of such a system rather questionable.

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