For Netherlands radio astronomy group, see ASTRON.
View from above of the Astron which was operated for the Atomic Energy Commission.
The Astron is a type of fusion power device pioneered by Nicholas Christofilos and built at the Lawrence Livermore National Laboratory during the 1960s and 70s. Astron used a unique confinement system that avoided several of the problems found in contemporary designs like the stellarator and magnetic mirror. Development was greatly slowed by a series of changes to the design that were made with limited oversight, leading to a review committee being set up to oversee further development. The Astron was unable to meet the performance goals set for it by the committee; funding was cancelled in 1972 and development wound down in 1973. Work on similar designs appears to have demonstrated a theoretical problem in the very design that suggests it could never be used for practical generation.
Christofilos is best known for independently inventing the concept of strong focusing, a feature used in particle accelerators. He had first started work along these lines in the late 1940s while running an elevator installation company,[1] and in 1948 he wrote a letter to what was then the University of California's Radiation Laboratory at Berkeley outlining several ideas on accelerator focusing. When they returned his letter pointing out several problems, he solved these and wrote them again. This second letter was ignored. In 1950 Christofilos filed a patent application, which was granted in 1956 as US Patent 2,736,799.[2]
Around the same time, Ernest Courant, Milton Stanley Livingston, and Hartland Snyder of Brookhaven National Laboratory were considering the same problem and devised the same solution, writing about it in the 1 December 1952 issue of Physical Review.[3] When he saw the paper, Christofilos arranged a trip to the US, arriving two months later. Making his way to Brookhaven, he angrily accused them of stealing the idea from his patent. He also met with members of the Atomic Energy Commission, and after meeting with his attorneys, they paid him $10,000 for the patent.[4]
Astron proposal
With the patent purchase came some fame and enough money that Christofilos was able to enter the US physics world. In April 1953 he attended a meeting of Project Sherwood, and presented another idea he had been working on in Greece, the Astron.[5]
The basic idea was to inject high-energy electrons into a magnetic mirror (the "tank"). The electrons would be captured in the mirror, and build up a layer of current near the outside surface of the tank volume, which he called the "E-layer". The E-layer would itself produce a powerful magnetic field as it built up, and once the current reached a critical density, the fields would "reverse", and fold into a new configuration of closed lines that formed a continuous confinement area. Once the E-layer had successfully formed, fusion fuel would be injected into the area inside it, and heated by interactions with the E-layer to bring it to fusion temperatures.[6]
This arrangement solved one of the main problems with the basic magnetic mirror concept, which had open field lines at the ends. Fuel could follow these lines right out of the reactor. Mirrors thus naturally leaked plasma, although designers believed they could address this problem by operating the machines at very high temperatures. In practice the leakage proved to be even higher than basic theory suggested, and never operated at the levels they hoped to achieve.[7]
At the time, Sherwood was still secret, which presented problems when he first outlined the concept. Prior to his taking the stage, the formulas from the previous session on the blackboard had been carefully erased. As he filled the blackboard with his own equations, someone helpfully showed him the buttons that would raise it and reveal a fresh one underneath. This one had not been erased and led to a rushed effort to prevent any sensitive material leaking. Looking to avoid a repeat event, Christofilos was given a job at Brookhaven, where he could continue working on the Astron theory.[8]
Astron testing
A side view of the Astron device, circa 1966. A single-layer solenoid, 92 feet long, is built around an aluminum vacuum vessel.
In 1956, Christofilos finally received his security clearance, and he immediately moved to what was now the Lawrence Livermore National Laboratory (LLNL) to start work on the Astron concept. After two years enough progress had been made that he was able to present the idea at the 1958 Atoms for Peace conference in Geneva, along with a model of the system they proposed to build. This consisted of two main parts, the magnetic bottle where the plasma would be held, and a particle accelerator that provided the relativistic electrons.[9]
In spite of his success, Christofilos was always an outsider at the lab. Time reported that "He still has no degree in physics, and his Greek accent, Greek volubility and love of passionate argument keep him an outsider."[10] This led to friction within the physics establishment, and early calls for termination of the Astron program. A 1963 review of the entire Project Sherwood effort led to formal calls for cancellation. However, the program had backers within the management of the controlled fusion program, notably Glenn Seaborg and John S. Foster, both with strong ties to LLNL. Foster, in particular, was concerned about groups in Washington dictating development to the labs. After considerable argument, it was decided that the program would be allowed to continue, but would need to demonstrate field reversal by 1965.[11]
By 1963 the team had designed and built a new type of linear induction accelerator with the required properties. The accelerator design led to interest as a particle beam weapon studied under Project Seesaw.[12] However, during construction the team realized that the electrons were free to travel back into the accelerator area. Christofilos solved this by introducing resistor wires that slightly slowed the electrons after entering the tank, so they no longer possessed the energy needed to flow back out.[13]
After some work ironing out bugs, the first results were published in June 1964. The accelerator worked, operating at 4 MeV and 120 amps, and a stable E-layer was confirmed, albeit generating only 2 A/cm of current, just 0.05% of the diamagnetic field required to reverse the field.[14] Work continued to meet the 1965 goal of reversal, but ultimately failed. However, the electron layer was stable, so the Herb-Allison committee recommended it continue to the next milepost.[15]
By 1967 this had been improved to 6%, but was still a long way from the stable E-layer the device needed to achieve. In 1968, Christofilos and T. Kenneth Fowler wrote a report asking for a more powerful accelerator, and upgrades to the tank.[16]
Scrutiny
Funds for the upgrades were eventually granted, but only at the cost of direct oversight by an Ad Hoc Panel created by the AEC. By this point the "conventional" designs, the stellarator and magnetic mirror, had long been working on real-world plasmas and were slowly increasing the pressures and temperatures. Astron, on the other hand, was still a long way from building its first useful E-layer, a prerequisite for plasma experiments.[17]
The Ad Hoc Panel returned a negative report, complaining that far too much effort had been put into operational issues like accelerator performance, with little or no effort into theoretical studies on whether or not the plasma would ever be stable even if an E-layer could be formed.[18] Moreover, the panel pointed out that no one had seriously studied whether or not an operating and stable Astron would require more power to operate than it would release. This was a serious concern in Astron, because its relativistic electrons would radiate away large amounts of power due to electron synchrotron radiation.[19]
Christofilos had already considered this and suggested that an operational design would use protons in place of the electrons, and would not suffer from the same level of energy losses. However, no such accelerator existed at the time, and the panel was highly skeptical that it would be simple to build.[20]
Upgrade
The upgrades to Astron went ahead and started operation in 1969. During this period, following the advice of the Panel, the theoretical divisions at LLNL started taking a much more serious look at the concept. Building computer models of the system, they first attacked the problem of "stacking", that individual pulses of electrons from the accelerator did not build up in the E-layer as expected. Bruce Langdon demonstrated that stacking simply would not work.[21]
However, a suggestion by Fowler proved to save the Astron from this problem. He had noted that adding a second magnetic field running down the centre of the tank would reduce the amount of external field needed to create the E-layer. Christofilos went ahead with this change and started testing in 1971; this demonstrated greatly improved performance both with the reduction in current and success in trapping the electrons. This also allowed two pulses to be stacked, raising the field to 15% diamagnetic strength.[21]
While Astron worked towards multiple pulses, a team at Cornell University had been working on a similar design. However, this Relativistic Electron Coil Experiment (RECE) used a single long pulse of electrons rather than the stacking concept. Late in 1971 they announced they had achieved complete field reversal. Christofilos was unimpressed; this design would not be useful for a steady state fusion generator, only by continually adding pulses could the machine maintain itself.[22]
Cancellation
Faced with the continued problems with Astron, and the seeming ease that the RECE team had managed to reach the goals they had originally suggested in 1968, a second Ad Hoc Panel published a scathing report. Among the problems they noted that the Astron team had been looking "for ingenious ways to avoid or circumvent difficulties rather than to understand them."[23]Roy Gould, head of the AEC's controlled fusion program, was specific in allowing the Astron project to continue, but only if it met a series of goals on a specific timeline.[24]
When Robert Hirsch took over the AEC's controlled fusion arm in 1972, he instituted a sweeping review to classify the approaches under study and eliminate duplication and low-payoff projects. Given the exciting results with the tokamak released in 1968, Hirsch favoured a program with relatively few projects each given much larger budgets.[25] Many programs like Astron simply didn't appear to have any near-term payoff, and Hirsch was keen to cancel them.
On 24 September 1972, Christofilos met with James Schlesinger of the AEC, but no record of the meeting remains. After a long day, he went to a local Holiday Inn to save a long commute home. That night he suffered a massive heart attack and died.[24]
Richard Briggs took over direction of the project until its planned shutdown date in June 1973. Under his direction, Astron returned to study of the new stabilizing field introduced by Fowler, and using single larger pulses the device hit 50% diamagnetic strength, much greater than Christofilos' efforts with pulse chains. Their final report stated that "buildup of the E-layer by multiple-pulse injection was generally unsuccessful" and noted that at the time of the shutdown they still did not understand what physics problem was limiting the buildup.[26]
After Astron
Although Astron shut down, work continued with RECE at Cornell for some time. As part of their work, the team attempted to make the switch from electrons to protons. However, as some suspected, the "P-layer" proved difficult to build, and field reversal with protons was never achieved. The last version of this effort, FIREX, shut down in 2003, having demonstrated what appears to be a purely theoretical reason why the Astron concept will never work.[27]
The relativistic electron ring also played a part in the bumpy torus design. This was another attempt to "plug the ends" of mirrors, by linking a number of mirrors end-to-end to form a torus. Electrons were driven to high energies not through direct injection, but external microwave-driven electron-cyclotron heating (ECH).[28]
Description
The Astron device consisted of two sections, the linear accelerator and the magnetic mirror "tank". These were constructed at right angles, with the accelerator's output firing into the side of the tank at one end.[29]
The tank was a relatively simple example of the magnetic mirror concept, consisting largely of a long solenoid with additional windings at both ends to increase the magnetic field in those regions and form the mirror.[30] In a simple mirror the ions in the fuel plasma were injected at an angle so they could not simply flow right out of the ends where the field was roughly linear. However, there was an annular region on either end where ions of the right energy could escape, and various calculations demonstrated the rate would be fairly high.
By injecting electrons into the mirror before the fuel, the E-layer would create a second magnetic field that would cause the annular areas to fold back into the center of the tank. The resulting field was shaped like a tube, and very similar to the Field-Reversed Configuration, or FRC.[30] The main difference between these devices is the way the field reversal is achieved; with the E-layer in the Astron, and by currents in the plasma for the FRC. Like the classic mirror, Astron injected the electrons into the mirror at a slight angle to ensure they would circulate into the center of the mirror.[29]
Today, the Astron is often considered a sub-class of the FRC concept.[31]
Related Research Articles
A stellarator is a plasma device that relies primarily on external magnets to confine a plasma. 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, such as the Sun. It is one of the earliest fusion power devices, along with the z-pinch and magnetic mirror.
A tokamak is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. As of 2016, it was the leading candidate for a practical fusion reactor. The word "tokamak" is derived from a Russian acronym meaning "toroidal chamber with magnetic coils".
A magnetic mirror, known as a magnetic trap in Russia and briefly as a pyrotron in the US, is a type of magnetic confinement fusion device used in fusion power to trap high temperature plasma using magnetic fields. The mirror was one of the earliest major approaches to fusion power, along with the stellarator and z-pinch machines.
Cyclotron radiation is electromagnetic radiation emitted by non-relativistic accelerating charged particles deflected by a magnetic field. The Lorentz force on the particles acts perpendicular to both the magnetic field lines and the particles' motion through them, creating an acceleration of charged particles that causes them to emit radiation as a result of the acceleration they undergo as they spiral around the lines of the magnetic field.
A linear particle accelerator is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear beamline. The principles for such machines were proposed by Gustav Ising in 1924, while the first machine that worked was constructed by Rolf Widerøe in 1928 at the RWTH Aachen University. Linacs have many applications: they generate X-rays and high energy electrons for medicinal purposes in radiation therapy, serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles for particle physics.
Aneutronic fusion is any form of fusion power in which very little of the energy released is carried by neutrons. While the lowest-threshold nuclear fusion reactions release up to 80% of their energy in the form of neutrons, aneutronic reactions release energy in the form of charged particles, typically protons or alpha particles. Successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as damaging ionizing radiation, neutron activation, reactor maintenance, and requirements for biological shielding, remote handling and safety.
A field-reversed configuration (FRC) is a type of plasma device studied as a means of producing nuclear fusion. It confines a plasma on closed magnetic field lines without a central penetration. In an FRC, the plasma has the form of a self-stable torus, similar to a smoke ring.
Project Sherwood was the codename for a United States program in controlled nuclear fusion during the period it was classified. After 1958, when fusion research was declassified around the world, the project was reorganized as a separate division within the United States Atomic Energy Commission (AEC) and lost its codename.
ZETA, short for Zero Energy Thermonuclear Assembly, was a major experiment in the early history of fusion power research. Based on the pinch plasma confinement technique, and built at the Atomic Energy Research Establishment in the United Kingdom, ZETA was larger and more powerful than any fusion machine in the world at that time. Its goal was to produce large numbers of fusion reactions, although it was not large enough to produce net energy.
The Christofilos effect, sometimes known as the Argus effect, refers to the entrapment of electrons from nuclear weapons in the Earth's magnetic field. It was first predicted in 1957 by Nicholas Christofilos, who suggested the effect had defensive potential in a nuclear war, with so many beta particles becoming trapped that warheads flying through the region would experience huge electrical currents that would destroy their trigger electronics. The concept that a few friendly warheads could disrupt an enemy attack was so promising that a series of new nuclear tests was rushed into the US schedule before a testing moratorium came into effect in late 1958. These tests demonstrated that the effect was not nearly as strong as predicted, and not enough to damage a warhead. However, the effect is strong enough to be used to black out radar systems and disable satellites.
Neutral-beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. Neutral-beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices. To date, all NBI systems were based on positive precursor ion beams. In the 1990s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative-ion-based NBI systems at LHD (H0, 180 keV) and JT-60U (D0, 500 keV). The NBI designed for ITER is a substantial challenge (D0, 1 MeV, 40 A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), ion cyclotron resonance heating (ICRH), and lower hybrid resonance heating (LH).
Migma, sometimes migmatron or migmacell, was a proposed colliding beam fusion reactor designed by Bogdan Maglich in 1969. Migma uses self-intersecting beams of ions from small particle accelerators to force the ions to fuse. Similar systems using larger collections of particles, up to microscopic dust sized, were referred to as "macrons". Migma was an area of some research in the 1970s and early 1980s, but lack of funding precluded further development.
The Perhapsatron was an early fusion power device based on the pinch concept in the 1950s. Conceived by James (Jim) Tuck while working at Los Alamos National Laboratory (LANL), he whimsically named the device on the chance that it might be able to create fusion reactions.
The bumpy torus is a class of magnetic fusion energy devices that consist of a series of magnetic mirrors connected end-to-end to form a closed torus. It is based on a discovery made by a team headed by Ray Dandl at Oak Ridge National Laboratory in the 1960s.
The Tandem Mirror Experiment was a magnetic mirror machine operated from 1979 to 1987 at the Lawrence Livermore National Laboratory. It was the first large-scale machine to test the "tandem mirror" concept in which two mirrors trapped a large volume of plasma between them in an effort to increase the efficiency of the reactor.
The Princeton Large Torus, was an early tokamak built at the Princeton Plasma Physics Laboratory (PPPL). It was one of the first large scale tokamak machines, and among the most powerful in terms of current and magnetic fields. Originally built to demonstrate that larger devices would have better confinement times, it was later modified to perform heating of the plasma fuel, a requirement of any practical fusion power device.
The Princeton field-reversed configuration (PFRC) is a series of experiments in plasma physics, an experimental program to evaluate a configuration for a fusion power reactor, at the Princeton Plasma Physics Laboratory (PPPL). The experiment probes the dynamics of long-pulse, collisionless, low s-parameter field-reversed configurations (FRCs) formed with odd-parity rotating magnetic fields. FRCs are the evolution of the Greek engineer's Nicholas C. Christofilos original idea of E-layers which he developed for the Astron fusion reactor. The PFRC program aims to experimentally verify the physics predictions that such configurations are globally stable and have transport levels comparable with classical magnetic diffusion. It also aims to apply this technology to the Direct Fusion Drive concept for spacecraft propulsion.
Heavy ion fusion is a fusion energy concept that uses a stream of high-energy ions from a particle accelerator to rapidly heat and compress a small pellet of fusion fuel. It is a subclass of the larger inertial confinement fusion (ICF) approach, replacing the more typical laser systems with an accelerator.
Theta-pinch, or θ-pinch, is a type of fusion power reactor design. The name refers to the configuration of currents used to confine the plasma fuel in the reactor, arranged to run around a cylinder in the direction normally denoted as theta in polar coordinate diagrams. The name was chosen to differentiate it from machines based on the pinch effect that arranged their currents running down the centre of the cylinder; these became known as z-pinch machines, referring to the Z-axis in cartesian coordinates.
The toroidal solenoid was an early 1946 design for a fusion power device designed by George Paget Thomson and Moses Blackman of Imperial College London. It proposed to confine a deuterium fuel plasma to a toroidal (donut-shaped) chamber using magnets, and then heating it to fusion temperatures using radio frequency energy in the fashion of a microwave oven. It is notable for being the first such design to be patented, filing a secret patent on 8 May 1946 and receiving it in 1948.
↑ Ernest Courant, M. Stanley Livingston and Hartland Snyder. "The Strong-Focusing Synchrotron—A New High Energy Accelerator", Physical Review Volume 88 (1952). p. 1190.
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