page_title: Tokamak
section_title: Heating: US takes the lead
hierarchical_section_title: Tokamak / History / Heating: US takes the lead
caption_attribution_description: English: Overhead view of the Princeton Large Torus in 1975. The PLT was an extremely successful tokamak fusion device that set numerous records and demonstrated that the temperatures needed for fusion were possible.
context_page_description: A tokamak is a device which uses a powerful magnetic field to confine a hot 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 is the leading candidate for a practical fusion reactor.
  Tokamaks were initially conceptualized in the 1950s by Soviet physicists Igor Tamm and Andrei Sakharov, inspired by a letter by Oleg Lavrentiev. Meanwhile, the first working tokamak was attributed to the work of Natan Yavlinsky on the T-1. It had been demonstrated that a stable plasma equilibrium requires magnetic field lines that wind around the torus in a helix. Devices like the z-pinch and stellarator had attempted this, but demonstrated serious instabilities. It was the development of the concept now known as the safety factor that guided tokamak development; by arranging the reactor so this critical factor q was always greater than 1, the tokamaks strongly suppressed the instabilities which plagued earlier designs.
  The first tokamak, the T-1, began operation in 1958. By the mid-1960s, the tokamak designs began to show greatly improved performance.
context_section_description: Experiments on the Symmetrical Tokamak began in May 1970, and by early the next year they had confirmed the Soviet results. The stellarator was abandoned, and PPPL turned its considerable expertise to the problem of heating the plasma. Two concepts seemed to hold promise. PPPL proposed using magnetic compression, a pinch-like technique to compress a warm plasma to raise its temperature, but providing that compression through magnets rather than current. Oak Ridge suggested neutral beam injection, small particle accelerators that would shoot fuel atoms through the surrounding magnetic field where they would collide with the plasma and heat it.
  PPPL's Adiabatic Toroidal Compressor (ATC) began operation in May 1972, followed shortly thereafter by a neutral-beam equipped Ormak. Both demonstrated significant problems, but PPPL leapt past Oak Ridge by fitting beam injectors to ATC and provided clear evidence of successful heating in 1973. This success "scooped" Oak Ridge, who fell from favour within the Washington Steering Committee.
  By this time a much larger design based on beam heating was under construction, the Princeton Large Torus, or PLT. PLT was designed specifically to "give a clear indication whether the tokamak concept plus auxiliary heating can form a basis for a future fusion reactor". PLT was an enormous success, continually raising its internal temperature until it hit 60 million Celsius (8,000 eV, eight times T-3's record) in 1978. This is a key point in the development of the tokamak; fusion reactions become self-sustaining at temperatures between 50 and 100 million Celsius, PLT demonstrated that this was technically achievable.
  These experiments, especially PLT, put the US far in the lead in tokamak research. This is due largely to budget; a tokamak cost about $500,000 and the US annual fusion budget was around $25 million at that time. They could afford to explore all of the promising methods of heating, ultimately discovering neutral beams to be among the most effective.
  During this period, Robert Hirsch took over the Directorate of fusion development in the U.S. Atomic Energy Commission. Hirsch felt that the program could not be sustained at its current funding levels without demonstrating tangible results. He began to reformulate the entire program. What had once been a lab-led effort of mostly scientific exploration was now a Washington-led effort to build a working power-producing reactor. This was given a boost by the 1973 oil crisis, which led to greatly increased research into alternative energy systems.
caption: Overhead view of the Princeton Large Torus in 1975. The PLT was an extremely successful tokamak fusion device that set numerous records and demonstrated that the temperatures needed for fusion were possible.
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