tokamak

tokamak
Name	Tokamak
Caption	Schematic diagram of a tokamak, showing the toroidal and poloidal magnetic fields, the central solenoid, and the plasma current.
Classification	Magnetic confinement fusion device
Inventor	Igor Tamm, Andrei Sakharov
First built	T-1, 1958
Related	Stellarator, Reversed-field pinch

tokamak. A tokamak is a toroidal apparatus designed for achieving controlled thermonuclear fusion by using powerful magnetic fields to confine a high-temperature plasma. Its design, pioneered in the Soviet Union, has become the leading configuration for magnetic confinement fusion research worldwide, forming the basis for major international projects like ITER. The primary goal is to heat the plasma of hydrogen isotopes like deuterium and tritium to conditions where fusion reactions occur, releasing net energy.

History and development

The concept was first proposed in the early 1950s by Soviet physicists Igor Tamm and Andrei Sakharov, inspired by earlier work on magnetic confinement by Oleg Lavrentiev. The first working device, the T-1, began operation in 1958 at the Kurchatov Institute under the leadership of Lev Artsimovich. Soviet results remained largely unknown in the West until 1968, when a team from the Kurchatov Institute presented remarkably improved plasma performance data at a conference in Novosibirsk, stunning researchers from institutions like Culham Laboratory and Princeton Plasma Physics Laboratory. This "tokamak surprise" led to a global shift in fusion research, with facilities such as the ST at Culham and the Alcator A at the Massachusetts Institute of Technology quickly adopting the design. Subsequent decades saw the construction of larger, more powerful devices like the Joint European Torus (JET) in the United Kingdom, the JT-60 in Japan, and the TFTR at Princeton Plasma Physics Laboratory, which collectively advanced the understanding of high-temperature plasma physics.

Operating principles

A tokamak operates by generating a combination of magnetic fields to create a stable, toroidal magnetic bottle. A strong toroidal field is produced by currents in D-shaped or circular toroidal field coils that surround the vacuum vessel. A central solenoid (or ohmic heating coil) induces a powerful electric current within the plasma itself; this plasma current generates a complementary poloidal magnetic field. The resultant twisted magnetic field lines, forming nested magnetic surfaces, confine the charged plasma particles. Additional heating systems, such as neutral beam injection and radiofrequency heating like ion cyclotron resonance heating, are used to raise the plasma temperature to the tens of millions of degrees required for fusion.

Major components

The primary structure is a toroidal vacuum vessel, typically made of stainless steel or Inconel, which houses the plasma and maintains an ultra-high vacuum. Surrounding the vessel are the toroidal field coils, often superconducting in modern machines, which generate the dominant confining field. Inside the central column is the ohmic heating solenoid, a large transformer that drives the plasma current. Critical auxiliary systems include divertors or limiters, such as those in the X-point configuration, to manage heat flux and plasma impurities. Other essential components are massive cryostats for superconducting magnets, complex diagnostic ports for measuring plasma parameters, and robust first wall materials designed to withstand intense neutron bombardment.

Plasma confinement and stability

Confinement quality is measured by the energy confinement time, a key parameter in achieving the Lawson criterion. Stability is challenged by various instabilities, such as major disruptions caused by kink modes or localized sawtooth oscillations. The achievement of the H-mode (High-confinement mode), first discovered on the ASDEX Upgrade tokamak at the Max Planck Institute for Plasma Physics, significantly improved confinement by forming a steep pressure gradient at the plasma edge, known as a transport barrier. Active control systems using feedback from magneto-hydrodynamic sensors and precise adjustments of electron cyclotron resonance heating are employed to suppress instabilities like neoclassical tearing modes.

Experimental and operational milestones

Landmark achievements include the first production of significant fusion power. In 1991, the Joint European Torus (JET) team used a deuterium-tritium mix to produce 1.7 MW of fusion power. This record was surpassed in 1997 when JET achieved 16 MW. The TFTR at Princeton Plasma Physics Laboratory also produced over 10 MW of fusion power. In 2021, the Experimental Advanced Superconducting Tokamak (EAST) in Hefei, operated by the Institute of Plasma Physics, sustained a 120-million-degree Celsius plasma for 101 seconds. The record for plasma current is held by JT-60SA in Naka, a joint project of Japan and the European Union. These experiments have provided vital data on alpha particle heating and burning plasma physics.

Future projects and research directions

The flagship international project is ITER, currently under construction in Cadarache, France, as a collaboration between the European Union, India, Japan, China, Russia, South Korea, and the United States. ITER aims to demonstrate a tenfold gain in fusion power (Q≥10) and the integrated operation of technologies required for a fusion power plant. Parallel designs for a demonstration power plant, or DEMO, are being developed by agencies like EUROfusion. Other major initiatives include the SPARC project, a collaboration between Massachusetts Institute of Technology and Commonwealth Fusion Systems, and the DIII-D National Fusion Facility's research on advanced plasma shaping. Research directions also focus on developing divertor solutions for extreme heat loads, qualifying irradiation-resistant materials like tungsten and ferritic steel, and exploring alternative configurations such as the spherical tokamak, exemplified by the MAST Upgrade at Culham Centre for Fusion Energy.

Category:Fusion power Category:Plasma physics Category:Russian inventions