Editing Tokamak (section)

== Plasma heating ==

In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature as fresh [[deuterium]] and [[tritium]] are introduced. However, in the startup of a reactor, either initially or after a temporary shutdown, the plasma will have to be heated to its [[operating temperature]] of greater than 10 keV (over 100 million degrees Celsius). In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature, and constant external heating must be supplied. Chinese researchers set up the [[Experimental Advanced Superconducting Tokamak]] (EAST) in 2006, which can supposedly sustain a plasma temperature of 100 million degree Celsius for initiating fusion between hydrogen atoms, according to a November 2018 test.

=== Ohmic heating ~ inductive mode ===

Since the plasma is an electrical conductor, it is possible to heat the plasma by inducing a current through it; the induced current that provides most of the poloidal field is also a major source of initial heating.

The heating caused by the induced current is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater. The heat generated depends on the resistance of the plasma and the amount of electric current running through it. But as the temperature of heated plasma rises, the resistance decreases and ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20–30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used.

The current is induced by continually increasing the current through an electromagnetic winding linked with the plasma torus: the plasma can be viewed as the secondary winding of a transformer. This is inherently a pulsed process because there is a limit to the current through the primary (there are also other limitations on long pulses). Tokamaks must therefore either operate for short periods or rely on other means of heating and current drive.

=== Magnetic compression ===
A gas can be heated by sudden compression. In the same way, the temperature of a plasma is increased if it is compressed rapidly by increasing the confining magnetic field. In a tokamak, this compression is achieved simply by moving the plasma into a region of higher magnetic field (i.e., radially inward). Since plasma compression brings the ions closer together, the process has the additional benefit of facilitating attainment of the required density for a fusion reactor.

Magnetic compression was an area of research in the early "tokamak stampede", and was the purpose of one major design, the ATC. The concept has not been widely used since then, although a somewhat similar concept is part of the [[General Fusion]] design.

=== Neutral-beam injection ===
{{see also|Neutral beam injection}}
Neutral-beam injection involves the introduction of high energy (rapidly moving) atoms or molecules into an ohmically heated, magnetically confined plasma within the tokamak.

The high energy atoms originate as ions in an arc chamber before being extracted through a high voltage grid set. The term "ion source" is used to generally mean the assembly consisting of a set of electron emitting filaments, an arc chamber volume, and a set of extraction grids. A second device, similar in concept, is used to separately accelerate electrons to the same energy. The much lighter mass of the electrons makes this device much smaller than its ion counterpart. The two beams then intersect, where the ions and electrons recombine into neutral atoms, allowing them to travel through the magnetic fields.

Once the neutral beam enters the tokamak, interactions with the main plasma ions occur. This has two effects. One is that the injected atoms re-ionize and become charged, thereby becoming trapped inside the reactor and adding to the fuel mass. The other is that the process of being ionized occurs through impacts with the rest of the fuel, and these impacts deposit energy in that fuel, heating it.

This form of heating has no inherent energy (temperature) limitation, in contrast to the ohmic method, but its rate is limited to the current in the injectors. Ion source extraction voltages are typically on the order of 50–100 kV, and high voltage, negative ion sources (-1 MV) are being developed for ITER. The ITER Neutral Beam Test Facility in Padova will be the first ITER facility to start operation.<ref>{{Cite web|url=https://www.igi.cnr.it/www/sites/default/files/home201511/SchedaNBTF_MIUR_EN.pdf|title=Neutral Beam Test Facility|access-date=9 October 2016|archive-date=10 October 2016|archive-url=https://web.archive.org/web/20161010212050/https://www.igi.cnr.it/www/sites/default/files/home201511/SchedaNBTF_MIUR_EN.pdf}}</ref>

While neutral beam injection is used primarily for plasma heating, it can also be used as a diagnostic tool and in feedback control by making a pulsed beam consisting of a string of brief 2–10 ms beam blips. Deuterium is a primary fuel for neutral beam heating systems and hydrogen and helium are sometimes used for selected experiments.

=== Radio-frequency heating ===
[[Image:Gyrotron plateforme.jpg|thumb|Set of hyperfrequency tubes (84 GHz and 118 GHz) for plasma heating by electron cyclotron waves on the [[Tokamak à configuration variable|Tokamak à Configuration Variable]] (TCV). Courtesy of SPC-EPFL.]]

{{see also|Radio frequency heating|Dielectric heating}}
High-frequency electromagnetic waves are generated by oscillators (often by [[gyrotron]]s or [[klystron]]s) outside the torus. If the waves have the correct frequency (or wavelength) and polarization, their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma particles, thus increasing the temperature of the bulk plasma. Various techniques exist including [[electron cyclotron resonance]] heating (ECRH) and [[ion cyclotron resonance]] heating. This energy is usually transferred by microwaves.