How scientists push fusion fuel to 150 million degrees — the three main techniques for heating plasma far beyond the temperature of the Sun's core.
Fusion reactions require temperatures on the order of 100 to 200 million degrees Celsius — roughly ten times hotter than the center of the Sun. At the Sun's core, immense gravitational pressure compensates for lower temperatures. In a laboratory reactor, with no such gravitational advantage, temperature must do all the work. Getting a plasma to these extreme temperatures requires sophisticated heating systems, and modern fusion devices typically use three complementary methods.1
In a tokamak, the first and simplest source of heat is ohmic heating — the same principle that makes a toaster's wire glow. A strong electrical current is driven through the plasma (induced by the central solenoid), and the plasma's electrical resistance converts some of that current's energy into heat.
Ohmic heating is effective at lower temperatures, but its usefulness fades as the plasma gets hotter. This is because a hotter plasma actually becomes a better electrical conductor — its resistance drops roughly as the temperature rises to the power of 3/2. Beyond about 30 million degrees, ohmic heating alone cannot push the plasma any further. Additional heating methods must take over.2
Neutral beam injection is conceptually straightforward: accelerate a beam of atoms to very high energy and fire them into the plasma. When these fast atoms enter the plasma, they are quickly ionized (stripped of their electrons) by collisions, becoming fast ions trapped by the magnetic field. These energetic ions then share their energy with the surrounding plasma through further collisions, raising its temperature.
In practice, producing a neutral beam is a multi-step process. First, ions (usually deuterium) are extracted from a source and accelerated through a high-voltage grid to energies of 50 to 1,000 keV. The fast ions then pass through a gas cell where they pick up electrons, becoming neutral atoms that can cross the magnetic field unimpeded. Any remaining ions are magnetically deflected into a beam dump.3
ITER will use two neutral beam injectors, each delivering 16.5 MW of heating power with beam energies of 1 MeV — the most powerful ever built. NBI also provides a tool for driving plasma current and fueling the plasma with fresh deuterium atoms.
The third major method uses electromagnetic waves — essentially high-powered radio or microwave signals — to transfer energy directly to particles in the plasma. Different frequency ranges target different particle populations:
Ion Cyclotron Resonance Heating (ICRH) uses radio waves in the 30 to 120 MHz range, tuned to the frequency at which ions spiral around magnetic field lines. When the wave frequency matches the ion cyclotron frequency (or a harmonic), ions absorb energy efficiently and are accelerated to high speeds. ICRH is particularly effective at heating minority ion species and can create populations of very energetic ions.4
Electron Cyclotron Resonance Heating (ECRH) uses microwave beams at much higher frequencies, typically 50 to 170 GHz, matching the cyclotron frequency of electrons. ECRH has the unique advantage of being highly localized — the microwave beam deposits its energy in a narrow layer where the magnetic field strength produces the right resonance condition. This makes ECRH a precision tool for controlling the plasma's temperature profile and suppressing instabilities.
Lower Hybrid Heating (LH) uses waves in an intermediate frequency range (1 to 8 GHz). Lower hybrid waves are particularly efficient at driving plasma current — a critical need for steady-state tokamak operation — and provide modest direct heating as well.5
Modern fusion experiments rarely rely on a single heating method. A typical large tokamak pulse might begin with ohmic heating to establish the plasma, then layer on NBI for bulk heating and ECRH for profile control and instability suppression. ITER, for example, will combine 33 MW of NBI, 20 MW of ECRH, and 20 MW of ICRH for a total external heating power of 73 MW.
In a future power plant, much of the plasma heating will come from the fusion reactions themselves: the alpha particles (helium-4 nuclei) produced by deuterium-tritium fusion carry 3.5 MeV of kinetic energy and, being charged, remain confined by the magnetic field, depositing their energy back into the plasma. When this alpha heating dominates, the plasma is said to be "burning" — the ultimate goal of fusion energy research.