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Plasma heating

Methods to heat the tokamak plasma. Click on the image to enlarge.

Confinement no longer being a major issue in such a tokamak configuration, one can subsequently address the second - high temperature - condition to be satisfied for fusion to spontaneously take place. Even for the "easiest" fusion reaction that is sufficiently abundant to make its commercial use to produce electricity possible, temperatures of the order of 100 million degrees are required. This is achieved in 3 ways: by Ohmic heating, by neutral beam injection and by resonant interaction of electromagnetic waves with charged particles.

Ohmic or Joule heating is what heats up a metallic wire when one sends a current through it: Running a household appliance at a too high current for the net 220V voltage results in a security fuse in your home fuse box melting away, a safety measure taken to avoid the same effect heating up your electric cables themselves and starting a fire. To the exception of superconducting materials no conducting medium, such as a metallic wire in which a fraction of the electrons can jump from one atom to the next fairly easily, can carry a current without part of the energy being converted into heat. This happens because the charged particles lose some of their momentum by bumping into each other. Plasma is a very good but is not a perfect conductor. Its resistance is of order of a millionth of an Ohm. Small as it may be, this resistance allows low-density plasmas such as those in a tokamak to be heated to temperatures of the order of a million degrees. Ohmic heating occurs in a natural way in present-day tokamaks: the poloidal magnetic field component needed for the plasma's stability is created by passing a toroidal current through the machine relying on the transformer principle. A primary current at the heart of the torus drives a secondary current in the plasma. A problem of Ohmic heating is that it becomes less efficient the hotter and the better conducting the plasma gets. As a consequence, Ohmic heating itself is insufficient to reach fusion in current man-made machines.

To fill the gap between the temperatures that can be reached by Ohmic heating and those required for fusion, one option is to fire very energetic particles into the machine. The principle is as follows: Just like in a classical lamp amplifier, charged particles are accelerated in between an anode and a cathode. The applied voltage difference between the two is typically several tens of thousands of electron-volts (an electron-volt is the energy an electron or a proton acquires when being accelerated across a voltage difference of 1 Volt). Recall that charged particles in a tokamak gyrate around guiding centers that are roughly aligned with the magnetic field lines spiraling around the machine. When fired into the machine as charged particles, they will start gyrating around the first, outermost magnetic field lines they encounter and thus it is unlikely that they ever reach the center of the machine. To prevent these energetic particles to be confined to the edge of the plasma, they are first sent through a cloud of neutral particles. Through what is known as charge exchange, they capture some of the electrons of the neutral gas. The particles that manage to rip away enough electrons to become neutral, are fired into the tokamak. Those that do not are deflected using magnetic force and are stopped on the walls of the neutralizer chamber.

A second method of auxiliary heating to reach fusion relevant temperatures is radiating electromagnetic waves into the plasma. Carefully choosing the frequency of the generator to coincide with a characteristic frequency of the plasma, the power electromagnetically radiated into the plasma is transferred to the charged particles. In the case of radio frequency heating, the specialization of the Belgian Laboratory for Plasma Physics of the Association "Euratom - Belgian State", the chosen frequency is that at which the ions gyrate around their guiding center.