There are several methods for heating plasmas. These include Ohmic Heating, Neutral Beam Injection, Magnetic Compression, Radio-Frequency Heating, and Inertial Compression
…. Each of these is discussed below. 1: Ohmic Heating "Since the plasma is an electrical conductor, it is possible to heat the plasma by passing a current through it; in fact, the current that generates the poloidal field also heats the plasma. This 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 current. But as the temperature of heated plasma rises, the resistance decreases and the 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." 2: Neutral-Beam Injection "Neutral-beam injection involves the introduction of high-energy (neutral) atoms into the ohmically -- heated, magnetically -- confined plasma. The atoms are immediately ionized and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, thus increasing the plasma temperature." 3: 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 system 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 an additional benefit of facilitating attainment of the required density for a fusion reactor." 4: Radiofrequency Heating "In radiofrequency heating, high-frequency waves are generated by oscillators outside the torus. If the waves have a particular frequency (or wavelength), 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." 5: Inertial Compression This is similar to magnetic compression in that decreasing the gas volume causes the temperature to rise, but in the inertial approach the compression is achieved by using laser or particle beams to heat the outer layer of a target pellet; the outer layer vaporizes and the pressure that the vaporized layer exerts back on the core of the pellet accelerates the plasma inward on itself, and the inertia of the imploding atoms in the pellet allows the pellet to be compressed (for a very short time), and thus heated. (MORE) 
