PhD and MPhil Thesis Classes - Université Libre de Bruxelles

1.5 Plasma HeatingP kin = n i k B T i + n e k B T e (1.2)to the magnetic pressureP mag = B22µ 0(1.3)where n i,e , T i,e are the density and temperatures of plasma ion and electron respectivelyand k B is the Boltzmann constant, B = |B| is the magnetic field strength, and µ 0 isthe permeability of free space. This ratio is defined as the beta parameter, β, and is ameasure of how effectively the magnetic field guarded the thermal motion of the plasmaparticles. A high beta would be most desirable, but it is also known that there existsa system-specific β max at which plasma fluctuations start to destroy the confinement.That is why for confinement purposes, we requireβ maxB 22µ 0≥ n i k B T i + n e k B T e (1.4)The maximum plasma pressure is thus determined by available magnetic fields.This criteria introduces the magnetic field technology as a limit on plasma confinementin Tokamaks.1.5 Plasma HeatingOhmic heating results from the plasma toroidal inductive current and therefore dependson the the resistance of the plasma. As the temperature of plasma rises however,the resistance decreases and the ohmic heating becomes less effective. The maximumplasma temperature attainable by ohmic heating in a tokamak appears to be 20-30 milliondegrees Celsius. Therefore, to reach still higher temperatures additional heatingmethods must be used (see figure 1.3).Neutral beam injection is one of these methods which involves the introduction ofhigh-energy (neutral) atoms into the ohmically heated, magnetically confined plasma.The atoms are immediately ionized and are trapped by the magnetic field. The highenergyions then transfer part of their energy to the plasma particles in repeated collisions,thus increasing the plasma temperature.5