Abstract
A summary of fast ion experiments in the DIII-D tokamak is given. Most of the experiments involve ~80-keV deuterium beam ions. Deceleration of dilute fast-ion populations is accurately described by coulomb scattering theory. Fast waves with frequencies several times the deuterium cyclotron frequency interact with beam ions when the product of wave number and gyroradius k⊥ρi is ≿ 1.4. Global confinement of fast ions is often excellent although sawteeth, tearing modes, and beam-driven instabilities can cause additional transport. Intense beam-ion populations often drive instabilities. Toroidicity-induced Alfvén eigenmodes (TAE) and somewhat lower frequency modes (originally called beta-induced Alfvén eigenmodes) are often observed in a wide variety of plasma conditions. Over 50% of the beam power is lost during strong activity. Damping mechanisms such as mode coupling or radiative damping are needed to explain the observed TAE stability threshold. The most unstable toroidal mode number agrees well with theoretical expectations, but the radial and poloidal structure of the mode and the observed beam-ion transport have not been adequately explained. The modes with frequencies below the TAE are probably two types of energetic particle modes: the resonant TAE and the resonant kinetic ballooning mode.
Highlights
A typical tokamak plasma contains thermal electrons, thermal ions, and a population of suprathermal fast ions produced by fusion reactions, neutral-beam heating, or radio-frequency heating in the ion cyclotron range of frequencies ~ICRF!
It is well known that turbulent transport of fast ions associated with short-wavelength fluctuations is one to two orders of magnitude smaller than thermal transport,[1] presumably because the large fast-ion gyroradius of ;2 cm decorrelates the fast ions from fluctuations with a scale length comparable to the thermal-ion gyroradius of ;0.5 cm
The highest fusion yield produced on DIII-D of 2.2 ϫ 1016 n0s is within 10% of the calculated value.[20,21]
Summary
A typical tokamak plasma contains thermal electrons, thermal ions, and a population of suprathermal fast ions produced by fusion reactions, neutral-beam heating, or radio-frequency heating in the ion cyclotron range of frequencies ~ICRF!. This suprathermal ion population often provides most of the power that sustains the plasma. Is the anisotropic beam-ion distribution associated with injection of ;80-keV deuterium neutrals into the torus. For typical plasma parameters, ,10% of the injected power is lost to charge exchange and prompt orbit losses. Fusion reaction products and energetic ions accelerated by ICRF have been the object of study. Most studies employ conventional diagnostics such as the volumeintegrated neutron rate[7] and neutral particle analyzers.[8]
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