Abstract
The pitch-angle scattering rate of a dilute population of 75 keV deuterium ions is measured in a well-diagnosed, relatively quiet, magnetically-confined deuterium plasma. Neutral particle diagnostics detect the fast-ion density in velocity space following a short 10 ms pulse of injected beam ions. The data are compared to the classical theory of diffusion in velocity space caused by many, small-angle, Coulomb-scattering events. Within uncertainties of ≲15%, the data confirm the classical theory.
Highlights
Coulomb scattering is a fundamental process in plasma physics that causes an initially monoenergetic beam to decelerate and diffuse in velocity space
Small-angle scattering events predominate, so a statistical theory governs the evolution of the distribution function in velocity space
Figure 6͑acompares the measured exponential decay time obtained from fits to the neutron signal following a beam blip to the exponential decay time calculated by TRANSP. ͑The same least-squares fitting procedure is used for both ‘‘signals.’’͒ The agreement is satisfactorycorrelation coefficient r ϭ 0.87)
Summary
Coulomb scattering is a fundamental process in plasma physics that causes an initially monoenergetic beam to decelerate and diffuse in velocity space. Small-angle scattering events predominate, so a statistical theory governs the evolution of the distribution function in velocity space.. ͑Self-collisions are assumed negligible.͒ The speed of the fast ions v is intermediate between the electron and ion thermal speeds, veӷvӷvi. Consider a dilute population of fast ions in a background plasma with Maxwellian electrons and ions. For this simple case, the evolution of the fast-ion distribution function f is described by the following Fokker– Planck equation:
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