ATOMIC COLLISION PROCESSES RELEVANT FOR HEAVY-ION BEAM PROBES

Atomic collision processes of fast Tl and Cs ions with particles in a high temperature fusion plasma are investigated. At low beam energies (< 5 MeV), ion impact collisions and charge exchange processes can be neglected compared to electron ionization processes. At beam energies above 5 MeV and high plasma ion temperatures, collisions with ions sta rt to contribute significantly to signal generation and attenuation. A lso, collisions with the neutral background gas in the beamlines can a ttenuate the ion beam significantly and lower the signal level, if the vacuum pressure is above 10(-4) Torr. For the heavy ion beam probes o perating today, only electron impact ionization processes are importan t and accurate predictions of the secondary signal level and electron density profile measurements are possible because of the good knowledg e of electron impact ionization cross sections for Cs+ and Tl+ ions.