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Quasi-linear theory calculation of ion tails and neutron rates during lower hybrid heating in Alcator-A

The steady-state fast ion distribution function and the resulting neutron rate are calculated for the conditions of the Alcator-A lower hybrid heating experiment from quasi-linear theory. First, ion orbit losses are ignored and the steady-state ion distribution function is calculated. It is found that in this case the experimental neutron rates and neutron rate decay times are consistent with only a small fraction of the incident radio-frequency (RF) power being absorbed in the plasma centre. This absorbed power is centred in the regime 3.5 < n‖ < 4.5. A Monte-Carlo ion simulation technique incorporating ripple orbit losses is then presented which calculates the steady-state ion distribution function in the presence of the RF. The results of this simulation require approximately 50% of the incident RF power to be dissipated in the plasma core in order to obtain agreement with the experimental results. Most of this RF power is lost to ripple-trapped ions. These calculations demonstrate the importance of orbit losses in determining lower hybrid heating efficiencies and additionally provide a technique for calculating them.