Lower hybrid heating in the Alcator A tokamak

Abstract

The results and interpretation of the modest nower ( 90 kW) lower hybrid heating experiment on Alcator A are presented.The expected results from linear waveguide-plasma coupling theory are outlined, and the possible effects of parametric instabilities, scattering from density fluctuations, and imperfect energetic ion confinement are addressed.It is found experimentally that good coupling and the absence of rf breakdown are achieved with a double wavecuide array 2 at available rf power densities Prf < 2.5 kW/cm , the waveguide vacuum windows being outside the toroidal field magnets; a waveguide array having vacuum windows near the waveguide mouth so that the w = wc. layer can be pressurizedshows no breakdown at P > 8 kW/cm2 when a single waveguide is energized. Energetic ion wroduction and a factor of 50 increase in the :usion neutron rate are ozserved to take place at well defined values of central plasma density; below these densities electron heating occurs. The ion tail production is found to be independent of the relative phase of the double waveguide array employed. This ion heating occurs at a lower density than theoretically expected; together with the electron heating this indicates waves having n 1 5 being absorbed near the plasma center. Probes at the plasma edge observe a frequency downshifted and broadened rf pump signal that .is strongly attenuated as the plasma density increases through the neutron production band. These anomalous heating results and probe signals can be explained by a parametric decay of the pump wave into higher n lower hybrid waves near the plasma edge. An alternate qualitative explanation would be the poloidal scattering of the lower hybrid waves at the plasma periphery due to density fluctuations; the n,, of the scattered lower hybrid waves would then increase as they propagated inward due to magnetic shear. The neutron rate decay times imply that the rf creates ion tails having a substantial fraction of their energy above 50 keV.The neutron decay times and rates strongly depend on plasma current and indicate the expected influence of ion confinement on rf heating efficiencies. Finally the rf heating efficiencies are assessed.2

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

“…In Ref. 6 it was shown that these minimum values for T7 and E,.Z were required by the slowing down of the tail ions on electrons. For a machine such as Alcator A, energetic ion orbit losses almost certainly affect this energetic ion distribution function.…”

The behaviour of fast ions in tokamak experiments

“…The perpendicular wavenumber kl predicted by the cold plasma dispersion relation was too small for direct interaction between the LH and the beam ions. Without beam injection, many experiments have noted a direct correspondence between parametric decay of the fundamental LH wave and tail formation [158,166,167,171,172,241,2421. In these experiments, the tail develops at a lower density than that predicted for direct ion heating by LH waves (that obey the cold plasma dispersion relation).…”

Engineering Aspects of Lower Hybrid Microwave Injection into the Alcator C Tokamak

“…Lower hybrid wave heating and current drive experiments are now being carried out at the I MW power level on the Alcator C tokamak to explore these issues. [1][2][3][4][5][6] Besides having successfully demonstrated current drive and electron heating, these experiments have shown the practicality of using large, multirow waveguide arrays to couple large RF power densities (P f/A -9 kW cm-2 ) into the tokamak plasma.…”