Measurements and modeling of Lower Hybrid Driven fast electrons on Alcator C-Mod

Abstract

A Lower Hybrid Current Drive (LHCD) system has been implemented on Alcator C-Mod with successful coupling to the plasma of up to 1 MW of power. Nearly fully non-inductive current drive has been achieved for several current relaxation times at low but ITER-relevant densities. One major advantageous feature of the C-Mod LH system is its phasing flexibility, allowing it to produce spectra with a wide range of peak parallel refractive index (nI). Theory predicts that LH power deposition location is strongly dependent on ng, as well as on other parameters such as electron temperature, electron density, and plasma current. Several diagnostics exist on Alcator C-Mod which can measure the effects of LHCD power on the plasma. The primary diagnostic for measuring LH-driven fast electrons is a hard x-ray (HXR) camera that measures fast electron Bremsstrahlung emission. This is a horizontallyviewing diagnostic with 32 spatial chords that span the plasma cross-section. Each chord ends at a cadmium zinc telluride (CZT) detector that detects individual x-ray photons as current pulses. The output from these detectors is shaped and digitized. Post-processing of the raw pulse train allows for flexible time and energy binning. Another diagnostic for detecting fast electrons is the Electron Cyclotron Emission (ECE) diagnostic. This diagnostic is designed to measure electron temperature in a Maxwellian plasma that is optically thick in the second harmonic. However, the LHdriven fast electrons have relativistically downshifted electron cyclotron frequencies and contribute to additional emission at frequencies just below the second harmonic, where the plasma is optically thin. A third useful diagnostic for LHCD operation is the Motional Stark Effect (MSE) diagnostic, which measures the magnetic field pitch angle profile and therefore can be used to infer the current profile inside the plasma. This current profile is the sum of ohmic current, LH current, and bootstrap current and has been observed to change when LH power is applied to an ohmic plasma. GENRAY/CQL3D is a ray-tracing/Fokker-Plank code package that solves iteratively for a self-consistent electron distribution function in the presence of LH waves, given a plasma scenario and LH wave spectrum. This code includes synthetic diagnostics that can be compared to experimental HXR and ECE measurements. Although an MSE synthetic diagnostic does not currently exist in CQL3D, the inferred current profile from MSE can be compared with the current profile output by CQL3D. Modeling has been carried out for multiple plasma scenarios to benchmark the code package and to further our understanding of how to interpret the experimental results. An experiment in which LH power is square-wave modulated on a time scale much faster than the current relaxation time does not significantly alter the poloidal magnetic field inside the plasma and thus allows for realistic modeling and consistent plasma conditions for different nII spectra. Inverted hard x-ray profiles show clear changes in LH-driven fast electron location with differing nil. Boxcar binning of hard x-rays during LH power modulation allows for 1 ms time resolution, which is sufficient to resolve the build-up, steady-state, and slowing-down of fast electrons. Ray-tracing/Fokker-Planck modeling in combination with a synthetic hard x-ray diagnostic show quantitative agreement with the x-ray data for high ni cases. The time histories of hollow x-ray profiles have been used to measure off-axis fast electron transport in the outer half of the plasma, which is found to be small on a slowing down time scale. This work is supported by the US DOE awards DE-FC02-99ER54512 and DE-AC02-76CH03073.