Millimeter-wave techniques for measurement of radio frequency plasma waves for advancing understanding of reactor-relevant helicon current drive and fast-ion driven instabilities

The goal of the project is to advance the scientific basis for helicon current drive as a reactor-relevant current drive capability using a new millimeter-wave Doppler Backscattering (DBS) diagnostic [S. Chowdhury, Rev. Sci. Ins. submitted March 2023] for localized internal measurements of high-power (~ 1 MW) helicon waves (476 MHz) injected into DIII-D by an external antenna [B.V. Compernolle, Nucl. Fusion 2021]. The new DBS system, which has the demonstrated capability to measure radio frequency density fluctuations in DIII-D with frequencies f < ~500 MHz, including helicon waves, will be used to measure the helicon wave field amplitude and spatial structure. This will contribute in several ways to the DIII-D effort for development of helicon current drive, in the process demonstrating DBS capabilities for measurement of radio frequency plasma waves that could be readily extended to a burning plasma environment. One of the goals for helicon current drive is to achieve spatially localized modification of the plasma current profile, a capability that has been predicted in simulations using codes for helicon wave propagation and absorption such as AORSA [C Lau, Nucl. Fusion 2018] and GENRAY [R Prater, Nucl. Fusion 2014]. Measurements of the helicon wave field amplitude and spatial structure with the DBS system will be used to validate these codes. The DBS system has several features that will be exploited for this task. First, the millimeter-wave operating frequency is set by a low-noise, remotely controllable, programmable synthesizer. The measurement location, which depends on the operating frequency and plasma density and magnetic field profiles, can be varied over a broad range during a single discharge by programming the synthesizer to step through frequencies, allowing for efficient measurements of spatial structure. Second, the DBS millimeter wave beam couples quasi-optically into a system for launching ECH power into DIII-D [M. Cengher, IEEE Trans. Plasma Sci. 2016] via a waveguide switch that allows it to exploit the ECH launcher steering mirror. The steering mirror tilt can be adjusted to change the location and wavenumber of the density fluctuations probed by the DBS system. The tilt is remotely controllable by a motorized system that can be programmed to change the tilt over a broad range during a signal plasma discharge, further contributing to the DBS measurement capability efficiency. The location of the ECH launcher is also a key enabling feature of the DBS system for this task. Full wave modeling predicts a complex 3D, localized spatial structure for the helicon wave field [C Lau, Nucl. Fusion 2018]. The use of the ECH launcher places the DBS system advantageously relative to the helicon antenna for investigation of this complex spatial structure during helicon current drive experiments. Other goals for helicon current drive are efficient coupling of power from the helicon antenna to the core plasma and localization of the current drive. The DBS is potentially sensitive to slow waves launched from the antenna and will be used in experiments to assess unwanted coupling of antenna power to slow waves [R Prater, Nucl. Fusion 2014], which potentially reduces current drive efficiency. The DBS system is capable of simultaneously measuring both turbulence and helicon wave density fluctuations and will be used in experiments to investigate the interaction of the helicon waves with turbulence, which potentially affects both efficiency and current drive localization [C. Lau, Nucl. Fusion 2021]. Finally, methods will be explored for incorporating DBS measurements of helicon wave spatial structure and amplitude obtained in experimental discharges into analysis to determine the localization of helicon current drive in those discharges. In the course of advancing the development of helicon current drive, the funded research will demonstrate DBS capabilities for measurement of radio frequency plasma waves that could be readily extended to a burning plasma environment and to other kinds of radio frequency plasma waves, including, for instance, fast-ion driven instabilities [S. Chowdhury, Rev. Sci. Ins. submitted March 2023]. Millimeter wave diagnostics, including DBS, are considered well-suited for a reactor environment [N. C. Luhmann, Jr., Fusion Sci. and Tech. 2008] and are already under development for other types of measurements in ITER (e.g. [G. Wang, Rev. Sci. Ins. 2017]). The proposed research will be performed by Drs. Satyajit Chowdhury and Neal Crocker and Prof. Troy Carter of UCLA.