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# 27th IAEA Fusion Energy Conference - IAEA CN-258

22-27 October 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
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## Fast wave experiments in LAPD in support of fusion

25 Oct 2018, 14:00
4h 45m
Mahatma Mandir Conference Centre

Poster

### Speaker

Dr Bart Van Compernolle (University of California, Los Angeles)

### Description

Recent work on ICRF physics at the Large Plasma Device (LAPD) at UCLA has focused on deleterious near-field antenna effects, such as RF rectification, sputtering, convective cells and power lost to the plasma edge. Plasma parameters in LAPD are similar to the scrape-off layer of current fusion devices. The machine has a 17 m long, 60 cm diameter magnetized plasma column with typical plasma parameters $n_e \sim 10^{12} – 10^{13}$ cm$^{-3}$ , $T_e \sim 1 – 10$ eV and B$_0 \sim 1000$ G. A new high-power ($\sim$150 kW) RF system and fast wave antenna have been developed for LAPD, enabling the generation of large amplitude fast waves. Evidence of rectified RF sheaths is seen in large increases ($\sim 10 \ T_e$ ) in the plasma potential on field lines connected to the antenna, and in copper deposition on plasma facing components due to sputtering at the antenna. The rectified potential scales linearly with antenna current. The rectified RF sheaths set up convective cells of local **E** x **B** flows, measured indirectly by potential measurements, and measured directly with Mach probes. At high antenna powers substantial modifications of the density profile were observed after the RF antenna is powered up. The density rearrangement is asymmetric with a decrease in plasma density near the top of the antenna and an increase near the bottom. The plasma density profile initially exhibits transient low frequency oscillations ($\sim 10$ kHz) and settles into a quasi-steady state profile for the remainder of the RF pulse. RF antenna current is constant during the pulse. The amplitude of the fast wave fields in the core plasma is modulated at the same low frequency, suggesting fast wave coupling is affected by the density rearrangement. At low antenna powers, the parasitic coupling to slow waves in the low density region in front of the antenna is being studied. Detailed wave field measurements show coupling to both the short wavelength slow wave and the long wavelength fast wave if the density at the antenna is low enough. Coupling to lower hybrid waves was demonstrated for a range of normalized frequencies, from $1 < f / f_{ci} < 30$. Performed at the Basic Plasma Science Facility, supported by the National Science Foundation and the Department of Energy.
Country or International Organization United States of America EX/P6-30

### Primary author

Dr Bart Van Compernolle (University of California, Los Angeles)

### Co-authors

Dr Cornwall Lau (Oak Ridge National Laboratory) Dr Dirk Van Eester (LPP-ERM/KMS) Dr Elijah Martin (Oak Ridge National Laboratory) Dr John Caughman (Oak Ridge National Laboratory) Dr Kristel Crombe (Ghent University, Belgium) Dr Michael Martin (University of California, Los Angeles) Dr Patrick Pribyl (University of California, Los Angeles) Rory Perkins (Princeton Plasma Physics Laboratory) Dr Shreekrishna Tripathi (University of California, Los Angeles) Dr Stephen Vincena (University of California, Los Angeles) Prof. Troy Carter (University of California, Los Angeles) Prof. Walter Gekelman (University of California, Los Angeles)