Plasma and Fusion Research

Volume 7, 2402031 (2012)

Regular Articles

Observation of Spectral Broadening of Lower Hybrid Waves in Alcator C-Mod
Seung-Gyou BAEK, Ronald R. PARKER, Cornwall LAU, Syun'ichi SHIRAIWA and Gregory M. WALLACE
Plasma Science and Fusion Center, MIT, Cambridge, MA, 02139, USA
(Received 28 November 2011 / Accepted 3 March 2012 / Published 10 May 2012)

Abstract

Observations of the spectral broadening of lower hybrid (LH) waves in Alcator C-Mod are presented. We report the dependence of the pump broadening on two plasma parameters: plasma density and magnetic topology. As the plasma density was raised, we observed a significant increase of the pump broadening at 10 dB below the maximum peak. However, at the constant plasma density, when the magnetic topology was changed from the diverted plasma to the inner wall limited plasma, the spectral broadening was reduced. In an attempt to understand the causes of this variation, we report the change of density profiles in front of the LH launcher in two different magnetic topologies. The model analyses indicate that both the scattering process and parametric decay instability can contribute to the spectral broadening. Further experimental investigations are necessary to identify the causality between spectral broadening and the degradation of lower hybrid current drive efficiency at high density plasmas.

Keywords

spectral broadening, lower hybrid wave, scattering, parametric decay instability

DOI: 10.1585/pfr.7.2402031

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This paper may be cited as follows:

Seung-Gyou BAEK, Ronald R. PARKER, Cornwall LAU, Syun'ichi SHIRAIWA and Gregory M. WALLACE, Plasma Fusion Res. 7, 2402031 (2012).