Experimental study of reversed shear Alfvén eigenmodes during the current ramp in the Alcator C-Mod tokamak

Abstract

Experiments conducted in the Alcator C-Mod tokamak at MIT have explored the physics of reversed shear Alfvén eigenmodes (RSAEs) during the current ramp. The frequency evolution of the RSAEs throughout the current ramp provides a constraint on the evolution of qmin, a result which is important in transport modeling and for comparison with other diagnostics which directly measure the magnetic field line structure. Additionally, a scaling of the RSAE minimum frequency with the sound speed is used to derive a measure of the adiabatic index, a measure of the plasma compressibility. This scaling bounds the adiabatic index at 1.40 ± 0.15 used in MHD models and supports the kinetic calculation of separate electron and ion compressibilities with an ion adiabatic index close to 7 4.

Energetic particle physics in fusion research in preparation for burning plasma experiments

“…The adiabatic index or specific heat ratio, , plays an important role in the description of AEs. The suitable value of γ is discussed in detail in the analysis of the minimum frequency during the RSAEfrequency sweeping in NSTX and C-Mod [280,481]. The best γ values fitted to experimental data were inferred to be 1.40 ± 0.15 for RSAEs in ICRF heated low beta plasmas on C-Mod, and 1.34-1.38 in high beta beam heated plasmas of NSTX.…”

The effect of toroidal plasma rotation on low-frequency reversed shear Alfvén eigenmodes in tokamaks

“…For the description of RSAEs in a low-β tokamak plasma this is likely to be an overestimate and a value of γ ≈ 1.4 seems more appropriate [33]. Since √ γ T is the sound speed c s in an ideal gas, we may write ω 0 = c s /qR 0 and replace c s with a more general expression:…”

A combined Phase Contrast Imaging and heterodyne interferometer for multiscale fluctuation measurements in tokamak plasmas

“…PCI has been used in the study of electrostatic turbulence and transport, with studies of including L-H transitions [5], ELMs [6], QH-mode [7], ITG/TEM instabilities and comparisons with modeling [8][9][10][11][12], I-mode [13], and the effects of ECH heating [14]. PCI has also been used in the study of MHD instabilities [15][16][17]. The absolutely calibrated PCI…”