Alcator C-Mod

Alexandre Parisot

Thesis title: Mode Conversion Current Drive experiments on Alcator C-Mod

Updated: 07/14/06

Advisor: Prof. R. R. Parker
Supervisor: Dr. S. J. Wukitch

Plasma waves are an attractive and flexible tool to achieve heating and current profile control in tokamaks. In Alcator C-Mod, up to 6 MW has been coupled to the plasma from three antennas, with frequencies close to the cyclotron frequency of the various ion species (Ion Cyclotron Range of Frequencies or ICRF), namely 40-80 MHz in C-Mod. The geometry of the radiating straps is such that most of the power is coupled to fast magnetosonic waves, which then propagate towards the plasma core. While the fast waves can interact directly with ions and electrons, they can also undergo a process called mode conversion, in which the incoming wave excites other plasma waves with different character in a narrow region in the plasma core. The excited waves, labeled mode converted waves, propagate away from the mode conversion region and can transfer their power and momentum to ions and electrons, usually through stronger absorption processes than for the fast wave. As a result, strong plasma heating, and possibly current drive, can be obtained in the immediate vicinity of the mode conversion region. Mode conversion electron heating (MCEH) has been observed in almost all large and medium size tokamaks, and could be considered as an efficient and flexible tool for localized electron heating in a reactor. Mode conversion current drive (MCCD), on the other hand, has proved more complicated to obtain. Indications of current drive have been obtained only on the TFTR tokamak. The results have not been satisfactorily explained using simple MCEH models.

Recent experiments on Alcator C-Mod, as well as advances in numerical modeling, allowed to develop a more detailed understanding of ICRF mode conversion. Density fluctuations associated with the mode converted waves were observed using a Phase Contrast Imaging (PCI) diagnostics, revealing propagation characteristics of the mode converted waves. MCEH deposition profiles and PCI measurements were compared with numerical simulations using the wave propagation code TORIC, and very good agreement was obtained. The simulations predict mode conversion from fast wave to Ion Bernstein Waves (IBW) and Ion Cyclotron Waves (ICW). While IBW are predicted in slab models, mode conversion to ICW is tied to more complicated effects involving the magnetic topology and the evolution of wavenumber parallel to the magnetic field. Since the parallel wavenumber is a key parameter for current drive, these effects are crucial in MCCD physics, and the agreement between PCI measurements and numerical results suggests that they are now correctly captured in simulations.

These results motivated further studies of MCCD on Alcator C-Mod, which is the object of this PhD thesis work. Experimental scenarios were determined from TORIC simulations and are being investigated based on loop voltage differences, changes in the period of sawtooth oscillations, and Alfven cascade activity in the early part of the discharge. The loop voltage is a measure of how much current is supplied inductively to the tokamak through the central solenoid ; therefore, when holding the total plasma current constant, changes in loop voltage can reveal current drive from non-inductive techniques, e.g. here MCCD. Sawtooth oscillations are periodic MHD events in the plasma core, and their period is affected among other things by changes in the current profile in a narrow resonant surface. By moving the mode conversion region through this resonant surface, MCCD can be studied by monitoring changes in the sawtooth period. Reverse shear Afven eigenmodes (RSAE) or Afven cascades can be also used to study changes in the current profiles induced by MCCD.

As of spring 2006, two series of experiments have been conducted along these lines. Significant differences in the sawtooth period were obtained experimentally and are indicative of localized current drive in the vicinity of the mode conversion layer. The results are in qualitative agreement with TORIC modeling. Data has been collected on loop voltage differences and is being analyzed. Initial experiments on RSAE were not successful, as Alfven cascade activity could not be obtained in the MCCD target plasmas.