Alcator C-Mod

Andrea Schmidt

June 16th, 2006

Most of today's tokamaks (toroidal fusion reactors) are inherently pulsed devices. This is because they need a toroidal current to sustain a stable magnetic field configuration. The toroidal current is usually provided by a DC inductor, and thus pulse lengths are limited by the amount of magnetic flux available.

One goal of the fusion program is to move towards more steady-state technologies. Lower hybrid current drive (LHCD) is one such technology that has the potential to provide non-inductive current drive in tokamaks for steady-state operation. In an LHCD system, RF waves of a particular resonant frequency are injected into the tokamak through wave guides with asymmetric spectra. The waves Landau damp on fast electrons, accelerating them to relativistic velocities. The asymmetry in the spectrum causes the waves to damp preferentially in one direction, creating a current.

Lower hybrid current drive has recently been demonstrated on Alcator C-Mod. The LHCD creates a small population of fast elections which compose the toroidal current. My research focuses on characterizing the spatial and velocity distributions of these fast electrons.

There are two instruments on C-Mod which give information about the fast electrons. The primary one is the hard x-ray (hxr) diagnostic, which was designed and built by a previous graduate student, John Liptac, for the purpose of analyzing LH-driven fast electrons. This instrument detects the bremsstrahlung radiation due to the fast electrons and bins counts into time and energy intervals. It has 32 channels which correspond to different line-integrated spatial chords.

The electron cyclotron emission (ECE) diagnostic was designed to determine electron temperature using a measurement of ECE spectra. Relativistic electrons have downshifted ECE frequencies. When the LH-driven fast electrons are present, the ECE diagnostic detects emission at these lower frequencies. This extra portion of the spectra can be used to a limited extent to determine the energy and spatial location of the fast electrons.