Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue Feb 14 08:59:06 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Feb. 13, 1995

Alcator C-MOD is continuing regular operation. Four run days were scheduled
and completed this week. Principal experiments included investigation of
H-mode thresholds, in support of ITER Urgent Research task 3.1; PEP mode
studies; increased plasma current; and localized gas puff experiments.

H-mode threshold studies were carried out in a standard lower SN discharge
with Ip=850kA, Bt=5.3T, kappa~1.65. For different densities, the RF power was
stepped in small increments during the pulse to determine the power threshold
for the L-H transition; RF powers up to 2.4 MW were used. Particular attention
was paid to the low density regime, which is of greatest interest to ITER. We
observe a hard lower density limit in the range of 0.8to 1.0e20/m3, with no
L-H transitions observed even for normalized powers P/(nBS) up to 4 times the
threshold observed for moderate densities. These discharges were sawtoothing
and showed no evidence of locked modes. Threshold powers were determined with
approximately 100kW resolution for densities up to about twice the low-density
limit. These data are being analyzed.

PEP mode studies were carried out using the Li pellet injector. A target
plasma density scan was conducted with 2MW of ICRF power at 850kA. Density was
scanned from 1.0 to 2.4e20 m-3. The maximum neutron rate during the PEP phase
was in the range 4e13 to 5e13 per second, and was nearly independent of
density. On the other hand, the stored energy increase was largest at
intermediate density, nebar=1.7e20 to 2.0e20 m-3, and deteriorated at higher
and lower densities. Towards the end of the run the current was raised to
1.1MA at a density of nebar=1.9e20 m-3. The stored energy was higher, but the
neutron rate was lower compared to 850kA shots at the same density.Different
pellet sizes and timing were tried. Further optimization to enhance the PEP
performance at high currents is required.

The standard operational range of the C-MOD tokamak was extended to plasma
currents up to 1.2 MA in a dedicated discharge development run on Wednesday.
Disruption and halo current behavior were carefully monitored during this
process. Pre-established limits on halo current magnitudes were not exceeded.
In the course of the PEP mode experiments on Friday, the flattop time was
extended to 0.8 sec, an increase of 0.25 sec over previous practice; total
pulse lengths of up to 1.7 sec were produced.

An experiment to investigate the effects of fueling from different poloidal
and toroidal locations, using the capillary tube system (NINJA), obtained less
useful data than anticipated, due to technical difficulties. This experiment
will be re-scheduled.

Dr. C.H. Ma of ORNL visited MIT to work on setting up his Faraday rotation
diagnostic in conjunction with our existing CO2 interferometer. The dedicated
run to test this new diagnostic system has been scheduled for early March.

Dr. Earl Marmar attended a meeting of the ITER Diagnostics Expert Group, on
the topic of Divertor Diagnostics, at the Joint Work Site in Naka, Japan. Dr.
Bruce Lipschultz participated in a meeting of the ITER Divertor Expert Groups
at the Garching, Germany, Joint Work Site.

Dr. Martha Krebs, Director of the D.o.E. Office of Energy Research, visited
MIT this week, and took a tour of the Alcator C-MOD facility conducted by
Project Head Prof. Ian Hutchinson and Prof. Miklos Porkolab, Director of the
M.I.T. Plasma Fusion Center.