Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue Feb 21 17:37:25 1995
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Feb. 21, 1995

Plasma operation on Alcator C-MOD continued last week. Four runs were scheduled
and completed. Principal experiments included a study of H-modes at low-q,
L-mode transport studies (ICRF), H/D isotope exchange experiments, and an
investigation of impurity transport and screening with non-recycling
impurities. Over 110 plasma shots were produced during the four run-days.

Ohmic H-modes were studied at reduced toroidal field (B=3.5T) and plasma
currents up to 850kA (q values down to 2.7). A number of long-duration
ELM-free H-modes were produced, and several shots exhibited multiple L-H and
H-L transitions, providing a good indication of the "hysteresis" in the power
threshold. Overall, threshold values of the P/(nBS) coefficient were in the
range of 0.02 to 0.03, which is typical for C-Mod and better than our previous
ELM-free results. At the lower q-values we obtained ELM-free ohmic H-mode at a
density of > 2e20/m3.

Studies of L-mode transport with ICRF were continued. The outer gap was
reduced to 1cm in an attempt to prevent H-mode transitions; this was partially
successful, although the highest power pulses (P_RF=3MW) did go into H-mode.
Power and density scans at one current (0.85MA) were carried out during this
run.

Thursday was devoted to an isotope exchange experiment (MP#082). The purposes
were to measure the evolution of the isotopic composition in the plasma both
during a discharge and from shot to shot when the fueling is changed from
deuterium to hydrogen and back again; to compare the Mo sputter rate in
hydrogen and deuterium; and to obtain confinement data in hydrogen for the
database. These goals were substantially accomplished. The changeover from D
to H took about 20 shots to go from <5% H/(H+D) to >90% H/(H+D); the reverse
changeover from H to D was approximately symmetric.

The behavior of non-recycling impurities was studied using N2 puffing, both
from a midplane piezo valve and using the NINJA capillary tube system. Short
pulses from the piezo valve confirmed that nitrogen indeed acts as a
non-recycling impurity, as had been observed on ASDEX. Puffing small amounts
of N2 through the capillary system provided data on impurity screening with
respect to impurities arising in the private flux zone, in the SOL at the
inner and outer divertor plates, and at the inner and outer wall in the main
chamber. Larger injected amounts were used to study the effects of nitrogen
radiation on SOL properties and on divertor detachment. One particularly
interesting observation, from the wide angle CCD camera at A-port, was a
plume of visible radiation from the inner wall when we puffed N2 there. The
plume followed field lines around the inner wall in one direction only. This
indicates that there is strong plasma flow towards the divertor dragging the
low charge state impurity ions.

The week of February 20 is a scheduled maintenance week. A clean vent (helium
backfill) was performed on Saturday, Feb. 18. The fast scanning probe was
removed for replacement of the probe head. In addition, new reflectometer
windows were installed at A-port, and a borescope inspection of the inside of
the vessel was performed. The overall condition was found to be good with the
following changes noted. The guard limiters on the antenna showed some erosion
of the boron carbide coating. The Faraday shields had a faint darkening
across the bottom half that appeared to be added material. No damage to the
antennas could be seen. The A-B outside limiter showed no damage except for
some shinny remelt near the center. The inside limiter and divertor tiles
seem to be unchanged with no missing, cracked or otherwise damaged tiles. The
machine was pumped down and left under vacuum over the weekend, then brought
back up to helium on Tuesday for re-insertion of the fast scanning probe, and
cleaning of windows on the interferometer and Thomson ports. The machine is
now back under vacuum.

Dr. Bob Granetz participated in an ITER Expert Group Meeting at the Garching
Joint Work Site, where he presented recent results from studies of disruptions
and halo currents in Alcator C-MOD.