Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Wed Feb 14 12:00:53 1996
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Feb. 12, 1996

Plasma operations continued last week on Alcator C-MOD. Four runs were
scheduled, and despite some technical difficulties which delayed the start
times on three days, all the scheduled experiments got good data.

Tuesday's run was a continuation of our studies of h-mode characteristics with
boronized walls. A fresh boron layer was put down overnight on Monday. A
vacuum incident Tuesday morning necessitated additional D2 ECDC and delayed
the start of this run. Since wall conditions clearly affect the accessibility
to H-mode and the resulting quality, there was concern that this run would not
be successful. However, after a few conditioning shots high quality H-modes
were obtained, and all the run objectives were addressed. Small sweeps of the
TF field were performed during a series of shots to provide a fine-scale
spatial scan for the ECE Grating Polychromator, to obtain better resolution on
the edge Te profile. The flattop toroidal field was then varied over several
shots from the nominal 5.3T up to 6.0T, placing the resonance up to 9cm
off-axis. Good heating was observed at all these fields, in contrast to
earlier results in which substantial degradation in heating was observed for
off-axis heating. Li pellets were injected on two shots, but good PEP modes
were not obtained. Extra D2 puffing in the divertor was tried as a means of
increasing the divertor dissipation during the H-mode. Operating at 5.8T we
raised the current to 1.2MA (q95~3.4) and again observed improved
confinement, with stored energies over 200kJ.

On Wednesday we continued our 7.9T H-mode studies, using D(He3) minority
heating. The start of the run was again delayed, this time due to a fault in
control circuitry at the alternator. Once the run started, good H-modes were
obtained, including some at a target density around 1.4e20, which is the
lowest value yet obtained at this field, though still higher than the
low-density limit at 5.3T. H-factors up to 1.5 (assuming 100% power
absorption) were obtained; attempts to determine the actual absorbed power
fraction by notching off the RF and observing the change in slope of the
diamagnetic and MHD equilibrium stored energies indicated the actual absorbed
power was somewhat less.

Thursday's run involved a study of plumes from trace gas injection using the
capillaries in the divertor and inner wall. This run was in support of PhD
thesis research. Attempts to observe plumes in NI using a newly-installed
filter were unsuccessful, although the line was observed on the Chromex
spectrometer. HeII and NII plumes were observed, and moved toward the x-point
as expected from previous experiments. However, plumes on upper x-point shots
appeared more symmetric, with a possibility of flow-reversal, in contrast to
carbon (methane) plumes observed in upper x-point discharges last year. Plumes
from the divertor capillaries were observed with the B-top camera. In one
shot, nitrogen was puffed on the top of the inner and outer divertor noses
simultaneously, and the resulting plumes were observed to go in opposite
directions (CW and CCW, as appropriate), towards the corresponding strike point.

Friday's run was the first of a series of runs aimed at optimizing dissipative
divertor operation in h-mode operation with boronized walls. The idea is to
introduce radiating impurities into the divertor to increase divertor
radiation, in some cases induce detachment, thus reducing the power flow to
the target plate. In this run, Neon was used as the radiating impurity. Future
runs will employ Ar and N2. This run was also delayed, because of a problem
with the Hybrid plasma control computer, eventually traced to a faulty ribbon
cable. Once the run started a total of 21 successful shots were obtained.
Detached divertor operation was obtained with up to 2MW entering the SOL.

The Time-of-flight neutral particle analyzer is now under vacuum. The
instrument is a collaboration between LANL and MIT and is expected to be
installed during the upcoming shutdown. Systems are being checked and brought
online.

Bruce Lipschultz is at GA this week, serving on the DIII-D Program Advisory
Committee.

Plasma operations are scheduled to continue next week, with continuations of
h-mode and dissipative divertor studies at 5.3 and 7.9T.