Date: Tue, 13 Jan 1998 9:15:56 -0500
From: WOLFE@CMOD.PFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Message-ID: <980113091556.3fc002e7@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT Plasma Fusion Center


Alcator C-Mod Weekly Highlights
Jan 12, 1998

Physics operation continued on Alcator C-Mod last week. Four run days were
scheduled and completed. Over 100 discharges were produced in support of five
miniproposals.

Tuesday's run was dedicated to qualification and calibration of two diagnostic
systems, the helium gas probe and the reflectometer. The helium gas probe
consists of a helium puff from a capilary tube located in one of the outboard
limiter modules, which is viewed spectroscopically. This diagnostic permits
measurement of density and electron temperature profiles in the SOL and
separatrix regions by maeans of He line ratios. Tuesday's experiment (MP197)
was to compare results from the He measurement with measurements using the
fast scanning Langmuir probes, in ohmic and RF-heated (H-mode) plasmas. Good
data was obtained, and results are being analyzed. The last part of the run
was devoted to spatial calibration of the reflectometer system for density
profile measurements (MP196A). The point of this MP was to drive the density
up at the edge so high we could feel confident that all the reflectometer
channels were reflecting from nearly the same location -- the limiter. The
experiment included scans of the outer gap, including a brief period with the
gap at zero, i.e. outboard limited. The run sequence plan was completed
successfully.

Wednesday's run was in support of MP#163, "High q_parallel dissipative
divertor". This proposal is an attempt to achieve simultaneously a good
confinement H-mode with low Zeff and a detached divertor. The goal is to
determine the threshold amount of impurity gas necessary to detach the
divertor. The run was successful in producing enhanced Dalpha (EDA) H-modes.
These H-modes were 'steady-state' with H-factors reaching 1.6 with up to 3MW
of RF power. With the radiation levels being low, the target plasmas were
indeed high q-parallel (greater than 400 MW per square meter). However, when
nitrogen was puffed at these plasmas, the H-modes became ELMfree and impurity
accumulation led to the demise of the H-mode. Nitrogen was puffed through a
piezo valve located in the divertor and through capillaries in the private
flux region. Puffing through a capillary was more benign but still resulted
in loss of H-mode. In comparing this run with that of 960227 (the first time
detached H-mode was achieved) it was noted that the radiation in the divertor
and core plasmas is similar although the H-factor is not as high (1.6
vs. 2.0). The increase in divertor radiation was only about 150kW compared to
500kW. It was also noted that the edge temperature pedestals were not very
high for a 'good' EDA H-mode. It is not clear as to why these H-mode plasmas
would not tolerate any impurity puffing, compared to the results from 960227.
This awaits further analysis.

Thursday's run was devoted to MP#186, "Cold pulse experiments", proposed by
Ken Gentle of U. Texas, who served as session leader for this run. The
objective was to produce temperature perturbations by the laser ablation
injection of impurities suitable for analysis of thermal transport. The
specific goal was to find discharges similar to those in TEXT and TFTR in
which edge cooling induced core temperature rises. These were circular
limited ohmic discharges, generally at low density. These core temperature
rises are particularly difficult to explain and offer a unique set of
conditions for testing transport theories. Target plasmas were inner-wall
limited circular equilibria at low density. Plasma current was varied between
0.3MA and 0.5MA, at a fixed toroidal field of 5.4T. Various impurities --
carbon, scandium, niobium, etc. -- were tested for their ability to induce
suitable increases in radiation and reduction in edge temperature. To search
for the effect under conditions as close to those in which it had been found
in TEXT, the current was lowered to 0.3 MA. Neither carbon nor scandium
produced significant perturbations, but both iron and niobium did. Core
temperature increases were not found under any conditions. Comparison of the
injections on these circular, limited discharges with standard 1 MA diverted
reference shots on this run and niobium injection from run 971210 suggests
that injection efficiency is equivalent for the two configurations. The
niobium and iron injections are suitable for electron thermal transport
analysis, but the analysis will be complicated by the need to include impurity
transport for the space-time profiles of radiation losses.

On Friday, the run was dedicated to MP#168, "Exploration of the Effect on Core
Confinement of Different Impurities". Neon and krypton were puffed into
steady-state EDA H-modes, increasing the core radiation to between 50% and 80%
of input power, and confinement degradation was observed. Degradation in the
confinement was correlated with a decrease in the pedestal
temperature. Profiles of radiation emissivity were obtained and are being
analyzed. Initial analysis of the edge profiles indicates that the decrease in
pedestal Te was indicative of the pedestal width narrowing as opposed to the
overall profile dropping everywhere. It appeared that the Te slope through
the pedestal and in the core stayed fairly constant.

The ICRF systems performed well last week. FMIT#1 and #2 were utilized in
support of 3 divertor MP's (MP#197, MP#163, and MP#168) and piggybacked into a
fourth MP (MP#186). In general the RF system performed well coupling 2.5 MW
reliably (without faults) and <3.0 MW with some faults during RF injection.
All experiments employed H minority in deuterium plasmas. The H/[H+D] was
measured to be about 1-2% spectroscopically (J. Weaver U. Maryland) and 2-4%
according to PCX (R. Boivin) for a typical discharge. Each of the divertor
experiments posed challenges for the RF system. In MP#197, the outer gap was
slowly scanned from 15 to 5 mm. For routine plasma operation, the outer gap
is typically 10 mm, but the larger and variable gap was tolerated without
excessive coupling problems for these discharges. During impurity gas puffing
of the other two MP's, heavy gas puffing appeared to decrease the voltage
standoff of the antenna. Some antenna arcs occurred, but the operations were
not compromised due to the system's arc protection. For MP#168, the field was
raised to 5.7 T to ensure the GPC was not cutoff at high density. This moved
the H resonance to the low field side, about 10 cm. The absorption was not
significantly different compared to 5.4 T H-modes from this campaign.

Work was also done on a new screen overcurrent pullback circuit. A prototype
is being tested off line and will require more testing before it becomes
operational. A combiner was also built and tested for the ECDC source. This
should allow higher powers for ECDC if desired.

Progress continued on development of the DNB for C-Mod. Water plumbing for
both the modulator/regulator and for the cryo compressors was completed and
tested. The low capacitance transmission line duct sections were assembled
to the point at which the duct enters the cell. Installation of safety
interlocks for the DNB test lab was begun with installation of door
switches and some grounding sticks.

This week will also be a physics run week. Four runs are scheduled.