From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma
Organization: MIT


Alcator C-Mod Weekly Highlights
Nov 1, 1999

Plasma operations continued on Alcator C-Mod last week. Three run days were
scheduled and completed. A total of 42 plasma shots were produced, with a
reliability of greater than 85%.

Following the week's operation, the Alcator C-Mod bus was reconfigured over
the weekend in preparation for a week of reversed field operation in which
SOL, x-point, and pedestal measurements will be made during both normal and
upper x-point operation.

Wednesday's run was in support of MP#154, and involved a study of H-mode
thresholds in helium discharges, with a goal of elucidating the role of
neutrals in the L->H transition and pedestal formation. Although the RF
was heating the core, it was very difficult to achieve H-mode. As expected we
had a fair amount of radiation, but the edge remained pretty hot
nonetheless. When the plasma did go into H-mode, the edge temperature
threshold was found to be higher than in deuterium, with the exact ratio to
be verified. We raised the current in an attempt to get more power with
no obvious effects. H-modes were again very short. Helium rotation data were
obtained, but further analysis is required.

H-mode studies continued on Thursday with an investigation of low target
density thresholds. No pre-operation discharge cleaning was done for this run,
which followed the helium plasma experiments of Wednesday. As a result, the
walls were unloaded, and there was a tendency for runaways to be produced
early in the discharge. As previously observed, the power and temperature
thresholds went up considerably at lower density. Nevertheless, we succeeded
in obtaining H-modes for target densities down to ~5e19/m3, albeit with
threshold values of P/nBS up to a factor of four higher than obtained at
higher density. We observed small ELMs at very low density, seemingly type
III. These may correspond to the low-density, high-temperature ELMS observed
on DIII-D. Edge temperatures, and consequent pedestals were very
hot. However, the density rise was slow and in some cases difficult to see.

The second half of Thursday's run was devoted to studies using the divertor
bypass (flappers). Localization effects were studied by opening and closing
one bypass sector at a time. We succeeded in observing a diffuse glow on the
TV camera view over the H-J flap when it alone opened. There was some evidence
that the B-bottom pressure can be reduced using only the flappers on either
side of B, but this was not completely clear.

The purpose of Friday's run was to determine the profiles of the impurity
diffusion coefficient and convection velocity during EDA H-mode. Complete
radial brightness profiles for He-like, Li-like and Be-like calcium lines were
obtained, in addition to a few time histories for Na-like calcium and Li-like
fluorine, following a sequence of CaF2 injections. Bolometer and edge x-ray
profiles were also collected. These data should allow for a complete
characterization of the impurity transport coefficient
profiles. Unfortunately, these were not very good EDA H-modes, with H factors
only slightly above 1, since the walls were not boronized and the molybdenum
and total radiated power levels were very high.

Physics and Analysis
--------------------

Investigations of ICRF minority heating with scans of the minority hydrogen
fraction were carried out on Friday, October 22. In all cases, the hydrogen
fraction was monitored with a high resolution spectrometer measuring the Balmer
alpha multiplets from hydrogen and deuterium, viewing emission from recycling
neutrals in the main chamber scrape-off plasma. The D and E port antennas were
used together for this run, with net forward power in the range from 2 to 2.5
MW for most shots. The best heating and confinement, and lowest radiated
powers, were obtained with no extra hydrogen puffing. The plasmas clearly
degraded as the hydrogen fraction was brought above 0.05. Based on H-factor
confinement enhancement, the optimum hydrogen level is at or below 0.025, the
lowest we have seen during this run campaign. The sawtooth reheat rate (not
adjusted for density differences) was also maximum for hydrogen fraction
between 0.025 and 0.06. The shots with 0.04 < n_H/n_D < .06 were ok during the
first part of the heating pulse, but eventually degraded from EDA toward type
III Elmy or Elmfree, with increased radiation and particle confinement, and
decreased energy confinement. Once the H fraction reached 0.08, even
quasi-steady H-modes were impossible; the radiated power increased
continuously, and the plasmas returned to L-mode. For fractions greater than
0.08, only the very briefest of H-Modes were obtained, with basically
steady-state L-mode the result. At the highest H level reached (n_H/n_D ~ 0.25)
there was little evidence of RF power absorption in the core plasma.

Recently the views of two CCD cameras have been changed to record high
resolution images of the J-port antenna. These views have been recorded since
October 18th, 1999. The cameras primarily viewed unfiltered visible light,
although some images were recorded with D_alpha or D_gamma bandpass filters.
During operation, injections were observed from the antenna, with the majority
of them originating from the top protection tiles. The injections were
correlated with increases in radiated power, as measured by the bolometers,
and with titanium influx, as measured by spectroscopy. Injections were also
observed when the J-port antenna was not being used, but were notably worse
when it was energized. In addition to the injections observed, the edges of
the Faraday-screen box are seen to glow when the antenna is energized. It is
still not clear what process is responsible for this glow.


ICRF System
-----------

On the last plasma shot of the day on 10/22/1999 we obeserved a fault and
crowbar of ICRF transmitter #1. Pulsing into a dummy load revealed anode high
voltage faulting of the output tube at decreasing levels of output rf power.
A tube hi-pot indicated excessive leakage current at 35 kV (60 kV holdoff
required), with no indication of poor tube vacuum, suggesting a damaged screen
grid structure. This 2274 tube was placed in service early in September, and
had been previously repaired and tested by the manufacturer. As a consequence
of the extensive transmitter repairs and upgrades of this past half year, all
of the tube protection circuits had been thoroughly checked out, and digitized
waveforms showed proper circuit behavior.

The tube is being returned to the manufacturer, since it is under warranty. It
has been replaced in transmitter #1 by a lower power (EIMAC #8973) tube. With
the improvements made over the summer we can still obtain relatively large
power (~1.5MW), albeit for shorter duration. The limitation is presently plate
disspation in the FPA tube. This configuration provided adequate power for
the physics experiments run last week. The two ICRF antennas at D- and
E-ports were used, coupled to transmitters #1 and #2, avoiding potential
complexities from J-port antenna impurity generation and conditioning for the
moment.

Diagnostic Neutral Beam
-------------------------
Progress on the DNB is consistent with the schedule presented at the September
review. The high voltage modulator/regulator was installed this week and is
being prepared for final testing prior to operation of the beam line in the
test stand. The preparations for final installation in the C-MOD cell are
underway during maintenance days when cell access is possible, and these
activities are ahead of schedule by a few weeks.


Travel and Visistors
--------------------

Last week Paul Bonoli and Ron Parker attended an IAEA Technical Committee
Meeting on Steady State Operation of Fusion Devices - Plasma Control and
Facing Components. Bonoli presented an invited paper entitled "Modelling of
Advanced Tokamak Scenarios with LHCD in Alcator C-Mod" (to be
published in a special issue of the journal Nuclear
Fusion).