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
Sep 27, 1999

Plasma operations continued at Alcator C-Mod last week. A total of 68 plasma
shots were obtained with a start-up reliability of 92% over 3 1/2 days. Two
mini-proposals were completed, and progress made on a third.

Monday's run was dedicated to completion of a study of the scaling of the rotation
velocity with plasma current, for otherwise constant H-mode target plasmas.
The range of currents achieved was from 1.36 down to
.42 MA, at nearly constant target electron density, and at 2 MW ICRF power.
There is a very clear trend of rotation velocity normalized to the
plasma stored energy decreasing with increasing plasma current. There
were some CaF2 injections as well, with some shell formation seen on the
top x-ray array, with very good time resolution. Some of the discharges
had all four ICRF transmitters operational, with a total of 4.3 MW achieved
on one shot.

Tuesday's run was in support of MP#230, "Effect of Divertor Baffling on H-mode
Power Threshold". Standard 0.8 MA, 5.4 T discharges with a variety of target
plasma densities (nl04 = 0.7 to 1.4) were used to investigate the H-mode power
threshold. RF heating was programmed to give slow linear ramps. Identical
discharges were run, first with flaps closed for the entire discharge and then
a comparison discharge with flaps open. Trace amounts of argon and helium were
used to investigate divertor screening. Results indicate that the flappers
appear to have no measurable effect on the H-mode threshold (within an error
of around 10%). As usual, with flapper open at moderate to high density, the
divertor neutral pressure is reduced by a factor of ~ 2, with no measurable
effect on the midplane pressure.

Wednesday's run was dedicated to MP#232, "Investigation of the EDA/ELM-free
operational boundary". The main focus of this run was the effect of
shaping. The average triangularity was scanned from 0.6 to 0.28 at several
values of q95 (Ip). Good pedestal data was taken with the new diagnostics and
fluctuation measurements were obtained during the transition time. The
pedestal widths from several diagnostics (edge soft x-rays, edge Thomson, VB)
varied directly with triangularity as did Halpha. The run was hampered by lack
of steady ICRF power at the desired 2-2.5MW level. It was possible to confirm
that low triangularity leads to ELMfree plasmas even at q95 high enough to be
EDA with our normal shape.

Thursday's run was cancelled in order to allow for repair of the arc in the
D-port transmission line that was responsible for some of the RF problems
encountered on Wednesday. This was accomplished.

The purpose of Friday's run was to determine the profiles of the impurity
diffusion coefficient and convection velocity during EDA H-mode (MP#218). A
satisfactory discharge was established, but the run was stopped early due to
an indication, based on resistance measurements, that one arm of the TF was
not cooling properly. This effect is being investigated.

This week is a maintenance week; no plasma runs are scheduled.

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

A record amount of ICRF power,4.3 MW total, was coupled into C-Mod. This was
distributed among the three antennas roughly as D-Port = 1.3 MW, E-Port = 1.4
MW, and the new PPPL antenna J-Port = 1.6 MW. In general, J-port antenna
conditioning is continuing while physics experiments emphasizing moderate
power from D- and E-Ports are conducted.

For a density scan in Ip=800 kA plasmas, J-port was run in piggy back and had
greater success in coupling to the lower density target plasmas than target
densities greater than nl~1.1e20 m^-2. Piggy back opportunities for J-port on
other experimental days were limited.

D and E-port antennas were run into plasmas with various plasma shapes. For
high triangularities, the match between the plasma and antenna shape was poor,
with 2 cm gaps at the antenna extremeties and 0.2-0.8 cm gaps on the midplane.
The E-port antenna was limited to <800 kW because of faults. At powers >800
kW, the E-port antenna faulted often. The D-port antenna ran at roughly 1 MW
before faulting on HV limits. After repairing an arc in the transmission line
from FMIT#1, the ICRF was operated into Ip~800 kA, nl~1e20 m^-2 plasmas.
Maximum injected power was 2.8 MW. D-port diagnostics indicated that for this
plasma the 1.4 MW injected was about maximum because of the antenna voltage
limit. E-port however was capable of higher power because the system was only
~75% of maximum.


Travel and Visitors
-------------------

Yijun Lin visited PPPL last week Monday and Tuesday. The main purpose is to
evaluate two channels (128-136 GHz and 136-144 GHz) of the frequency-swept
TFTR reflectometer. These two channels are proposed to be installed in C-Mod
in near future. The second purpose was to discuss with Dr. Raffi Nazikian on
the development of the code simulating reflectometry fluctuations measurement.

D. Winslow (U. Texas) completed a two week visit to begin driven tile
probe experiments. R. Chatterjee (U. Texas) completed a two week visit
to re-install a section of the ECE radiometer and get first results from
the ECE correlation system.