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
Feb 16, 1999

Plasma Operations devoted to machine clean-up and facility commissioning
continued on Alcator C-Mod last week. Four run days were scheduled and
completed. Startup reliability of C-Mod has been almost 95% over the last two
weeks of operation.

Tuesday's run completed calibrations of the VUV and x-ray spectrometers.
Reproducible, "constant" with auxilliary gas puffs of Ar and He were required
and achieved. The views, relative sensitivities, and instrumental spectral
resolutions of each of five x-ray spectrometers was calibrated. This was done
by having pairs of spectrometers view Ar lines from the same plasma region. In
addition the pairs' views were scanned so that the entire poloidal
cross-section is covered by the combination of all five spectrometers. For the
VUV spectrometer the sensitivity was calibrated using the so-called branching
ratio technique. This relies on observation along the same line-of-sight of
two spectral lines originating from the same upper state, one in the VUV and
one in the visible. The measurement of the visible line is made by an
instrument whose sensitivity has been calibrated by other means. In this way
the calibration in the visible is transferred to the VUV.

Wednesday's run was dedicated to calibration and testing of the fast-scanning
gas-injection probe. Controlled amounts of ethylene and freon gas were first
puffed into the vacuum chamber to correlate quantity injected with plenum fill
pressures and optimize valve timing. Plasma experiments were performed in
which freon was injected into 0.8MA discharges with line-averaged density
around 1.5e20 m-3. The amount of gas injected and the insertion depth of the
scanning probe were varied. Measurements of fluorine were made with the
McPherson and measurements of chlorine were made with HIREX.

High power conditioning and testing continued on D and E-port antennas on
Thursday and Friday. Up to 3MW total power was launched into the
plasma. Although the H/D ratio continued to be higher than optimum, some
heating was observed. On Friday several L-H transitions were obtained.
Reliable operation has been achieved with E-port antenna and its ICRF system.
D-port, however, continues to be inconsistent. The symptom is variable power
for the same input voltage. The problem seems to be in the low power
amplifier chain, not in the IPA, Driver, or FPA stages. All but the signal
generator were swapped. The symptom disappeared when a splitter was installed
between the signal generator and the mixer. The signal generator, however,
did not show a drift in the RF output or any other problems when tested on the
bench. We will continue to monitor this inconsistency.

Progress on tuning FMIT#4 to 78 MHz goes slowly. The maximum
frequency at which the RF output is 2 MW is 75.6 MHz. At this point
the driver reaches its minimum length and can be adjusted no
further. As the frequency is increased further, the reflected power
from the FPA input cavity increases and the output power decreases so
that at 78 MHz only 700 kW is produced. Chris Brunkhorse (PPPL RF engineer)
is visiting on Tuesday of this week to help troubleshoot the problem.

The first boronization of the machine during this run campaign was carried out
last Friday evening, extending into Saturday morning. After approximately 200
plasma discharges the machine was judged to be sufficiently clean to boronize.
Assuming uniform deposition, approximately 1900 Angstroms of boron was applied
to the vessel walls using ECDC with a fill pressure of 3 to 5 mTorr of B2D6
(diborane). Following boronization, we have run ECDC in helium to deplete
somewhat the deuterium on the walls.

A schedule for bring up the DNB has been prepared and reviewed. Principal
milestone dates are:
Operation of DNB on test stand complete - 5/19/99
DNB tests into CMOD plasma complete - 8/2/99
Present progress measured against this schedule is satisfactory.

Arrangements are being made to obtain supplemental engineering support from
UT during the final system testing phases immediately prior to the above
milestone dates.


Visits and Travel
------------------
Dave Johnson (PPPL) visited last Sunday and Monday to help with alignment of
the edge TS system.

Ron Parker, Dave Gwinn, and Steve Wukitch visited PPPL to meet with
Stefano Bernabei, Neville Greennough, and Elmer Fred to discuss,
survey, and assess the MIT lower hybrid equipment at PPPL and the
proposed LH experiment on C-mod.