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


Alcator C-Mod Weekly Highlights
August 9, 1999



Operations
----------

Alcator C-Mod operated for four days last week with the primary goal
of bringing all RF transmitters online. All transmitters have now
delivered approximately 1 MW of power to the plasma, limited primarily
by power supply transformer tap settings that will be raised once initial
testing is complete. With this increase in supply voltage, FMIT#1 and #2
will be ready for physics operation. A problem with the RF feedback control
system on FMIT#3 and #4 only allowed low power simultaneous operation of
these two transmitters. This problem is now being resolved.

Last week's runs were also used to clean up the machine after the up-to-helium
of the previous week. A low temperature bake at 55C during ECDC was
carried out over the weekend and into Tuesday morning. By Tuesday the
vacuum conditions were much improved over the previous Friday, and the
machine ran extremely well, though the H/D ratios were still too high for
efficient RF heating. Vacuum conditions and the H/D ratios improved
as the week progressed, until on Thursday they were sufficiently good for
boronization to take place. The run on Friday, following the boronization,
produced good H-modes at relatively low RF power thresholds. After
several shots were taken to judge the quality of the boronization,
the Friday run was stopped early so that work could proceed on the
FMIT#3 and #4 feedback circuitry.


Physics
-------

The inability two weeks ago to get a standard tokamak discharge lasting
longer than about 0.4 s, and the spectroscopic evidence of much
increased iron concentrations in these short plasmas were the reasons
for our TV search for any broken in-vessel hardware. As reported
previously, a broken SS gas feed line was found to be sticking into the
plasma volume. This tube was removed in the one day up-to-helium vent on 7/28.
The iron concentrations had been increased by about one order of magnitude
compared to normal levels. After the tube removal, these elevated
levels fell to about twice normal, indicating that, while most of
the SS source was gone, some SS still remained on the walls.
The oxygen concentration was also elevated in those discharges
with high iron levels. Spectroscopic measurements also documented the
cleanup of the discharges occurring after the up-to-helium vent. In those
clean-up discharges, the carbon and oxygen concentrations were significantly
greater than normal. The increased oxygen was no doubt due to water vapor
on the vessel walls. The source of the increased carbon is
unknown, but as mentioned above, after a mild bake and ECDC
the machine cleaned up rapidly, and both of these impurities
decreased.

Detailed Omegatron spectra were obtained on Thursday during ohmic fiducial
tokamak discharges immediately prior to boronization, with the deuterium
(M/Z=2) resonance dominant. Almost all other resonances could be
attributed to charge states of carbon-12, boron-10 and boron-11, as
well as hydrogen-1. A resonance at M/Z=8 was observed on multiple
discharges, which may have been due to doubly-charged oxygen-16. Ion
mass spectra were obtained again immediately after boronization on Friday
in fiducial ohmic tokamak discharges. Surprisingly the relative levels
of boron and carbon changed little compared to pre-boronization, but
the M/Z=8 resonance reduced to the background signal.

In a series of separate discharges it was found that the Omegatron
probe can be operated as a residual gas analyzer (RGA) on a tokamak.
Grid component biases were set in electron saturation mode, edge
plasma electrons were used to ionize the neutral gas inside the
omegatron, and the ion mass spectrometer discriminated on the basis of
M/Z. Neutral pressure of order 0.2 mTorr was calculated inside the
omegatron cavity, which was consistent with midplane neutral pressures
at the same time for the same shot. A rich spectrum of resonances was
obtained with M/Z=4 dominant, probably corresponding to singly ionized
molecular D_2; decreased by a factor of four or more but still easily
recognizable were resonances of M/Z=2,3,12,5,6 and others (in
decreasing intensities). Before using this preliminary Omegatron RGA data
to draw conclusions about the composition of the neutral gas outside the
Omegatron, it must be noted that the probe was designed to analyze edge
plasma ions and does not permit easy communication of neutral gas with the
surroundings.


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

Romik Chatterjee, UTexas, ended a three week visit during which he continued
work on the hardware for using the high resolution ECE radiometer to observe
Te fluctuations. The radiometer is now regularly taking profile data.
Perry Phillips, UTexas, will visit for the next three weeks and continue this
effort.

David Winslow, also UTexas, ended a two week visit during which he analyzed
divertor probe fluctuation data and prepared for a driven divertor probe
experiment.