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 22, 1999

Plasma operations continued on Alcator C-Mod last week. Due to the Monday
holiday, only three run days were scheduled, Wednesday through
Friday. Operation following the first boronization proved more difficult than
anticipated, with only 12 successful shots obtained on the first two runs.
Reliability improved markedly on Friday, with 26 successful plasma shots
obtained. EDA H-mode plasmas were reproducibly obtained at 800kA and 1MA, with
performance similar to previous campaigns.

The Transmitters #1 and 2 (D- and E-port two-strap antennas) were used to couple
up to 2.8 MW into EDA H-mode plasmas at 800kA. However, the power feedback
gain is insufficient to compensate for changes in total gain of the
transmitter as the plasma loading changes, resulting in lower power and RF
faulting for other target conditions. We attempted to modify the feedback
during the run, but this was unsuccessful (resulted in power oscillations).
This problem will be addressed during this week's operation.

Temperature pedestal measurements made using small Bt ramps during EDA H-modes
on Friday's run were quite consistent with those from the last campaign, with
widths ~ 11 mm, and the foot of the pedestal located at the separatrix. The
pedestal height was moderate (~ 350 eV) and varied with input power.

Romik Chatterjee (UT-FRC), John Heard (AU) , and Perry Phillips (UT-FRC)
completed the installation of the high spatial resolution ECE radiometer.
Thirty of the thirty-two channels are operating with low noise, and shakedown
data is being acquired at the "slow" 10 kHz rate. Qualitatively, this data is
consistent with that expected of plasma temperature during current rise and
sawteeth. Spatial localization of the channels is not consistent with
expectations, and the cabling, filtering, and other problems which might
contribute to this discrepancy will be investigated this week. Some CAMAC
highway problems arose which interfere with use of the fast CAMAC channels.
These do not affect shakedown yet and should be worked out in parallel with
the rest of the shakedown activies this week. Romik Chatterjee returned to
UT-FRC to assemble and test amplifiers -- two will replace the inoperative
ones now installed, and the remainder will be spares. When these problems are
resolved, development of the fluctuation measurement can proceed.

Tuning FMIT#4 to 78 MHz is progressing slowly. We had Chris Brunkhorst
(RF engineer from PPPL who has operated these transmitters to 80 MHz)
visit and work with the MIT RF engineers on this problem. Further
work with PPPL support is planned for the upcoming week. We also have
contingency plans to begin operation of J-port antenna at 75 MHz so as
to begin testing the rest of the system as soon as possible.

The DNB high voltage power supply was operated successfully for the first time
last week with up to 4 amps of current at 55 kV into a dummy load. Ten amps
into the dummy load will be tried as soon as a modification to the dummy load
is complete, and debugging of some of the instrumentation is finished.

We continue to work on the DNB, new J-port antenna feedline installation,
improving the reliability of the RF feedback systems, tuning of the #3 and #4
transmitters for operation at 78 MHz, installation of our new pump vent
system, and installation of new diagnostics. Equipment for the lower-hybrid
system is being returned from storage and options for system installation are
being discussed.

Physics:
-------

The first H-modes gave interesing results for the two high resolution edge
x-ray arrays, which view the soft x-ray emission of the edge plasma at the top
and outboard midplane, In H-mode, a pronounced pedestal is seen on both
arrays, as expected. The center of the pedestal is 10-12 mm inside the
separatrix at the outboard midplane, consistent with previous measurements at
the outboard midplane. However, it is located within 2 mm of the separatrix at
the top of the machine. This implies that the x-ray emissivity varies
significantly on EFIT flux surfaces just inside the separatrix. Previous
measurements by J. Rice et al. found large up-down asymmetries just inside the
last closed flux surface for hydrogen-like argon in Alcator C-Mod.

Power-law fits to the reflectometry fluctuations power spectra show systematic
differences depending on plasma conditions. If we assume flucuations power
density S(f) is proportional to f^(alpha), the parameter alpha is
significantly different in L mode plasma and H mode plasma. For ohmic L-mode,
the spectra are usually flat for f < 100 - 150 kHz, -0.5 < alpha < 0.0, but
steeper for f > 100 -150 kHz, where -2.5 < alpha < -1.5. In H-mode case (ELM
free), the spectra show a robust 1/f (-1.1 < alpha < -0.9) characteristic for f
> 1-10 kHz to the frequency where system noise dominates. In EDA H-mode, a
frequently occuring coherent mode degrades this power law. The physics behind
these power-laws and its effect on plasma transport is still under
investigation.

Radiated power measurements have been obtained in the last 2 weeks. The
systems were all relocated during the last shutdown and numerous changes have
been made. Three of the 4 systems are functional. 2 of these are global
measurements, one using a standard foil bolometer, the other using a wideband
silicon detector (AXUV diode series). The 3rd system is based on 2 arrays of
wideband detectors (AXUV diodes). The diode single view agrees with the
integrated radial emissivity profile, within a few percent. However, a
significant difference (up to a factor of 2-3) is found between the standard
foil bolometer and the diode. On a single impurity injection event, though,
the difference amounts to approximately 30% only, indicating that a measurable
amount of power is carried by neutrals (which are not measured by the
diodes). Difficulties arise with neutrals as the plasma is not "optically
thin" and careful inversion/analysis is required. Additional checks and
calibrations are under way, and a future run in helium is planned which should
help in elucidating the difference.

Components of the currently unused divertor region Thomson scattering
experiment have been adapted to measure electron temperature and density at
the upper separatrix. This new edge system began operating during the week of
February 9-12, obtaining appreciable signals from scattered photons. The
signals from four spectral channels are used to compute electron temperature
in each of 17 scattering volumes. Currently the accuracy of the results is
unclear due to incomplete characterization of the transmission in each
spectral channel. However, tests of the analysis code using modelled data
indicate that once a reliable calibration is obtained, relatively accurate
measurements of edge temperature, as well as relative density profiles, can be
produced. Efforts to obtain the necessary calibration coefficients are
proceeding.

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

Chris Brunkhorst visited from PPPL to assist in tuning of the new
transmitters.

Bill Noonan, from the University of Maryland, visited C-Mod last week to work
on the high resolution visible/UV spectrograph diagnostic.

Gary Taylor (PPPL) continued to assist in ECE operations. The 38 channel GPC2
instrument is now fully operational.

Romik Chatterjee and Perry Phillips (UT-FRC) visited to work on
installation of the ECE radiometer diagnostic, as described above.