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
Feb 8, 1999


Operation:

The conditioning and facility commissioning campaign continued at Alcator
C-Mod this week. Four run days were scheduled and completed. The reliability
of the tokamak was excellent for this phase of the run period, with startup
reliability of greater than 90% during the last four days of operation.
The main tasks this week, aside from cleanup and conditioning of the machine,
were diagnostic alignment and calibration, conditioning of the D- and E-port
ICRF antennas, and commissioning of the divertor bypass flappers.

On Tuesday a series of reproducible 800kA diverted plasmas was run for the
purpose of aligning the ECE view. The HIREX x-ray diagnostic also
used this run to cross-calibrate its spectrometers, and the flappers were
exercised successfully for the first time during plasma operation. In addition,
the ICRF began conditioning activities, which continued as the main activity
on Wednesday.

On Wednesday the RF power was increased up to 3 MW for about 40 msec within a
few shots. The performance then degraded and power was reduced. The system
resumed running cleanly at lower power. New MW_coefficients and DC2
and stub calibration factors for D and E-port were obtained. RF conditioning
continued as a background task during the runs on Thursday and Friday. By the
end of the week, the D- and E-antennas were operating reliably at a total
power of 2.5MW.

Thursday's run was mainly devoted to carrying out the first experiments with
the new divertor bypass flappers, and related diagnostics. Three experiments
were conducted: an ohmic density scan to look at the effect of the bypass on
neutral pressures and Argon screening; a scan of strike point location at
fixed density; and a transient 'puff and pump' experiment in which the
discharge was initiated with the bypass open, and then the flaps were closed
at 1 sec into the shots. It was found that, for the same plasma density, the
main chamber pressures are unaffected by the flapper. This is consistent with
earlier conclusions that main chamber pressure is determined primarily by main
chamber recycling and not by neutral escape from the divertor. Divertor
pressure is reduced by about a factor of two with the flappers open, at all
densities. The Argon penetration factor as defined by # Ar in plasma / # Ar
atoms released is on average a factor of 2 higher with flapper open; this
difference is smaller than expected from previous results. The transient
experiments are still being analyzed.

On Friday, we began a two-day experiment devoted to calibration of several
spectroscopic diagnostics. The sensitivity of the McPherson spectrometer is
calibrated using the branching ratio technique. This technique 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
intensity ratio of such a pair of lines can be calculated quite accurately.
Thus a calibration in the visible is to be transferred to the VUV.
The line pairs used in Friday's run were:

D_0 1025 A (Ly_beta) & 6563 A (D_alpha)
D_0 972 A (Ly_gamma) & 4861 A (D_beta)
He_0 537 A & 5016 A
He+ 247 A (Ly_gamma) & 4686 A (Paschen_alpha)
He+ 256 A (Ly_beta) & 1640 A (Balmer_alpha)

Lines also tried were CIV 312 A & 5801+5812 A.

The identification of the weak visible lines 5801, 5812 A
in the Chromex spectrum is questionable, but there appears to be
something detectable. The McPherson calibration is about 50% complete.

For HIREX the views, relative sensitivities, and instrumental spectral
resolutions of each of five spectrometers was calibrated. The calibration was
done by having pairs of spectrometers view Ar lines from the same plasma region.
In addition the pairs' views are scanned as much as possible so that
the entire poloidal cross-section can be covered by the combination of all
five spectrometers. About 60% of this calibration work was completed.


Engineering:

We continued to work on installation of the DNB, new J-port antenna feedlines
and transmitters, and a new vent system for the vacuum pumps in the cell.
Over the weekend we backfilled the machine with hydrogen and then argon to
complete the core Thomson Scattering calibration. Following the calibration,
we brought the machine up-to-helium so that the new 2Pi bolometer could
be installed.


Visits and travel:

The C-Mod Program Advisory Committee met at the PSFC on Feb. 4-5. The PAC
members attending, D. Hill, E. Synakowski, E. Strait, B. Carreras,
C. Gormezano, and P. Efthimion, heard presentations on the status of the
C-Mod facility, near-term program, and long-range plans. Our OFES contract
monitor, R. Dagazian, also attended the PAC meeting. R. Hawryluk and
N. Sauthoff from PPPL participated in the discussions on the first day.

Gary Taylor (PPPL) is now at C-Mod working on the ECE system.

Stewart Zweben (PPPL) visited the PSFC this week, and presented an idea for a
2D turbulence imaging diagnostic, which could be implemented on C-Mod in the
next run campaign.

During the week of Feb. 1-5, 1999, Dr. Shunsuke Ide visited the
Alcator C-Mod Group at the PSFC. He worked with Paul Bonoli on
a JAERI / MIT collaboration related to implementing an improved
version of the ACCOME Lower Hybrid (LH) Module at JAERI. The code
was run through numerous physics checks after installation on
a UNIX workstation at JAERI. Several test cases were run for the
JT60/U LH current drive experiments, including experiments where
a reverse shear configuration was sustained by LH current drive.