Date: Tue, 3 Feb 1998 8:37:26 -0500
From: WOLFE@CMOD.PFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Message-ID: <980203083726.42e001bd@CMOD.PFC.MIT.EDU>
Subject: Alcator C-Mod Weekly Highlights
Organization: MIT Plasma Fusion Center
Newsgroups: sci.physics.plasma

Alcator C-Mod Weekly Highlights
Feb 2, 1998

Last week was a scheduled maintenance week at Alcator C-Mod. No runs were
scheduled. Power systems tests were carried out in support of the
modifications in the alternator excitation regulation circuitry described
below.

A Peer Review of the proposed Alcator C-Mod Fusion Research Program for
1999-2003 was held at M.I.T. January 27-28. The Reviewers heard presentations
by C-Mod researchers from M.I.T. and collaborating institutions. This review
was combined with the annual meeting of the C-Mod Physics Advisory Committee
(PAC), which continued through Jan. 29.

The voltage regulator that controls the alternator exciter current has been
modified for better frequency response. This change will allow somewhat
faster ramp-up rates for the TF field and less heat load on the TF magnet.
In addition, changes have been made to the fault summation relay so that
nearly all faults result in exciter supply inversion rather than pulling the
exciter breaker. These changes were made in response to the review
recommendations following the alternator fault last June.

A series of gas puff tests were carried out to evaluate the pumping speed of
the divertor cryopump for nitrogen, in the absence of plasma. With all other
pumps closed off, N2 pumping speeds of about 800 l/sec were obtained. These
tests used the J-bottom piezo valve. A new "duty-cycle" driver board has been
installed on this valve, which uses a pulse-width modulated output rather than
a proportional output, to allow a more linear valve response.

During the maintenance week, a circuit was implemented in one of the ICRF
transmitters (FMIT#1) to avoid grid overcurrent faults, which cause the
transmitters to be shutdown during the pulse without a retry. A filter
circuit was also added to the grid high voltage supply. These modifications
will be evaluated during operations before being implemented on the second
transmitter (FMIT#2). We continue to work on a pull back circuit because this
circuit will solve most of the overcurrent situations and we still need to
implement it for high voltage faults. We also started tuning FMIT#3 and #4 to
higher frequency. They were originally tuned to 40 MHz, and we would like to
have them tuned to as close to 80 MHz as possible. The criteria is for
reliable 2 MW output from each transmitter. Once this frequency is known we
can finalize the resonant loop design.

Development of the DNB for C-Mod continued. Mechanical assembly and
plumbing of the Mod/Reg cabinet continued. A technique was developed to
apply a conducting coating to interior of the high capacitance section of
the HV transmission line. The coating will facilitate a graded voltage
scheme to prevent electrical breakdown inside the transmission line.

A mini-review of the MSE optical system was held. Norton Bretz and Bob
Parsells of PPPL presented an outline of the current design to members of
the MIT and UT-FRC staffs.

Dr. Steve Wukitch visited Tokamak de Varennes (Canadian Center for Magnetic
Fusion) last week for two days. The purpose was to examine a working
LHRF system and discuss their operational experience.

On Jan. 29-30, Paul Bonoli travelled to PPPL to attend the PhD thesis defense
of John Wright. Paul was one of the thesis readers and had suggested the
problem which eventually became Wright's thesis topic - formulating the
quasilinear diffusion coeffcient of Kennel and Engelmann to include poloidal
mode coupling of fast ICRF waves in toroidal geometry. Dr. Bonoli also gave a
seminar on Advanced Tokamak modelling in Alcator C-Mod. The talk emphasis was
on the lower hybrid current profile control studies that had been discussed at
the C-Mod Five Year Plan Review. Dr. Bonoli also continued working with Cynthia
Phillips and Dan Clark (Princeton graduate student) on the development of an
interface between the TORIC ICRF code and the FPPRF code, currently deployed
in TRANSP.

Roger Bengtson, UT-FRC, and David Winslow, a UT-FRC postdoc, spent the week
completing the installation of the Texas Turbulence Probe on K-port. David
will remain on-site to operate the probe for much of the remainder of the run
period.

Ron Bravenec, UT-FRC, visited to participate in the Program Review and to
continue shakedown of the detectors, amplifiers, and data acquisition
equipment that will form the bulk of the BES spectrometer.

Josef Neuhauser from ASDEX-U visited the PSFC last week. He gave a seminar on
"Results and Plans for Tokamak Improvement Research on ASDEX-U". He also
spent the day having discussions with Alcator personnel on a number of
subjects, including edge and core transport, atomic physics of the divertor
and MARFE, pedestal physics, MDS-plus, and ICRF heating.

Plasma operations are scheduled to resume this week. Four run days are
planned.