From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
Oct 20, 2003

Startup plasma operations continued at Alcator C-Mod last week, with three run
days scheduled and completed. Work also continued on Lower Hybrid and ICRF
Systems; diagnostic systems are being brought back on-line. Operations are
scheduled to continue this week.

Operations
----------

A total of 48 plasma discharges were produced last week, with a startup
reliability of 52%. The main focus of the operation was tokamak clean-up and
wall conditioning. Most of the plasmas produced were in standard lower null
equilibria at Ip=800kA. In addition, a number of upper null discharges were
produced in order to begin conditioning of the upper strike point regions in
preparation for upper and near-double null experiments planned for this
campaign. The RF Group made good progress in conditioning of the D- and E-port
antennas. The H/D ratio is in the 10-15% range, and needs to be reduced
further by about a factor of two for efficient H-minority heating.

Physics
-------

The latest passive MHD spectroscopy results on C-Mod show that in
addition to the usual Toroidal Alfven Eigenmodes, we now observe Alfven
Cascades (AC's) in the current rise with strong ICRF heating as well as
high frequency modes in flattop EDA H-mode discharges centered on the
gap of the Ellipticity-induced Alfven Eigenmode (EAE) frequency. Such
Alfven Cascades are observed in JT-60U and JET at the onset of ITB's
when they have reversed shear q profiles. Modeling with the MISHKA code
through our JET collaboration indicates that the q profile at the time
of the Alfven Cascades has slightly reversed or very flat shear with a
minimum q value of 3 at the minimum AC frequency. By fitting the
frequency evolution of the AC's, it is possible to model the evolution
of the q profile and show that the q profile remains flat or slightly
reversed to a minimum q value of 2.5 as the next n=2 AC appears. Then,
the modes evolve into TAE's indicating that the q profile becomes
monotonic.

In a few relatively low density (2 x 10^20 m^-3) EDA H-modes
a series of modes are observed with frequencies from 700 kHz to 1100 kHz
centered on the EAE frequency gap. The surprising feature of these
modes is that they rotate in the electron diamagnetic drift direction,
which is opposite to what would be expected for EAE modes with core ICRF
heating. If the fast ion profile were hollow, it would be possible for
EAE's to rotate in the electron direction.

Ron Bravenec (U. Texas) has been running the gyrokinetic turbulence codes GS2
and GYRO in an "apples-to-apples" comparison (equivalent input parameters,
same flux-tube domain). This is in preparation for simulating turbulence and
transport in C-Mod.


ICRF System
-----------

Transmitters 1 and 2 (D- and E-port antennas) operated nominally, with
conditioning of the antennas continuing throughout the week. Power levels up
to 2MW were coupled to the plasma.

Efforts to couple 50 MHz from transmitters 3 and 4 into the J-port antenna
were unsuccessful. Extensive troubleshooting of the low level RF drive chain
and the instrumentation, and open loop transmitter operation, verified all the
power room equipment is operating properly. The problem appears to be in the
matching of the antenna to the transmission line. Review and recommendations
for a solution are underway.

The J-port system will be reconfigured to 78 MHZ on Monday morning. Return to
50 MHZ will occur once a solution to the matching problem is complete and the
hardware is available.


Lower Hybrid System
-------------------

The filament of the s/n 102 rebuilt klystron was operated. The filament
voltage and current values were within expected ranges indicating both a good
filament and vacuum. The klystron was reinstalled and is ready for testing
to take place this week.

A statement of work for the final welding of the forward waveguide has
been reviewed and approved by both MIT and PPPL. The welding should
proceed this week. A statement of work for the brazing of the alumina
windows into the titanium couplers is under final review at MIT and
will be released this week. Modifications to the couplers, agreed to
by both MIT and PPPL at the recent review, are underway at PPPL that
will reduce stress on the windows during the brazing process. A final
cleaning process of the couplers to remove any braze material or alumina
residue is also underway.


Diagnostics
-----------

The NSTX Imaging X-ray Crystal Spectrometer was operational during C-Mod
plasma operations last week, and improved performance relative to last
summer's results were observed. The instrumental improvements made so far are
(1) installation of a new spherically bent crystal from Inrad Corporation, (2)
installation of brass apertures before and after the crystal to reduce
background due to X-rays scattered from the crystal holder, etc., and (3)
preliminary lead shielding around the detector. The instrument is presently
still equipped with the Korean detector, so the improved resolution is
primarily due to the new crystal. The Brookhaven detector, which has a factor
of 10 better spatial resolution (0.1 mm FWHM vs ~1mm), and significantly
improved uniformity, is expected to be installed shortly.

DNB System
-----------

Beam conditioning continued during C-Mod operation last week. Matt Sampsell
and Marco Valisa finished taking the data for several new tests of beam
performance, including studies of beam divergence, equilibrium, and component
optimization. Other studies are ongoing.

Collimation of the BES filters was completed and calibration of the
spectrometer was begun. A broken BES detector was replaced. Work on porting
and debugging the beam penetration code to the Linux platform was begun.


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

Manfred Bitter and Ken Hill were at MIT 10/14-17 and made progress with the
NSTX imaging X-ray crystal spectrometer on C-Mod.

Matt Sampsell (U. Texas) continued a 2+ week visit to C-Mod; he participated
in work on the BES diagnostic, characterization of DNB parameters, and DNB
operation. Collaborating with several of the co-authors, he also finished a
second draft of his APS Oral presentation "A Multi-Diagnostic Study of the QC
Mode on Alcator C-Mod".

Bill Beck, Bill Burke, and David Terry attended THE 20th IEEE/NPSS SOFE
MEETING in San Diego last week. Bill Beck presented a poster on "ICRF Antenna
Modifications on Alcator C-Mod". Bill Burke presented posters on "Broadband
Amplifiers for the Active MHD Diagnostic on Alcator C-Mod", and "Serial Fiber
Optic Links for the Lower Hybrid Current Drive Control System on Alcator
C-Mod". Dave Terry's poster was on "Lower Hybrid Low Power Microwave Active
Control System Design, Installation, and Testing on Alcator C-Mod". Two other
C-Mod posters, "Alcator C-Mod Non-Axisymmetric Control Coils Design and
Fabrication" by R. Vieira, et al. and "C-Mod Cryopump Design Evaluation" by
J. Zaks, et al., were also presented.






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