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
July 8, 2002

Plasma operations continued at Alcator C-Mod last week; two run days were
scheduled and completed, both in support of startup and conditioning
activities. A total of 19 plasma discharges were produced. Progress also
continued on ICRF, DNB, and Lower Hybrid Systems. Plasma operation is
scheduled to continue next week.

Operations
----------

Plasma operations were carried out on Monday and Tuesday of last week. The
primary focus remained tokamak conditioning. The key H/(H+D) ratio decreased
to ~10%; we anticipate that another one or two run days will be sufficient to
bring this value down to a level suitable for boronization.

During Tuesday's run the startup scenario was modified in an attempt to
improve startup reliability. By the end of the day a new program featuring
earlier position feedback was implemented; we will be evaluating the
robustness of this scheme during next week's operation.

The lithium pellet injector (LPI) is fully operational. During the 1020702 run,
6 pellets were injected into 4 discharges. Injector reliability was 100%. The
LPI will be used during this run campaign, in conjunction with an upgraded
Princeton Scientific Instruments ultra-fast camera, to image pellet ablation
trails and measure poloidal field profiles.

No runs were scheduled for Wednesday or Friday. Additional bake-out and ECDC
was carried out. In addition, ICRF conditioning and diagnostic alignment and
calibration activities requiring cell access were conducted.

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

The ECE system was calibrated on Friday, using a cold (LN2 temperature) load
to provide an absolute intensity calibration. Results do not appear to differ
significantly from the prior calibration carried out last year.

The Thomson scattering system was aligned through the machine, and both edge
and core systems, including eight new core spectrometers, should be
operational for the physics campaign.

A thermoelectric cooling unit on the detector array used by the Two Color
Interferometer density diagnostic failed prior to Tuesday's run. The array has
been sent to the vendor for repair, and a single-channel detector will be
installed for near-term operation. A failed analog fiber optic link (AFOL)
unit connecting the visible bremmstrahlung diagnostic to the plasma control
computer was repaired, and the VB diagnostic is again available for use as a
density feedback signal if needed.

ICRF Systems
------------

Conditioning of the J-port four-strap antenna continued during plasma
operations last week. Up to ~0.5 MW at 70MHz was coupled to the plasma. Vacuum
conditioning of D- and E-port antennas was carried out between shots during
the runs and during the maintenance days on Wednesday and Friday.

Diagnostic Neutral Beam Systems
--------------------------------

We made significant progress toward repair of the DNB power supplies. Damaged
components in the accelerator capacitor bank were replaced. The charging
supply was verified to be undamaged, and the capacitor bank was successfully
charged to full operating voltage. Arcing damage was identified in invertor
and chopper modules which will be repaired at the beginning of this week.


Lower Hybrid MIE
----------------

Fabrication of Lower Hybrid launcher components at PPPL is continuing well.
At MIT, the TPS boards have been modified for the faster opto-couplers. The
pulse current response of the beam current monitors was tested, and showed
normal waveforms. New pulse transformers to measure fast body current are now
on-site, and will be installed shortly.


Physics
-------

The C-Mod Experimental Program Committee met on Wednesday to approve new
Mini-Proposals for the first phase of the upcoming physics campaign. These
experiments include several Advanced Tokamak and Internal Transport Barrier
studies which require two-frequency ICRF heating techniques for simultaneous
on- and off-axis heating. The ICRF system is presently configured to provide
70MHz from the four-strap J-port antenna (in heating phasing) and 80 MHz from
the two dipole antennas at D- and E-ports. In addition, experiments in the
transport and divertor/edge topical areas, and in support of the density
control task force, were approved.

At last week's Physics Meeting, Joe Snipes reprised some results on the
Quasi-Coherent Mode (QCM) which he presented at the EPS meeting in Montreux,
Switzerland on 17 June. The change in frequency of the QCM in EDA H-mode has
previously been observed to increase with increasing stored energy divided by
plasma current in much the same way as the core toroidal rotation scales.
Now, with double magnetic probe data, the average poloidal mode number of the
QCM has been measured for a number of EDA H-modes as a function of time. If
the toroidal rotation is assumed to be zero at the plasma edge, then the
measured change in QCM frequency as the H-mode evolves should be proportional
to the plasma ion poloidal rotation in the edge. The steep edge temperature
gradient in EDA H-mode drives a substantial neoclassical poloidal ion rotation
velocity in the ion diamagnetic drift direction. When the high edge
collisionality is included in the calculation, the neoclassical poloidal
rotation in the edge is consistent with the measured change in QCM frequency
times the poloidal circumference divided by the measured flux surface averaged
poloidal mode number of the QCM. The edge poloidal rotation velocity reaches
2 km/s corresponding to a QCM frequency of 50 kHz. Since the QCM rotates in
the electron diamagnetic drift direction, the larger the poloidal rotation in
the ion direction the smaller the QCM frequency becomes, consistent with
observations.


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

Paul Bonoli attended the NSTX Five Year Planning Forum at the Princeton
Plasma Physics Laboratory on June 24-26, 2002. He gave a talk at the
Plenary Session of the Forum on the Alcator C-Mod Program and C-Mod Plans
for Advanced Tokamak Scenario Development.




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