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
May 1, 2000


Operations
----------

Plasma operations continued last week at Alcator C-Mod. A total of 33 plasmas
were obtained during two run days. Substantial power was coupled to plasma
through each of the ICRF antennas, the lithium pellet injector was operated
successfully into plasma, and the diagnostic neutral beam was successfully
operated into a C-Mod plasma for the first time. Wall and antenna conditioning
are now satisfactory for us to carry out the first boronization of the
campaign.

Plasma operations are scheduled to continue this week.


Physics
-------

Lithium pellets were injected during both runs this week. Long-lived P-modes
were obtained, with sawtooth suppression lasting more than 150 ms and neutron
rate of 1e12/s. The Los Alamos camera very nicely showed the bremsstrahlung
torus that forms after the density peaks on each shot. Impurity (Argon)
peaking during the P-modes was studied using the HIREX x-ray spectrometer.

ECE Signal changes associated with Type III ELMs have been explained in terms
of refraction effects; the ELM perturbations are modeled as a poloidally
elongated density loss located just inside the LCFS. Based on the ECE
observations, the inferred geometrical dimensions were as follows: width of
the affected region ~2cm, minimum density 0.5 e20 m^-3, poloidal elongation
~1.5.

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

All four ICRF transmitters have been brought back into operation. Each
antenna has been vacuum conditioned to at least 20 kV, and conditioning in
plasma has started. The D-Port antenna is up to 1.25 MW, E-Port is up to 1.25
MW, and J-Port is up to 1.2 MW. Total power into the plasma has reached 2.2
MW so far. At net powers above 2MW, ELM-free H-modes were obtained.

At the highest powers obtained from J-port, RF pickup effects were observed on
several systems, notably in the fault-sensing circuitry of the EF4 power
supply. The source of the pickup is being investigated, and improved noise
rejection will be implemented in the supply.

DNB Systems
------------

The DNB was fired into C-Mod plasmas on Friday using deuterium as both the
beam fuel and the neutralization gas. Two 26kV, 100ms pulses were fired, each
drawing roughly 1A of ion current, and two 21kV, 50ms pulses were fired. The
effects on C-Mod of opening the DNB gate valve, both with and without source
gas puffing, were assessed. Analysis of the data from these pulses is
continuing. During C-Mod pulses, the arc current is significantly lower than
the arc current from an equivalent conditioning pulse. Plans for determining
the cause of the deficit were developed. Waveform generator #1 was repaired
and debugging of the new, isolated current measurement system continued. A
problem with suppressor timing which causes the suppressor to fire after the
arc starts was traced to overheating.


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

Rejean Boivin, Catherine Fiore, Amanda Hubbard, Dmitri Mossessian, Ian
Hutchinson, Ron Bravenec (UTx), Joe Snipes, Martin Greenwald, and Bill Rowan
(UTx) participated in the Transport Task Force (TTF) meeting in Burlington,
VT, last week.

Rejean Boivin also attended the APS Division of Plasma Physics ExComm meeting
in Long Beach, CA.

Randy Wilson (PPPL) came to MIT for the week to participate in the ICRF
startup experiments.

Romik Chatterjee of UT-FRC visited C-Mod for installation of the heterodyne
ECE diagnostic. The installation is complete and shakedown will start this
week.