From: WOLFE@PSFC.MIT.EDU
Reply-To: WOLFE@PSFC.MIT.EDU
Subject: Alcator C-Mod Weekly Highlights
Organization: MIT
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
April 24, 2000

Operations:
----------

Plasma operations resumed at Alcator C-Mod last week, following a maintenance
interval. Three run days were completed. A total of 63 plasma shots were
produced, with a startup reliability of over 98%. Most plasmas had a
nominal current of 0.8MA, with toroidal fields of 5.1 to 5.4 Tesla.

These runs were a continuation of the startup phase for the 2000 experimental
campaign. In addition to cleanup and conditioning discharges, operational
discharge development required for later physics runs was carried out, and
commissioning of the ICRF system was begun.

Plasma operations will continue this week.

Physics
-------

Wall condition was evaluated by running a sequence of special discharges in
which density feedback was turned off and a short fixed-length gas puff was
injected 1 second into the discharge. The plasma density before and after
the puff, and the density decay time after the puff, are indicators of the
extent to which the wall is "loaded". On Thursday and Friday we observed
pre-puff densities below 3e19/m3, indicating considerably lower loading than
in the previous operating weeks.

An upper x-point equilibrium was developed in support of physics experiments
to be carried out in the next months. These discharges also allowed us to do a
better job of conditioning the upper divertor region.

Ohmic H-mode was obtained using the technique of ramping the toroidal field
down at fixed density and plasma current. A brief (~100msec) ELM-free H-mode
phase was obtained at a qedge value of 2.7. This result is comparable to
our experience in the 1999 campaign, and is another indication that the
machine cleanliness and wall conditions are progressing satisfactorily.

The lithium pellet injector has been reconnected to the torus, and loaded with
a variety of pellets, including pure lithium, as well as Li pellets doped with
potassium chloride, silver and tungsten. The beam-line has been baked, and the
injector successfully test-fired. The system should now be ready for operation
into plasma.



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

We began vacuum conditioning D, E, and J-port antennas this past week and
managed to begin plasma conditioning of E and J-port antennas.

D-port vacuum conditioning was interrupted by a FPA by-pass capacitor failure.
The capacitor was last replaced in November 1996. The capacitor had water
marks and appeared to fail in the vicinity of one these marks (another area
was close to failure). The water is most likely from a pipe failure we had
last year. We should be able to continue D-port antenna conditioning after
replacement of this capacitor.

E-port successfully coupled 1.4 MW to the plasma. The vacuum conditioning and
initial plasma operation were not significantly affected by the presence of
the boron nitride (BN) tiles (there was concern that conditioning would
require significant more time). From the camera view of E-port, there were
hot spots on the moly protection tiles at the midplane and upper right corner
of the antenna for these low density, high H concentration plasmas. We will
monitor these hot spots carefully and plan for the replacement of these moly
tiles with BN in the next opening. During the run, we had a couple of
discharges where the screen xbar apparently fired without a FPA HV xbar. Upon
closer investigation, the screen xbar unit was found to have a shorted SCR
(one of two) and the FPA water cooling line had a small leak. The water leak
caused the screen by-pass capacitor to fail. This capacitor and the water
cooling line are being replaced.

The J-port antenna coupled 400 kW from FMIT#4 (running as a 2-strap antenna)
on the last shot of Friday. FMIT#3 had a control problem (false HV fault)
that prevented it from operating. Vacuum conditioning went extremely well
compared to last campaign (28 kV with little effort). We will continue with
making this antenna system operational this week.

Finally, the modified resonant loops (changed from a low impedance to high
impedance feed point) for D and E have performed well. In fact the voltage
for a given power and loading appears to have decreased and the match is more
stable.

Diagnostic Neutral Beam System
-------------------------------

Progress on the DNB continued. The DNB gate valve was opened using the
conditioning pulse software. The PLC and Torvac wiring was debugged and
the DNB conditioning software was checked. The fueling system was switched
to deuterium for the initial tests into C-Mod. The plasma source was
retested using deuterium with the new filament cabling.