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
Oct. 10, 2000

Plasma operations continued at Alcator C-Mod last week. Three run days were
scheduled, but only one was completed. A total of 22 plasmas were obtained,
with a startup reliability of over 90%. Operation was curtailed in order to
locate and correct a ground fault in the OH2L circuit, which was ultimately
determined to be due to a failing capacitor in the commutation bank. The bad
capacitor was identified and removed from the circuit on Friday. Experiments
to evaluate the performance of the four-strap J-port antenna with the newly
corrected phasing continued.

Plasma operations are scheduled to continue this week.

Physics
-------

ICRF experiments were carried out with upper null, inboard limiter, and
(normal) lower null target plasmas. The upper null and inner limiter targets
were employed in order to obtain good comparison discharges in L-mode, without
the complication of H-mode transitions. On one upper null discharge with 2MW
of RF power an EDA H-mode was observed. The threshold power for this L-H
transition was somewhat lower than expected, though higher than required to
obtain H-modes with the standard lower null geometry later in the day. The
edge electron temperature at the transition was observed to be significantly
higher for the upper null than for lower null discharges; this result is
consistent with previous experiments with reversed toroidal field and lower
null targets, which also have the ion grad-B drift direction away from the
X-point.

Last week, we obtained the first results from the neutral particle analyzer
since it was reinstalled in B port. The diagnostic, which looks at passive
charge-exchange, is a E||B-type analyzer and produces a 39 channel energy
spectrum of both the hydrogen and deuterium neutrals. Looking perpendicular
to the toroidal direction, the analyzer is located slightly above the
midplane, and looks 5 cm above the magnetic axis. First results indicate that
the new 4 strap RF antenna couples differently to the edge versus the two 2
straps. The measured H/D ratio is similar to the one obtained
spectroscopically but temporal differences seem to exist; a complete
comparison awaits further experiments and analysis.

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

In L-mode, we obtained good comparison discharges for evaluating the heating
efficiency of the four-strap J-port antenna relative to the dipole antennas at
D- and E-port; loading data were also obtained. High power operation up to 2MW
was obtained from J-port; total power with all three antennas operating
simultaneously reached 4.5MW into an H-mode plasma. Operation of the J-port
antenna above 2MW was hampered by arcing in either the transmission line or
the antenna. This problem is under investigation; additional conditioning may
be required to increase the power handling capability of the antenna.

The FMIT#3 (one of the two transmitters feeding J-port) screen crowbar has
been firing on RF trips. Installing a 480T5E (4800V, 5A) fuse (150 msec at 20
amps) in place of the screen crowbar unit allowed us to observe the screen
current spikes associated with antenna arcs. It appears that the screen
crowbar response is faster than the arc protection (both have ~5-10 usec
response time), and the crowbar is firing before the arc protection removes
the RF drive. The screen crowbar in FMIT#2 (E-port) will be instrumented in
the same manner to investigate whether it suffers from a similar problem.

It is clear from last week's operation that if one transmitter faults on J the
other needs to fault as well, so that monopole operation of the antenna, which
leads to impurity injections, does not occur. This change in the fault logic
has now been implemented, and will be tested in operation this week.

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

A meeting was held to discuss the recent damage to the DNB drift duct
and the low signal levels detected by the diagnostics. The damage to
the duct appears to be due to a combination of factors: reionization
of the beam and failure of the diverting magnet. Repairs to the
drift duct are underway. Experiments to look more closely at the
reduction of signal levels due to reionization and due to stray
fields were recommended, and plans for these are underway. Spectra
from the plasma source were acquired under typical conditions of
current and voltage in the source, but without extraction or
acceleration of a beam. They appear to be absent of impurities, but
this work is not yet complete.

Engineering
-----------

Final drawing of the inner divertor "girdle" rear plate was reviewed with the
manufacturer, and a final quote was obtained. 100% inspection of all 14
critical dimensions is included in the price. Delivery of the first plate for
MIT approval is scheduled for November 27, 2000, with the last of 12 pieces,
including two spares, to be delivered by Jan 17, 2001.


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

Ian Hutchinson, Earl Marmar, Bruce Lipschultz, John Rice, Miklos Porkolab, Jim
Hastie, and Bob Granetz are presenting papers based on Alcator C-Mod research
at the 18th IAEA Fusion Energy Conference in Sorrento, Italy. Also attending
the conference are Joe Snipes and Ron Parker. Earl Marmar also presented a
seminar on Alcator C-Mod results to the Tore Supra group in Caderache, France.

Montgomery Grimes, Ron Parker, and Dave Terry visited Thomcast AG in Turgi,
Switzerland for a design review of the Lower Hybrid power supply/modulator.