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
May 8, 2000

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

Plasma operations continued at Alcator C-Mod last week. Three run days were
scheduled and completed, although one run was delayed by a power outage. A
total of 40 plasma shots were obtained.

Prior to plasma operation, the first boronization of the current campaign was
carried out on Monday night. We used 127 psi of diborane which should produce
a film about 1860 A thick on average. The boronization was followed by 15:40
hours of ECDC in helium at 5.e-5 Torr.

The primary goals of the week's operation, re-establishing operation after
boronization, continuation of the ICRF commissioning up to high power, and
characterization of the interactions of the diagnostic neutral beam with
tokamak plasmas and initial checkout of DNB diagnostics, were all
accomplished. Over 5MW of ICRF power was injected into plasmas using all three
antennas by the end of Friday's run. EDA H-modes lasting up to 0.5sec were
obtained using the D- and E-port dipole antennas. H-mode was also obtained
using the J-port antenna alone.

We are now ready to begin the physics phase of the 2000 Campaign. Plasma
operations are scheduled to continue next week.

Physics and Diagnostics
-----------------------

Results from simple 1-D modelling of the neutral behavior in the C-Mod
divertor, compared with recent divertor bypass experiments, suggest that a
large fraction (~ 50%) of ions that strike the divertor plates recycle
directly to the midplane as neutrals via leaks in the outer divertor plate
structure (or through the bypass when open). The primary leakage path with
the bypass closed is the open divertor ports. The modelling explains the
experimental observation that increasing the present leakage conductance by
opening the bypass doesn't change the leakage flux significantly. The
resulting drop in divertor gas pressure is also explained by the model.

David Winslow (UTx) completed installation of the Texas probe; bakeout of this
probe vacuum system is in progress. Bakeout of the re-installed A-side
scanning probe (ASP), which had experienced an internal short last month, has
been completed, and this system is being readied for operation during this
week's runs.

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

Commissioning D, E, and J-port antennas continued this past week after the
first boronization of the campaign. D and E-port are operating up to 1.7 MW
each. For H-mode target plasmas, the maximum injected power is limited by the
antenna voltage to typically 1.7 MW and 1.2 MW for D and E-port, respectively.
We are currently investigating the reason for the differing voltage limits.

J-port phase feedback has become operational and the interference with
diagnostics and EF4 have been reduced. Impurity production by the J-port
antenna has been greatly reduced compared to the previous campaign. No large
impurity events, seen repeatedly last campaign above 1.2 MW, have been
observed up to 2.5MW. The camera views of the J-port antenna also suggest
that the arcing seen between front tiles has been eliminated. We have
obtained an H-mode with J-port alone and obtained 5 MW with D, E, and J-port
antennas (with brief pulses up to 5.7 MW). Further investigation of the
heating efficiency and impurity production for the different antennas needs to
be performed.

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

The DNB was operated into plasma discharges this week. Deuterium beams were
fired into deuterium plasmas. The typical operating voltages were between 40
and 45 kV. There was no indication that these were the available upper limits
for the beam operation. Improvements in cooling for both the suppressor and
the mod/reg led to more reliable operation. The beam operation continues to
suffer from reduced arc current during C-Mod shots which is likely caused by
sagging of the 480VAC due to other power supplies on the same service. This
will be followed up this week. The BES diagnostic observed the interaction of
the low pressure D2 gas in the C-Mod vacuum vessel during fizzles and during a
gas only shot. In the latter case, the beam was also observed with the LANL
fast framing camera. Beam plasma interaction was observed on the BES
diagnostic, but was not so clear -- if present -- on the CXRS diagnostic. The
CXRS problems may be due to the use of deuterium beams compounded by the arc
current problem mentioned above.


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

Bob Granetz, Joe Snipes, and Jesus Ramos participated in the FIRE Workshop at
PPPL last week.

Miklos Porkolab and Rejean Boivin attended the APS Division of Plasma Physics
Executive Committee Meeting in Long Beach, Ca., on April 29th.