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


Alcator C-Mod Weekly Highlights
April 26, 1999



Three plasma run days last week were focused primarily on bringing
up the four ICRF transmitters and conditioning the three antennas.
A forth run dedicated to the scaling of the rotation velocity with plasma
current, in H-mode discharges, was undertaken on Friday.


Physics:

The Tuesday, Wednesday, and Thursday runs were dedicated to RF conditioning
of the antennas, and the work required to debug and bring up the third and
forth transmitters during plasma operation. A total input power of 3 MW was
coupled to the plasma from the D and E antennas. Testing of the J-port
antenna into vacuum was begun and up to 140 kW of power has been injected.
Protection circuitry needs to be tested before significant power can
be applied.

On Friday a run to study the scaling of plasma rotation with plasma current
during H-modes was scheduled. The plasma current ranged from 0.4 to 1.4 MA.
However, the quality of the H-modes was erratic probably because of lack of
conditioning of the RF antennas following boronization done the previous
night. Some results were obtained, including edge temperature pedestal
profiles in H-mode at high current, and impurity confinement scalings with
plasma current. We are finding, for example, that the height of the edge
temperature pedestal clearly increases with Ip. Further analysis of the
rotation velocities will be forthcoming. Vacuum conditioning of all
antennas is proceeding in preparation for this week's set of runs.

The first images of carbon 'plumes' where recorded during
piggy-back experiments last week. In these experiments, trace
amounts of ethylene gas (approx. 10^17 molecules) are injected
over a 10 msec period by the F-port vertical scanning probe. Two fast-gated
(0.2 msec exposure) CCD cameras (on loan from PPPL and LANL) were used to view
the resulting emission patterns of CII or CIII light. One camera views the
plume from the top (along -Z direction) while the other views from the side
(along -R). Video images were captured using the newly installed Matrox
color frame grabber system and IDL-based widget interface. Based on the
success of this frame grabber system system, another one is begin put
together for divertor imaging.

Plumes of both CII and CIII emission appear to be extended along the
magnetic field line in both directions. Asymmetries in the pattern suggest a
reversed plasma flow (flow out of the divertor) for injections near the
separatrix. The plumes also indicate a strong cross-field plasma drift near
the separatrix in a region where the ExB poloidal drift is expected to be high.
These preliminary experiments show that the ethylene injection does not affect
the discharges in any detectable way. A comparison of CII signals before and
after a series of discharges with ethylene injection showed no change.
Consequently, these experiments may be continued in a 'piggy-back' mode of
operation.

Plans for further experiments with ethylene include a reduction
and optimization of ethylene puff and camera exposure. Progress is
underway towards a full 3-D reconstruction of the emission plume from
the two camera images.

We reported last week on the first operation of the inner wall
rangefinder diagnostic. Measurements reported at that time of 4 to 6
nm should have been reported as 4 to 6 um. Several weak disruptions
(halo currents<= 100 kA) during last week's operation have now produced
measurements in the 20 to 30 um range. Comparisons with the strain-gauge
at the same inner wall location are now being made.


Engineering:

All four transmitters are now operational and work is focused on conditioning
the antennas for plasma operation. Up to 3 MW of power was delivered to
the plasma last week from the D and E antennas. The J-port antennas has been
operated into vacuum at low power levels to test fault detection and condition
the antenna for plasma operation. Boronization on Thursday evening degraded
the performance of both D and E antennas. The cause of this effect is
under investigation. Vacuum conditioning over the weekend has prepared
them for operation this week.

Work on the DNB last week included bench tests of the mod/reg tube regulator
board. This system is now complete except for calibration. Details of the
mod/reg cage component mounting and wiring were completed. Work has begun
on the MCL/PLC HV supply and MOD/REG interface designs.


Travel and Visits:

Gary Taylor from PPPL was here last week to continue taking data with the GPC2
ECE diagnostic. He also brought on line new higher bandwidth Te data for this
instrument. Raffi Nazikian, also from PPPL, visited to work with Yijun Lin
on the C-Mod reflectometer. Raffi and Yijun also took a trip to Millitech
to discuss new hardware for the reflectometer upgrade to be done in
collaboration with PPPL.

The DOE C-Mod Quarterly Review was held last Thursday, 4/22, via teleconfence.
DOE, MIT, and UTexas were all linked together with talks presented from both
the MIT and UTexas sites. Talks on Ohmic H-modes, the outer divertor
flappers, high spatial resolution pedestal diagnostics, the status of the RF
and DNB systems, and the Lower Hybrid proposal were presented from MIT.
The status of the DNB diagnostics was presented by UTexas.