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
August 28, 2000

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

Plasma operations continued at Alcator C-Mod last week. Three run days were
scheduled and completed. The primary focus was on machine cleanup and wall
conditioning, and on conditioning of the ICRF antennas. A total of 52 plasma
shots were produced with a startup reliability of over 80%.

Good progress was made on both conditioning tasks. All three ICRF antennas
operated at high power. H/D ratios in the 0.2 to 0.3 range were obtained by
the end of the week. The next step in the wall-conditioning process is
boronization, which will be carried out on Monday evening. Plasma operation is
scheduled to continue this week.

Physics
--------

A clear correlation between the character of fluctuations in the scrape-off
layer (SOL) and the shape of the density and temperature profiles in the SOL
has been identified using the A-port scanning probe. In addition, snapshots of
the turbulence in the SOL from the newly installed turbulence imaging system
provide important information about the spatial structure of the turbulence in
the far SOL. Density and temperature profiles in the SOL typically exhibit a
steep gradient in the 'near SOL', a region within 5 mm of the separatrix.
Gradient scale lengths at this location range from 2 to 5 mm, depending on the
discharge. The steep gradient appears to continue smoothly across the
separatrix for at least a few mm. Beyond ~ 5mm in the 'far SOL', the density
and temperature profiles flatten considerably. Here the gradient scale
lengths can be 10 to 30 mm, depending on the discharge. Fluctuations in ion
saturation current and floating potential occur at higher frequencies in the
near SOL compared to the far SOL. Time series data in the near SOL exhibit a
steady stream of apparently random fluctuations with auto- correlation times
in the range of a few microseconds. Starting at the transition between the
near and far SOL regions (~5mm) and moving outward, the signals
acquire a lower-frequency, higher amplitude 'bursty' character.'Bursts' in ion
saturation current lasting 10 to 30 microseconds are seen. This behavior
persists and becomes more clearly defined out to the limiter radius in the
main chamber. Bursts in floating potential are also seen. However, floating
potential bursts tend to be negative-going, suggesting that an increased
electron temperature is associated with the fluctuation events.

Snapshots of the spatial structure of density fluctuations using the
turbulence imaging system (looking at HeI light) show the formation of
well-defined, spatially separated density 'blobs' in the far SOL. Isolated
blobs, which are viewed along magnetic field lines by optics, appear to have a
nearly circular form with a characteristic size of about 10 mm in
diameter. The probe data (bursts) combined with imaging data (blobs) suggest
that large density and perhaps temperature perturbations propagate across the
far SOL, convecting particles and energy to the main chamber limiters. It is
recognized that in order for the density perturbation to persist in the
presence of particle losses along field lines, the blobs must be sustained by
ionization. Simple estimates show that there exists sufficient neutral density
in the far SOL for this to occur, just as there is sufficient ionization in
the SOL of C-Mod to support the main-chamber recycling regime. Based on
particle balance, the effective cross-field velocity of the blobs is expected
to be on the order of 100 m/s leading to a transit time across the far sol of
order 150 microseconds. The collisionality of the far SOL plasma is
sufficiently high such that temperature perturbations that may be associated
with the blobs can transit the sol without being damped by conduction along
open field lines.

A new view has been added for the high resolution visible spectrograph
typically being used to measure the hydrogen minority fraction using line
ratios in the Balmer-alpha multiplet. The new view is vertical, and is
optimized for measurement of the total magnetic field at a particular major
radius, using the Zeeman splitting of deuterium Balmer-alpha. This was
motivated by the desire to have an alternate measurement of the field which is
independent of the magnetic loops, primarily because of the sensitivity of the
mapping of the ECE electron temperature measurements to major radius,
especially near the location of the H-mode barrier, where sub-cm accuracy is
important. Initial results indicate that the field measured in this way tracks
the magnetic measurements, with absolute agreement at the plus/minus 0.5%
level, which is within the presently estimated uncertainties of each of the
two measurement techniques.

First data were obtained last week with the upgraded plume-imaging system.
Beam-splitters are now installed on both side and top camera views to allow
for simultaneous acquisition on each camera of CII and CIII plumes generated
by injection of deuterated ethylene (C2D4) through the FSP. Piggy-back runs
were scheduled for both Wednesday and Friday. Initial results indicate: (1)
Camera exposure settings in the range of ~ 0.5 - 1.2 ms are necessary for
background carbon light to be seen in the absence of gas injection. This
criterion sets the operational exposure range. At the peak of probe insertion,
corresponding to the location of gas injection, the probe moves ~ 0.3 mm --
thus the motion of the probe is effectively "frozen" even at these exposure
levels. (2) At exposure ranges determined by background carbon levels, plenum
pressures of ~ 16-17 torr C2D4 are sufficient to produce visible emission.
These pressures correspond to ~ 3-4 x 10^16 molecules of C2D4 injected per
probe scan. (3) Neutral density filters are not necessary to equalize emission
intensity between the two legs of the beam-splitter. However, further analysis
may be required to determine if this is true over various discharges and at
various scrape-off layer depths. A ground loop problem causing false
triggering of the side view camera was discovered, and fixes implemented. The
system will continue to operate in piggy-back mode -- without gas injection to
monitor background carbon levels and effects of recycling off of the probe.
With gas injection, we will continue to optimize system parameters in
various plasma discharge conditions.

ICRF System
-----------

Following several hours of conditioning in vacuum, conditioning of the ICRF
antennas in plasma started on Wednesday last week. Individual antenna power
levels are up to 1-1.5 MW each, and up to 3.2 MW combined have been injected
into plasma for 0.7 seconds. Since the antenna conditioning is being
performed in parallel with machine discharge cleanup, plasma conditions are
not yet adequate for heating experiments and antenna comparisons. Disruptions
occur frequently, mostly triggered by the application of heating power, and
the hydrogen/deuterium ratio is still too high for hydrogen minority
heating. Internal surfaces heat up and outgas under these conditions. Wall
and antenna conditioning will continue this week after boronization of the
internal vacuum vessel surfaces.


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

The newly upgraded glow discharge system was tested on C-Mod last week. The
system was run in helium down to pressures of 8 to 10 mTorr, and with voltage
and current in the 300 volt, 3 amp range. No arcs were observable via the
wide angle TV system, though the new arc detection electronics indicated the
system was responding to arc events and cutting back the current in about 5
us.

We experienced a problem pumping out the low pressure side of the primary gas
regulator for the diborane system last week and were unable to boronize the
machine as planned. This problem was discovered while doing a full system
checkout in helium (no diborane installed). We made changes to the system
over the weekend to fix the problem, and expect to boronize early this week.
Boronization is clearly needed at this point to continue progress in RF
conditioning and physics experiments.

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

Ricky Maqueda (LANL) was at C-Mod last week, working with the new edge
turbulence imaging diagnostic which was installed during the last vent by
Stewart Zweben (PPPL). This diagnostic is continuing to produce images of edge
turbulence on a routine basis.