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

Alcator C-Mod Weekly Highlights
Mar 1, 1999



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

Plasma operations continued on Alcator C-Mod last week. Four run days were
scheduled and completed. Five physics experiments were carried out, along with
continued conditioning and development related to the D- and E-port ICRF
systems. A total of 85 plasmas were produced, with a reliability of better
than 85%.

Tuesday's run was devoted to study of ohmic H-modes. Two experiments were
carried out, both aimed at comparing behavior in ohmic H-modes with that in
the more commonly studied ICRF H-modes. Specifically, the goals of the
experiments were:

(1) Measure plasma rotation in ohmic H-modes to determine whether or
not the plasma rotation is due to RF specifically.
(2) Measure pedestal characteristics in ohmic H-modes to determine if
there are any differences due to RF fields in the edge region.

Ohmic H-modes were obtained at 1.0 MA and 0.8 MA, with a toroidal field of 3.5
tesla.

Toroidal rotation in the co-current (ion) direction was observed during the
ohmic H-modes. The stored energy and corresponding velocity increases are very
similar to the values found during some ICRF heated discharges, suggesting
that the rotation is associated with stored energy (or related some related
parmeter), regardless of the mechanism. Similar rotation for these discharges
has been inferred from magnetics measurements.

The soft x-ray pedestals in these ohmic H-modes are similar to those in
ELM-free H-modes with RF heating. At 1 MA current, there are no measurable
differences between RF- heated ELM-free H-modes and ohmic (ELM-free)
H-modes in terms of the width or position of the x-ray emissivity pedestals.

Wednesday's run was primarily aimed at developing more reliable simultaneous
high power operation of the D and E port ICRF systems. Operation above 2 MW
was achieved, but with faulting. FMIT#1's power feedback performed quite well
(maintained power within ~15% of requested for varying loads). Numerous
faults continued throughout the run. Vacuum conditioning was performed
following the run and it was found that the antennas were essentially
unconditioned. This was rather surprising since previous experience
suggests that the vacuum condition could be maintained for weeks of plasma
operation. This incident suggests the vacuum condition had become suspect
after a few days. It does however offer the prospect that the problems with the
RF can be solved with further vacuum conditioning.

Also on Wednesday, the plasma was moved vertically for evaluation of the edge
Thomson scattering system. Data were obtained on the edge Thomson system and the
soft x-ray array looking at the pedestal near the top of the plasma.

Some piggybacking with krypton puffing in support of the investigation of high
Z screening was also accomplished. Measurements were made with the divertor
RGA and HIREX. Data for atomic physics investigations of highly charged
krypton were also obtained on these shots, with both the McPherson and
HIREX. The krypton appeared to prevent access to EDA H-mode at ICRF powers up
to 2 MW, with type III ELMs being the norm.

The purpose of Thursday's run was to carefully measure conditions just before the LH
threshold in a range of conditions. We started with the standard 800 kA,
1.6e20/m3 discharge for a reference. Both scanning probe data and
Bt sweeps were obtained shortly before the transition.

A current scan was carried out at fixed density, up to 1.2 MA. The LH
threshold was at 1.2 MW RF at 1.0 MA and only 560 kW at 1.2 MA. Again, L-mode
Te profiles were obtained close to threshold. It appears there is at most a
weak Ip dependence of the Te threshold. We will have to look more closely at
this data, since sawteeth have a significant effect. The remainder of the run
consisted of a density scan at 1.2 MA. Several high density points, up to
nebar=2.5 e20/m3, were obtained. A surprise was that even a small density
increase, to 2e20, caused a large increase in the RF power required (to 1.75
MW from 0.5). This effect may have been related to the large gas fuelling required,
and high neutral pressures. The threshold temperatures seemed quite constant
with density. 2.2 MW was needed at the highest density. Data were obtained on
several edge diagnostics, including the soft x-rays and edge Thomson
scattering, in both L and H-modes.

Friday's run was in support of two experiments: MP#227, H-mode Ion Fueling,
and MP#229, Secondary Divertor Experimemts. During previous runs it has been
observed that the lower divertor neutral pressure "pumps out" during the high
dn/dt phase of an H-mode transition, particularly when the transition leads to
an EDA H-mode. Apparently, the H-mode is fueled, at least in part, by
neutrals in the lower divertor region. The goal of MP#227 is to determine
whether the fueling results simply from ionization of these neutrals above the
x-point region inside the last-closed flux surface or if the fueling involves
parallel plasma flows from the divertor to the core scrape-off layer. The
F-port and A-port scanning probes (FSP,ASP), which are outfitted with Mach
probe sensors, were used to look for changes in the parallel plasma flows
during the high dn/dt phase of L to H-mode transitions. Data were obtained in
the first half of the run at 0.8MA and densities of 1.6 and
2.0e20/m3. Additional relevant data was obtained during the second half of the
run, which was primarily dedicated to investigations of the effect of the
secondary separatrix.

Recent analysis of flows into the divertor chamber and ionization in the main
chamber indicates that most of the plasma in the main chamber recycles onto
main chamber surfaces, i.e, the limiters, the walls, and the top surfaces of
the lower divertor. Particle recycling fluxes in the lower divertor chamber
are large but well separated from the particle recycling that occurs on the
walls in main chamber. At first glance, this is not too surprising since the
closed lower divertor chamber is presently optimized for power flux handling
not particle flux handling. However, it has also been found that cross-field
particle transport radially increases with distance from the separatrix in the
SOL. This prompts the question whether the upper divertor, with its relatively
open geometry, could be used to influence the main chamber and divertor recycling
balance, or if the cross-field transport so high (relative to parallel flows
to the divertor) as to maintain the same coupling between midplane neutral
pressure and core plasma density, independent of divertor geometry. A related
question of operational interest is whether the neutral pressure in the
upper divertor is high enough for a cryopump located there to be used for
density control. In these experiments, we investigated the relationship
between neutral pressure in the upper divertor and SSEP, the separation of the
separatrices associated with the upper and lower x-points.

A series of discharges were run at moderate and low plasma densities and with
varying upper secondary separatrix position (SSEP). The principal diagnostics
were upper divertor, lower divertor, and midplane neutral pressures, and the
F-port and A-port scanning probes (FSP,ASP) to monitor SOL plasma conditions
and flows. For ne~1.6e20/m3, the upper divertor pressures were found to vary
with SSEP from around 1 mtorr with SSEP>15 mm to 20 mtorr with SSEP~0 mm,
i.e double null. These results imply that a cryopump in the upper divertor
could be used for dynamic density control. Detailed analysis of the probe data
is underway.

RF Systems:
-----------

Measurements of FMIT#4's input cavity indicate that this cavity had a problem.
The transmitter, which will be used with the J-port four-strap antenna, has
been completely disassembled for inspection. The same isolation capacitor
failed in a manner similar to that observed in June 1998. Several arc tracks
were found in the vicinity of the load isolation capacitor. The most
significant was located under the the collar, which makes up part of the
isolation capacitor, where the teflon insulator had been arced through. To
fix the problem, we need to replace the teflon tubing (which we have) and
repair the Al coax. The resassembly should be complete by Friday.

Measurements of test resonant coax configurations have been completed
on J-port. Determination of the proper lengths for 75 MHz and 78 MHz
will be completed shortly. Manufacturing of the coax can begin this
week.

Diagnostic Neutral Beam:
------------------------

Technical progress on the DNB remains satisfactory. The schedule is presently
four days behind the baseline due to underestimate of time required to lay out
circuit boards for low-end electronics for the high voltage modulator-regulator.

Physics Analysis:
-----------------

After the first boronization of this campaign, we observed the following
changes in the carbon and oxygen impurities: The carbon concentration was
reduced from the pre-boronization level by a factor of ~2.7, and the oxygen
concentration was reduced by a factor of ~6. These measurements were made
spectroscopically by looking at the impurity resonance lines. While the carbon
and oxygen emissions were decreased by the boronization, the atomic deuterium
emission from the divertor region increased after boronization, typically by a
factor of ~2.5 (at the same plasma density).

The first Enhanced D alpha H-modes of the 1999 campaign had some interesting
fast magnetic fluctuations that clearly increased during EDA and decreased
during the ELM-free portions of the discharges. Broadband activity increased
during EDA below 100 kHz and again between 150 and 200 kHz. There were also
coherent modes observed throughout most of the EDA phase with steady frequency
that varied from shot to shot. These coherent modes were typically between 20
kHz and 80 kHz. In some cases, there were two simultaneous modes at 47 kHz and
80 kHz during EDA. These modes were observed on the inner wall. The outboard
coils could barely make out these modes on some shots, perhaps indicating an
anti-ballooning structure or that the mode rational surface was closer to the
coils on the inner wall than to the outboard limiter coils and that the mode
amplitude decreased rapidly with distance from the mode rational surface due to
high poloidal mode numbers. On some shots, there were also high frequency
magnetic fluctuations at 300 kHz that were frequency modulated by the sawteeth.
In these cases, there were also observed simultaneous 800 kHz modes on the
outboard coils that were sampled at 2 MHz. Analysis of these fluctuations is
ongoing.

Travel and Visits:
------------------

Bob Granetz, Jay Kesner, and Jesus Ramos attended an MHD Crititcal Issues
Workshop at PPPL from Wed to Fri. While there, they saw Energy Secretary Bill
Richardson dedicate the NSTX, which just recently began operation.

Martin Greenwald and Tom Fredian attended a workshop on software sharing in the MFE
community at PPPL last week. The main area of discussion was the sharing of analysis
applications (such as TRANSP and EFIT), database applications (such as LOGBOOK and
LOCUS) and graphical tools (such as REVIEW+, FAS and IDL/RPLOT). This software
sharing has become feasible since many sites have standardized on the MDSplus
data system, a common relational database interface and the IDL data scripting
language.

On Monday (Feb. 22) Paul Bonoli was at PPPL for their Science Focus Group Meeting on
Wave Particle Interactions. He gave a 30 minute talk on non-inductive current
drive for Advanced Tokamak Applications.

Dave Johnson from PPPL reported on a new design for the X-point Thomson
Scattering diagnostic.