From: "Stephen M. Wolfe" <wolfe@psfc.mit.edu>
Reply-To: wolfe@psfc.mit.edu
Organization: MIT Plasma Science and Fusion Center
Subject: Alcator C-Mod Weekly Highlights
Newsgroups: sci.physics.plasma


Alcator C-Mod Weekly Highlights
May 19, 2003

Plasma Operations continued at Alcator C-Mod last week; three run days were
scheduled and completed. The first boronization of this campaign was
successfully carried out. Progress also continued on the Lower Hybrid System.
Plasma operations are planned to continue this week.

A DoE review of the C-Mod renewal proposal for the next five year period
(November 2003 - October 2008) was held at MIT on Tuesday and Wednesday of
last week. The review panel included: Jim Luxon (GA, Chair), Don Batchelor
(ORNL), George Cava (DoE Princeton area office), Dave Hill (LLNL), Bob Kaita
(PPPL), Sergei Krasheninnikov (UCSD), Fritz Leuterer (IPP Garching), Takahisa
Ozeki (JAERI), and Doug Post (LANL). Attending from DoE OFES were John Willis
and Rostom Dagazian. Viewgraphs from the presentations can be found at:
http://www.psfc.mit.edu/cmod/sciprogram/5_Yr_Review_03/00_agenda.html


Operations
----------

The C-Mod vessel was boronized on Monday, May 12, for the first time this
campaign. An average layer thickness of approximately 2000A was
deposited. Plasma operation resumed on Wednesday. Two days were primarily
devoted to ICRF antenna evaluation (MP#326), with additional experiments run
in "piggyback" mode. One day was devoted to low voltage startup scenarios
(MP#329). A total of 51 plasma discharges were produced. Excluding the
start-up studies run, startup reliability was over 85%.

Physics
-------

On Thursday, an operational development miniproposal was run which aimed at
obtaining a low-voltage startup without commutation of the precharge coil
currents. The objective was to obtain a current ramp that is better
controlled because of being at a slower ramp rate, and possibly to dispense
with the need for commutation. This would be very helpful if it gave improved
reliability and would certainly be useful for studies of profile tailoring by
current ramping. Field configuration control was demonstrated and successful
initiation of full plasma discharges was obtained in the new configuration.
However, the initiation is not yet robust and more development will be needed
before it can be used during routine operation.

During Friday's run, the PSI-4 camera was used to image the Li+ ablation
trails from injected lithium pellets. During these piggyback
experiments, pellets were injected into 17 discharges, during the early
part of the current rampdown, at 1.6 seconds. The PSI-4 camera was used
to take images at frame rates between 100,000 and 1 million frames per
second. The main goals of these experiments are to examine the fine
radial structure of poloidal plasma motion as the pellets cross
radially, and to measure internal magnetic field profiles. Evaluation of
the data is ongoing. These data were the first to be taken on C-Mod with
a new image intensifier installed on the PSI-4, and also allowed us to
bring into operation the linux-based data acquisition for the camera. We
expect to continue these experiments in the coming week, as well as to
use it to image fluctuation striations in background neutral deuterium
light at the outboard midplane, in collaboration with Stewart Zweben
(PPPL).

A systematic analysis of plasma profiles in the edge plasma has been
performed, drawing on high spatial resolution fast-scanning probe and edge
Thomson data collected over a wide variety of discharges including Ohmic
L-mode (forward and reversed magnetic field), and Ohmic H-mode (Elm-free and
EDA regimes). The scanning probe and Thomson data are found to match
reasonably well at the separatrix and show the same values of an increased
electron pressure gradient scale length at increased plasma densities (in
otherwise identical discharges). These observations lend confidence to the
data set. Focusing on data collected in the Near scrape-off layer (i.e., ~2
mm beyond the last closed flux surface), two interesting and perhaps
fundamentally important observations have been made:
(1) Despite the wide range of external control parameters in these discharges
(plasma current, toroidal field, density) the local pressure gradient scale
length in Ohmic L-mode discharges appears to depend solely on the local value
of the 'diamagnetic parameter', $\alpha_d$, a parameter that has been
identified to regulate the character of drift-resistive ballooning turbulence
in 3-D turbulence simulations.
(2) For fixed values of $\alpha_d$ in the Near SOL, the magnitude of the local
electron pressure gradient is found to scale as plasma current squared,
precisely the scaling required to maintain the MHD ballooning parameter,
$\alpha$, invariant. Thus the Near SOL plasma lies on a well defined
'trajectory' when plotted in ($\alpha$,$\alpha_d$) space. These observations
offer very strong evidence that gradients in the Near SOL are set
predominately by drift-resistive ballooning mode physics.

H-mode discharges appear as a jump to a new trajectory in
($\alpha$,$\alpha_d$) space, while reversed magnetic field discharges lie on a
trajectory with reduced values of $\alpha$ compared to normal magnetic field
discharges. The latter result suggests that other parameters influence the
($\alpha$,$\alpha_d$) state of the near SOL, such as poloidal velocity shear,
which is found to be reduced in reversed magnetic field discharges. Further
analysis is in progress.

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

Two days of plasma operation were run to investigate high power antenna
operation in support of MP326. The recovery from the boronization has
proceeded slowly. During H-modes, D and E-port have reached ~1 MW and J-port
~1.6 MW. Further vacuum and plasma conditioning is required. Some
measurement problems, particularly with D and J-port, require further testing
of the instrumentation, which is planned for next week.

Lower Hybrid System
-------------------

The circulator has been mounted on Cart 3, klystron V1, and will be ready for
testing next week. The LH Low Power Microwave Rack cabling (except for new
microwave relay chassis cables) was installed. Tests will start soon to
characterize each of the twelve channels (without SFOLs). The master
oscillator chassis has been reinstalled and connected. A test board was built
which will allow us to change phase and amplitude of one channel at a time
using the vector modulator. The I-Q detector outputs will be read directly
with a digital meter for initial tests. It is important to determine the
output levels of the completed system before the cables to the transmitters
are selected to insure that we have adequate drive.


DNB System and Diagnostics
--------------------------

A study of apodization effects [integration over a non-integral number of
waveform periods] on the analysis of data from the C-Mod Motional Stark Effect
(MSE) diagnostic has been performed and has led to improved precision of the
measurement. Apodization effects of a constant noise source have been removed
by zero-centering the data on each time window. The effect is to reduce the
variation in measured angle over successive measurement time intervals by a
factor 2-3 compared to the previous analysis technique. Using the new
analysis technique, the uncertainty in mean angle in the frame of the
polarimeter, as measured by the time variation in the measured angle
throughout the 50 ms beam pulse, is approximately 0.02 - 0.06 degrees. In
moderate density plasmas (nebar = 1.0 10^20 m^-3), the corresponding
uncertainty is 0.03 - 0.10 degrees.

Software has also been developed to remove the apodization effects
introduced by the beat frequency of the two PEM drives, but this effect
appears to be small and does not appreciably improve the measurement
precision.

On Friday, May 9, the DNB began to operate with very low beam current, ~2
amps, despite no changes in settings from the previous run day. Tests
indicated a sensitivity to the anode gas timing. The anode gas valve in the
source was removed and replaced, but the low beam current persisted. After
consultation with Budker, the source pressure was reduced from 1.5atm to
~0.8atm, resulting in improved beam current, up to 4 amperes. Several shots
were fired into C-Mod plasmas late on Friday. Additional tuning of both the
anode and cathode gas pressures may further improve the beam performance. The
reason for the change in beam behavior is not yet known.