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
Mar 8, 1999

Operations
----------
Plasma physics operations continued on Alcator C-Mod last week. Four run days
were scheduled and completed. A total of 76 plasma shots were produced with a
startup reliability of 84%. These runs supported six principal physics
experiments.

Last week concluded the first block of physics runs of the 1999 campaign. The
facility is now in a scheduled maintenance period. A major focus of this
period will be readying the J-port ICRF system for operation.

Physics Runs:
-------------

The runs on Tuesday and Wednesday of last week were devoted to studies of the
effects of neutrals on H-mode behavior, including an experiment on thresholds
and one involving the EDA/ELM-free boundary. The divertor bypass flappers were
used to control the backflow of neutrals from the divertor to the main
chamber. Conditions in the pedestal/edge region were carefully monitored using
a variety of diagnostics. Evaluation of the plasma behavior on Tuesday
indicated that the effects of the boronization carried out on February 12,
1999, were wearing off. A fresh boronization was conducted on Tuesday night,
with an average layer thickness of 980A deposited; the ECDC discharge
resonance was run well out past the limiters and antennas, to a radius of
1.03m. Flapper experiments then continued in the boronized machine on
Wednesday. RF power was limited to about 1.5MW on both days due to problems in
the D-port transmission line. Preliminary conclusions from these runs are that
the flappers do not influence midplane pressures or Halpha, or confinement.
Divertor pressures and compression ratios are affected, with divertor pressure
reduced by a factor of 2 with the flappers open. Based on this week's runs,
the EDA/ELM-free nature of the discharge is not affected by opening the
flappers. More work needs to be done to investigate the effects on the L-H
threshold.

Thursday's run was divided between two half-day experiments. The first
concerned the dependance of pedestal width on dI/dt. A secondary goal of this
experiment was to measure the electron temperature pedestal width at two
different currents on the same shot. The secondary goal was accomplished,
with the expected result that the Te H-mode pedestal width is indeed
independent of plasma current. The primary goal was only partially
accomplished, with some observations of a dIp/dt dependence of the xray width
at 0.8 and 1.0 MA. The measurements are not as clear as hoped, primarily
because the maximum available RF power (1.0 MW) was not enough to give
long-duration H-modes.

The second half of the run was devoted to a detailed cross-comparison of the
various edge diagnostics. To this end, we wanted to obtain a complete set of
edge profiles with all available edge diagnostics, the primary ones being,
asp, fsp, ECE, Edge Thomson and Kaiser helium "beam" probe. Good EDA H-modes
were obtained with ICRF power of 1MW and target densities (pre-Hmode) of 1.2
and 1.6e20/m3; a L-mode case at nebar of 2.1e20/m3 was also obtained. The
primary edge diagnostics (including the new Kaiser spectrometer- helium beam)
all worked. Comparisons of results over a range of densities, and in ohmic
and EDA H-mode plasmas are now being carried out.

The run on Friday was also divided between two half-day experiments. The first
was to complete the rotation studies in ohmic H-modes which were begun last
week, by extending the range of currents studied. Good rotation data was
obtained at 1.0 and 1.1 MA. Magnetics rotation data was also obtained at 0.8MA
but the Argon line intensity was very low, making the xray Doppler measurenent
problematic. A He-like line may be needed for the low current data.

The second half of the run was dedicated to MP #222, "Impurity Compression and
Enrichment". This proposal is an attempt to extend the compression and
enrichment database to gases other than argon and investigate the effect of
the divertor bypass on impurity behavior. It was also hoped that argon
compression scaling with density in Ohmic plasmas from the 1997-98 run
campaign could be reproduced. The plan for this half-day was to perform a
density scan with puffed argon and krypton in Ohmic plasmas, starting with low
density and increasing until the detachment threshold was passed; and to pick
an intermediate density and do a divertor bypass scan, i.e., bypass closed,
open, closed->open, and open->closed. The run was successful in covering the
above points. Argon and krypton were puffed into Ohmic plasmas with nebar
ranging from 1e20 to 2.25e20/m3 with Ip=0.8MA. The detachment threshold for
these plasmas is ~2.2e20/m3. The scaling of argon compression with density is
similar to that obtained in the previous run campaign. However, the
compression values are reduced by a factor of ~2. The krypton data is yet to
be analyzed. The core krypton levels are awaiting some modelling. The
divertor bypass does indeed have an effect on the impurity behavior. The
difference on the argon compression with the bypass open and bypass closed is
a factor of about two. The compression is higher with the bypass closed.
This is due to less argon in the core and more argon in the divertor. Opening
and closing the bypass during a discharge reproduces these results
dynamically.

Diagnostics and Analysis:
-------------------------

The new edge Thomson scattering diagnostics is now running, measuring
electron temperature and density profiles around the separatrix. Although some
further calibration and analysis is needed, the preliminary analysis shows that
the system is capable of measuring the profiles in L-mode as well as H-mode.
Formation of H-mode pedestal is observed, the Ne pedestal width being typically
of 4-6 mm, centered on the separatrix.

Ohmic H-mode runs at low q provided a good opportunity to measure the core
plasma rotation with sawtooth postcursor oscillations from fast magnetic
pick-up coils. Because the sawteeth were somewhat larger at low q and the q=1
surface is closer to the pick-up coils at the wall, the m=1, n=1 mode coupled
out to the edge with sufficient amplitude to measure the rotation frequency and
direction in both L-mode and H-mode. The n=1 postcursor frequency varied from
about -3000 Hz in L-mode to nearly 8000 Hz in ELM-free ohmic H-mode. The HIREX
rotation frequency from the same shot, dividing the rotation velocity by
2*pi*67 cm, varied from about -10 kHz to nearly 7 kHz. So, the change in
rotation is similar, though not quite the same with the two diagnostics.
Taking into account the error bars and possible offsets, the agreement is
reasonably good. The high time resolution of the magnetic pick-up coils
shows that the rotation changes sign within 8.5 msec, from one sawtooth to the
next, on either side of the L-H transition. The core plasma rotates in the
electron direction in L-mode and in the ion direction in H-mode.

Comparison of data from two scanning Langmuir probes shows that parallel
Ohm's law holds in the SOL of ohmic plasmas. The data was taken during last
year's run campaign by the fast scanning probes on F-bottom port (FSP) and
A-side port (ASP), during simultaneous scans on which the two probes lay on
the same field line. The two probes measured different pressure profiles,
different temperature profiles, and different plasma potential profiles,
yet the parallel Ohm's law, which relates the parallel gradients in these three
quantities, was approximately satisfied.

The simultaneous measurements of the soft x-ray emissivity at two poloidally
separate parts of the plasma edge show that the x-ray emissivity varies
significantly along flux surfaces in the region just inside the separatrix
during H-mode. These results are obtained by inverting the chord integrated
brightnesses assuming that the emissivity IS constant on a flux surface. Thus,
a systematic error is introduced. The size of this error has been estimated by
using simulated data which have a poloidal variation of the emissivity along a
magnetic field line similar to that seen in the experiments, and is found to
be up to 15% for the absolute value of the emissivity. The position and width
of the pedestal are reproduced with less than 3% error, if one chooses to
compare at the location where the viewing chords are tangent to the surface
corresponding to the bottom of the emissivity pedestal.

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

D and E-port ICRF systems continued to struggle this week. Arcing in D-port was
identified: a mechanical problem in a 9" coaxial elbow was causing arcing and
the stub tuner rod had arced. The elbow problem can be solved by reducing the
anchor bullet teflon thickness to allow better electrical connection. The
tuner problem occured several years ago and the problem was thought to be
local heating in the drive rod material. The original material was changed to
G-7 which has a lower loss tangent. The design of the connection however was
unchanged. Several design changes are being considered to reduce the E-field
enhancement that may contribute to breakdown at this location. The antenna
conditioning problem also resurfaced this past week. Unlike previous
campaigns, the antenna voltage condition is deteriorating during the course of
a day and deteriorates after boronization. The faults that consistently
appear are phase balance faults. Discussions are focused upon gas retaining
material in the antenna box.

For the J-port system, the 78 MHz resonant loops were constructed and the
resonant frequency is 77.8 MHz. The required decoupling stub length is 11.3"
(electrical length). The 60 MHz loops were measured and the resonant
frequency is 59.8 MHz. Four straight coaxes need to be manufactured to
complete this loop. The decoupling stub length will be determined by week's
end. Both decoupling stubs need to be manufactured. Measurements for the 75
MHz will be also be made this week. Reassembly of FMIT#4 is at the final
cavity assembly stage. The isolation capacitor was rebuilt and measured to
have 1.3 nF (it should be nominally 1 nF). Analysis by Chris Brunkhorst
(PPPL) suggests that this will have no adverse affect on tuning the
transmitter to 78 MHz. Reassembly should be done early this week.

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

Technical progress on the DNB continuse to be satisfactory. We will take
advantage of the present two week shutdown to complete some tasks in the
power system room and the C-MOD cell.

Travel:
-------

Tom Fredian is visiting the Australia National University where he will
be installing the MDSplus data system for the H-1 Heliac device. His
local contact is Boyd Blackwell. With this installation, MDSplus will
be in use on four continents.