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
June 19, 2000

Operations
------------

Plasma operation continued on Alcator C-Mod last week. Four run days were
scheduled and completed, with a total of 80 plasma shots produced, with a
reliability of 90%. Experiments were carried out in support of MiniProposals
submitted by the Core Transport, Pedestal Physics, and High Performance task
groups, as well as to investigations of ICRF antenna performance, and first
operation into C-Mod of the DNB running a hydrogen beam.

Plasma operations are scheduled to continue this week.

Physics and Analysis
--------------------

In preparation for the AT experiments on C-Mod, we will need to be able to run
at lower density. However, the presence of H-modes during Lower Hybrid Current
Drive may complicate matters seriously. Experiments carried out on Tuesday
were designed to try to understand and document the H-mode low density limit,
and project this case to AT discharges. These experiments will also increase
our understanding of the implications/effects of neutrals at low density. The
machine was prepared by overnight ECDC (electron cyclotron discharge cleaning)
in Helium, as opposed to deuterium which is normally used, in order to reduce
the gas load in the walls. We succeeded in obtaining low density discharges,
down to 3.5e19/m^3. As the density was reduced below 1.e20/m^3 the H-mode
threshold first decreased, and then again increased for nebar< 7e19/m^3. At
those densities the H-modes were brief, with little increase in density, but
the edge electron temperatures and Dalpha showed clear H-mode signatures. It
was clear that the H-modes were not being fuelled efficiently if at all and
the H-mode phase was quite transient. Attempts to go lower in density, below
the low-density limit, pushed us to very low values (for C-Mod),
nebar~3.5x10^19/m^3. At those densities, we encountered a regime in which the
sawteeth were stabilized, Zeff went up, but radiation stayed low (or even
decreased a bit); in this regime both particle and energy confinement were
poor and disruptivity was high. Operation at this low a density was therefore
problematic, at least with the present wall condition. Data from these
experiments are being analyzed.

A study of spontaneous formation of an Internal Transport Barrier (ITB)
following H-L transitions was carried out on Thursday. The run was successful
in reliably producing ITB's at a range of plasma currents, toroidal fields and
RF power levels. Some of these barriers were long-lasting (close to
0.1sec). Unfortunately, CAMAC problems prevented us from obtaining ion
temperature and rotation profiles under these conditions. The available data
is being analyzed, and related additional experiments are planned.

The High Performance task force carried out an experiment to maximize stored
energy and neutron rate using all available RF power from the D- and E-port
antennas. The purpose was to benchmark the current performance and to document
core and pedestal profiles in a series of EDA H-mode discharges. We varied the
target density and varied the gas puff program during the H-mode phase; good
EDA H-modes were obtained at currents of 1, 1.1, and 1.2MA. Stored
energies above 200kJ were obtained, for the first time in the current
campaign, at a plasma current of 1.2MA and RF power of ~3.5MW. Radiated power
levels were relatively high (>50%), which was not unexpected in view of the
fact that the last boronization was two weeks ago; gas puffing during the
H-mode seemed to reduce P_rad somewhat. On some shots, coherent MHD activity
was observed on the fast magnetics diagnostics.

Experiments were run last week to test the hypothesis that the density limit
in tokamaks is due to a "transport collapse" rather than a radiative
collapse. Measurements of edge fluctuations were made with a
scanning probe as the density limit was approached. An increase in
the level of fluctuations (dn/n and dphi/Te) was observed inside the
separatrix. Further analysis showed that a region with high levels
of turbulence and long correlation times moved from the far SOL
region into the edge plasma. Additional planned experiments
should allow direct measurement of the particle and energy flux
convected by these fluctuations.


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

A major part of one run day was dedicated to a series of measurements
comparing the performance of PPPL's J-Port ICRF antenna to the antennas at D-
and E-Port. All three antennas were separately ramped from 1-250 kW and these
ramps were repeated with different outer plasma gaps, from 0.4-2.5 cm. In
L-mode discharges, the heating efficiency of the D- and E-Port antennas were
measured to be 75-80%, while the heating efficiency of the J-Port antenna was
50-55%.

In addition, all three antennas were pulsed in the vacuum conditioning
mode with BT~3 T and ~1 mtorr of deuterium present. This test
attempted to determine if an internal glow discharge could be
responsible for the apparently missing power in J-Port. Some internal
plasma was produced by all three antennas as observed by light
emission.

In support of the high performance MP, the RF power from D and E antennas set
a new record of 3.6 MW of injected power. The reliability of the RF systems
was very good, but at these power levels the power supplies, which are
operating near the maximum they can deliver, caused tripouts of one
transmitter on some discharge attempts.

We have now reconfigured the J-port antenna as a 2-strap antenna by shorting
straps 1 and 4 at a high voltage point and feeding straps 2 and 3. We will
repeat the same experiments we made last week in this configuration. This
experiment should help in indentifying the parasitic mechanism responsible for
the lower heating efficiency of the 4-strap array.

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

Bob Childs traveled to Wall Colmonoy Corp. in Dayton, OH 6/14-17 to H2 fire
new copper felt metal sheets to be used in the fabrication of TF Magnet spare
components. .

Joe Snipes is at JET this week to participate in an experiment, scheduled for
this Wednesday, to try to access the EDA H-mode regime, as seen on C-Mod.

Raffi Nazikian (PPPL) came to MIT 6/13-14 for discussions on the C-Mod
reflectometer upgrade he is preparing. Stew Zweben (PPPL) came 6/15-16 and
brought all the optical components of the new edge turbulence imaging
diagnostic in order to prepare them for in-vessel installation during the July
opening.