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
Sep. 7, 1999

Plasma operations continued at Alcator C-Mod last week. Three runs were
carried out in support of four miniproposals. The principal experiments were
focussed on edge and divertor issues, with continued ICRF development carried
out in background mode. The tokamak continued to operate reliably, with 69
plasma shots produced at a start-up reliability in excess of 90%.

Tuesday's run was dedicated to investigation of SOL transport during H-modes.
Although MP#228 did not originally call for ohmic H-modes, it was recognized
that ohmic H-modes provide a nice way to study H-mode transport under a
variety of toroidal fields which are not compatible with ICRF heating. We were
successful in getting a number of good H-modes during this run with the
scanning probes recording profiles of n, Te, parallel flow, and fluctuations
up to the separatrix. On a number of discharges, the scanning probes appeared
to poke into a 'shear layer'-like zone where the floating potential (Vf)
dropped dramatically (100 V/mm) and the fluctuations appeared to be
suppressed. The 'shear layer' Vf gradient also appeared to be much stronger in
the 'elm-free' phase. Observations of C+2 plume dispersal on a few selected
shots showed a qualitatively similar behavior: during 'elm-free' phases there
was some very strong poloidal dispersion while in EDA-like phases the poloidal
dispersal was much smaller. One should keep in mind that all these 'on the
fly' observations are preliminary and need to be carefully checked.

Wednesday's run was concerned with MP#223 and #224 - "Recombination and
Ionization in the C-Mod Divertor" and "Measurement of the plasma parameters
and their scalings in recombining regions of Alcator C-Mod".The run was
successful in accomplishing the following goals:

1) measuring the spatial extent and magnitude of the
volume recombination over a range of densities and "degrees" of
detachment
2) measuring the effect of the bypass flaps on the detachment
threshold (their state had no effect on the threshold)
3) measuring the effect of the bypass flaps on the main plasma
fueling rate
4) measuring the effect of N_2 impurity puffs on the detachment
characteristics.

These shots were all 1MA, 5.4T discharges, with densities scanned between 2
and 4.7e20/m3.

Thursday's run continued studies of the effect of the bypass flaps on divertor
impurity compression (MP#222). In contrast to previous work, a non-recycling
impurity (N2) was used for these experiments. We operated at three nominal
discharge densities (1.1, 1.6, and 3e20/m3). The N2 gas puffing position was
varied from the outside midplane, to the outer divertor, to the private flux
region, to the inside midplane. Discharges were run with either the flappers
open or closed for the entire discharge. No major differences were observed
in any discharges comparing flaps closed with flaps open. We focussed
primarily on nitrogen spectroscopy, i.e. chromex (N II) in the divertor and
main chamber and the MacPherson in the main chamber (li-like). Nitrogen
signals were also obtained on the divertor RGA, the omegatron, and visible
bremmstrahlung (which looks right at the AB limiter gas puff location). This
null result was obtained despite the usual factor of 2 effect of the flapper
on the divertor neutral pressure, and suggests that nitrogen does not pass
through the bypass (as recycling gases do, e.g. argon); this is consistent
with nitrogen being a non-recycling species.

We were hoping to observe the effect of the deuteron flow that the flapper
(when open) supposedly induces towards the plate....this might manifest itself
as better screening when puffing N2 from the divertor.....however, no effect
was observed. The most striking results of the day were related to the
different screening properties at different N2 release locations. N2 released
from the AB limiter was very effective at reaching the core, while releases of
N2 from the outer divertor, private flux region and inner midplane were all
much less effective. The inner wall result is surprising, but consistent with
earlier C-Mod results which have shown preferred streaming of injected
impurities at the inside midplane towards the divertor.

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

Results of recent absolute calibrations of core and edge Thomson scattering
systems exhibit small, but non-negligible, differences from the prior
calibration. The latest results from edge TS density pedestal measurements
show no clear scalings of the H-mode density pedestal width with various
plasma parameters, although a dependance of the maximum width on such plasma
parameters as current, midplane neutral pressure and input power is
observed. The relationship between pedestal height and central density in
ELM-free and EDA H-mode requires further study.


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

We have continued to work towards having a full complement of transmitters and
antennas available for plasma operation. For FMIT#1, a rebuilt high power
tube has been installed and operated up to 1.8 MW for 20 msec pulses into
dummy load. This power level was achieved with low oscillations. For RF
power greater than this, the self-oscillations increased dramatically. In
FMIT#2, the self-oscillation has been further reduced by retuning the input and
output cavity. Since both transmitters oscillate, we have begun working with
PPPL to understand and reduce this parasitic oscillation to an acceptable
level (where filament heating is not an issue).

The J-port antenna continued to clean up slowly during plasma operation. Flat
top power levels of up to 1.8 MW were achieved in a piggyback mode. Plasma
heating is observed in a greater fraction of the discharges and under
different phasing settings.


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

Satarou Yamaguchi, Haruhiko Okumura, Takashi Eura and Yasuhiro Hasegawa visited
from NIFS to discuss data systems collaboration between Alcator C-Mod and LHD.

Martin Greenwald attended a meeting of the ESnet Steering Committee
(ESSC) on which he represents the interests of the fusion energy
community.