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
July 30, 2001

Plasma operations continued at Alcator C-Mod last week. Five run days were
scheduled and four and a half were completed. On what was locally reported as
the hottest day of the year so far, a water chiller failure caused
temperatures in the cell and power room to rise to levels that affected data
acquisition hardware and caused shutdown of the RF transmitters, forcing
Wednesday's run to be stopped early. Runs on Thursday and Friday were extended
to 7:00PM. A total of 73 plasma shots were produced last week, with a startup
reliability of about 75%. Experiments carried out included a study of neutral
particle control and recycling in Helium discharges, non-dimensional
similarity studies for comparison with ASDEX-Upgrade, L-H and H-L transition
dynamics, and continued studies of the double transport barrier mode with
off-axis ICRF.

Plasma operations will continue this week, which is scheduled to conclude the
2001 C-Mod Experimental Campaign.

Physics
--------

Following GDC in helium, boronization, and ECDC in helium carried out over the
July 20-22 weekend, Monday's run was devoted to a study of neutral particle
control, fueling, and gas inventory experiments using helium plasmas (MP297).
The goals of these experiments were to quantify the relative contributions to
the midplane neutral pressure of divertor neutral leakage and main-chamber
plasma recycling; and to perform full accounting of the fuel particle
inventory in the vessel including gas and plasma phases in the lower divertor,
main-chamber, and upper chamber regions. The experiment consisted of opening
and closing sets of the divertor bypass "flappers" with a period of 0.2sec
during the discharge, while monitoring the effect on plasma density and local
neutral pressures. Which sets of flappers were operated was varied from shot
to shot. There is a narrow operating window in helium plasmas where there is
sufficient helium pressure in the divertor (> 40 mtorr) to allow the flappers
to 'fuel' the plasma yet the divertor is not detached, and some time was spent
optimizing conditions. It appears that the response of helium plasmas to the
flapper opening is very similar to deuterium plasma, suggesting that wall
pumping does not affect the gas-fueling transient that occurs when the flapper
is opened. As in the deuterium plasma, the midplane pressure is more tied to
the line-averaged density than the magnitude of the local divertor bypass
conductance. While most of the shots were run with normal feedback on the
line-averaged density, we also did two shots with a fixed pulsed gas
programming. This resulted in a noticeably smaller perturbation in the
line-averaged density in response to the flapper modulation, which suggests
that lag in the pulsed gas affecting the plasma combined with the chosen 200
ms flap period might have amplified the influence of the flapper on the
line density. This effect will have to be taken into account in the analysis.

Runs on Tuesday, Wednesday, and Friday were devoted to additional studies of
the double barrier mode plasmas with off-axis and on-axis ICRF heating
(MP#289A). These experiments have been a major focus of this phase of the
current campaign, making use of the J-port four-strap antenna operating at
70MHz and the older dipole antennas at D- and E-port at 80 and 80.5MHz. This
week's runs investigated the effect of the relative timing and power levels of
the off- and on-axis heating in forming the internal transport barrier,
arresting the central density and impurity peaking, and heating within the
ITB.

Two experiments were carried out during an extended run on Thursday. The goal
of MP287 was To make high density, high power EDA H-modes in a shape identical
to that in ASDEX-Upgrade 'Type II ELM' discharges. This regime, which was
only recently obtained on AUG, appears rather similar to that obtained on
C-Mod when power is increased into EDA H-modes. At reduced powers, AUG
sometimes sees a 'quasi-coherent' mode. We would like to see if the same
behavior is observed on the two machines with the same dimensionless
parameters. AUG will then try to match the dimensionless parameters
(nu*,beta, rho*) of these discharges. We completed the first three parts of
the experiment. The machine conditions were much better than on 7/19, with
H/D typically about 6-8%. Once again, we found that C-Mod does not get
steady, EDA-type discharges at this shape, and also that RF coupling is much
more difficult (the two effects may be related). On some shots a weak
quasi-coherent mode was seen on PCI, but it was not strong enough to cause
large particle transport. The optimum density seemed to be nel=9.5e19 m-2; at
higher targets the gas pressure increased substantially. H-mode density
became steadier, but seemed to exhibit bursts of higher transport, reminiscent
of the typical behavior with Type III ELMs and a cold edge, though the ELMs
were not immediately obvious. An H-mode shot which was not steady-state but
had moderate and steady increases in density and radiation was obtained, which
should be suitable for ASDEX-Upgrade to attempt to match. We completed this
phase of the experiment by taking a slow power ramp to get the threshold power
and edge parameters at identical target conditons. This will be suitable for
AUG threshold comparisons.

The second part of Thursday's run was devoted to L-H-L hysteresis
studies. Plasmas were 800 kA and of a fairly standard C-Mod shape. All
discharges gave very steady EDAs. The RF was programmed with a triangular
waveform from 0.4 to 2MW, and then back to zero. Plasmas stayed in H-mode down
to very low RF power; there was typically a short period of type III ELMs
before the back-transition. Good edge TS data were obtained, which we will
compare to previous results with the ECE and visible bremsstrahlung. The goal
is to examine the hysteresis curve of input flux vs various gradients, such as
grad T and grad P/n. This is being compared to predictions of various H-mode
theories, in collaboration with Ben Carreras of ORNL. Results will be
presented at the Transport Barrier workshop in September.

More results have been obtained in our study of edge turbulence
using the ultra-fast framing camera. We have observed the evolution
of edge/SOL turbulence on a 4 microsecond timescale. Twelve sequential
images, each with high radial and poloidal resolution of the emission
from a gas puff at the plasma periphery, are taken. "Blobs" of emission are
seen moving poloidally and primarily outward radially. They appear to be
generated near the separatrix. Their lifetime is of order 10 microseconds
in which time they move of order ~1 cm, which is also their typical
size scale. Some of these facts were known previously, e.g. size and
lifetime, but what is new and intriguing is the observation of their
birth, transport, and disappearance. Similar 'blob' behavior is seen in both
L- and H-Mode confinement, at least in the SOL. MPEG movies, each showing
total time intervals of 48 microseconds, are available on the WEB at

http::/www.psfc.mit.edu/people/terry/MPEG_1010726007.mpg
http::/www.psfc.mit.edu/people/terry/MPEG_1010726011.mpg
http::/www.psfc.mit.edu/people/terry/MPEG_1010726015.mpg
http::/www.psfc.mit.edu/people/terry/MPEG_1010726017.mpg

Operations and Engineering
---------------------------

A new shunt for the OH2L bus was fabricated in-house, has been installed and
tested, and is now opertional. The commercial OH2L shunt which failed last
week has been returned to the manufacturer and is in the process of being
opened for an inspection.

The new bypass snubber resistors in the "EFC" chopper supply are holding up
well with no failures to report. Additional resistors to replace the
remaining old-style ones have been ordered.

ICRF Systems
------------

Post-boronization, the antennas were re-conditioned in He plasmas.
The J-port voltage maximum was observed to be ~30 kV for 70 MHz
operation compared to ~25 kV at 78 MHz. The J-port antenna coupled 3 MW and
appears to be reliable when the maximum voltage is <30 kV. The combined D, E and
J-antennas injected 5.5 MW last week.

The antenna fault system on J port was changed from voltage
probe input to current probe input. Phase limits were decreased from +/-90
degrees back to +/- 65 degrees. Spurious phase balance faults have been
virtually eliminated at J since this change.


DIagnostic Neutral Beam Systems
--------------------------------

The DNB did not run consistently this week. Due to the high heat in
the cell, the suppressor control circuitry failed. An intermittent
heat sensitivity had been observed, but we were unable to find the
source of the problem until this failure. The suppressor is expected
to be back in operation on Monday and to continue operation through
the final week of the campaign. The beam penetration/reionization
model was successfully bench marked against BES data. This will be
critical for pursuing beam mods during the shutdown period.


Inner Divertor Fabrication Project
----------------------------------

Fabrication of all the probe box and mock-up parts presently designed has been
completed. Also, modification of the Tile Support Plates that go into the
C-Plate pockets has begun; several mills of material need to be removed from
each side of the tile support plate to make plates fit loser into the
pockets. Mock-up building is moving forward and we continued to assemble
components on the Mock-up wall.

Lower Hybrid MIE Project
------------------------

EMI/RFI shielded racks for transmitter carts and CPS arrived and work to
install equipment into the racks started. Relocation of the klystron air flow
blowers on the carts has begun. One of the three Transmitter Protection
System (TPS) optical interface boards was sent out for fabrication. Wiring
for one of the TPS chassis was completed, and the chassis was installed in the
EMI/RFI rack.

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

Bruce Lipschultz attended the Int'l meeting on Plasma Edge Issues
(July 10-11th) at the the JAERI Naka site. There were 5 sessions at
this meeting with a number of talks. The sessions were on high
density operation, ELMs, main chamber recycling, SOL flows and high-Z
materials experience. Bruce Lipschultz gave a presentation on main
chamber recycling work at C-Mod and DIII-D as well as a presentation
on high-Z wall operation in C-Mod. He also chaired the main chamber
recycling session.

He also spent time (July 10th-13th) with collaborators N. Asakura and
T. Nakano at JT-60U on US-Japan collaboration FP3-5, "Comparison of
main chamber recycling in C-Mod and JT-60U'. The purpose of this work
was to compare and contrast JT-60U and C-Mod SOL data and plan
towards joint experiments on JT-60U and C-Mod. Diagnostics and
experiments were idenstified for the September run period on JT-60U.
Data is now being taken on C-Mod.

Professor Yuichi Takase from the University of Tokyo visited MIT on
Friday (July 20, 2001) and worked with Paul Bonoli on full-wave
modelling of high harmonic fast wave (HHFW) heating in the NSTX
device. They also discussed internal transport barrier experiments
(ITB) in Alcator C-Mod.

Paul Bonoli visited the Princeton Plasma Physics Laboratory (PPPL)
from Monday - Thursday (July 23-26, 2001). He worked with Dr. Cynthia
Phillips and Yuichi Takase on HHFW heating in NSTX and he gave a talk
at the weekly NSTX Group Meeting titled "Full-wave modelling of HHFW
heating in the NSTX device". Paul Bonoli also attended a 2 1/2 day
planning meeting for the RF Sci-Dac Initiative (Scientific Discovery
Through Advanced Computing). The meeting was hosted by Cynthia
Phillips (PPPL) and was attended by representatives from ORNL,
Lodestar, PPPL, Mission Research Corp., Comp-X, and MIT. The meeting
was extremely productive. A three year workplan was formulated and
physics issues related to our proposed problems were discussed in
detail.

Martha Redi and David Mikkelsen, PPPL, visited CMOD last week, July 23-27,
Martha worked with Catherine Fiore on gyrokinetic modelling of dual RF
experiments. David worked with Steve Wolfe to learn more about how EFIT can
be used to estimate the q profile inside C-Mod plasmas.

Dan Kellman of GA visited Josh Stillerman and Joe Bosco to discuss our CAMAC
system and the new CPCI-based system. He also met with Dave Terry and Bill
Cochran concerning power systems issues. Dan has worked at GA on their chopper
supplies and other power systems and has been a point of contact regarding our
own chopper supply.