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
August
6, 2001
Plasma operations continued at Alcator C-Mod last week, the
final week of the
2001 Experimental Campaign. Five run days were scheduled
and completed. All
runs were extended by an extra two hours in order to
maximize the data
obtained from each experiment. A total of 107 plasma
shots were produced with
a startup reliability of over 87%. Experiments
carried out included two MP's
related to double barrier/ITB formation, a
study of pedestal parameters and
the QC mode in EDA H-mode discharges,
exploitation of C-Mod's long-pulse
capabilities, and plasma operation at
8T for the first time since 1998.
Having completed the 2001
experimental campaign, C-Mod is now entering an
extended period of
maintenance, inspections, and facility upgrades.
Physics
-------
This
week was extremely successful from the standpoint of learning more about
plasma
conditions under which the density barrier and internal transport
barrier
(ITB) modes can be produced in C-Mod. The density barrier mode was
produced
with off-axis ICRF heating (70 MHz) on BOTH the tokamak high field
side
(HFS) at 3.9 T and the low field side (LFS) at 5.4 T. Preliminary
attempts
were also made at heating the LFS density barrier with on-axis ICRF
power
at 80 MHz.
The density barrier mode was first produced at lower B_t
= 3.9 T using 1.5 MW
of off-axis ICRF heating power from the J-port
antenna, driven at 70 MHz. The
central density was observed to rise to
ne(0) = 6 X 10^20 m^{-3} and the
toroidal rotation velocity (v_tor) from
HIREX decreased to zero and reversed
direction, becoming
counter-current. Interestingly, the
value of B_t where
the barrier formed at 70 MHz, scales nicely from the
critical B_t for barrier
formation at 80 MHz, that is:
B_t = (70 MHz / 80 Mhz) * 4.5 T = 3.93
T.
This places the minority hydrogen cyclotron resonance at the same
minor radius
on the high field side (12 cm) as it was at 4.5 T and 80
MHz.
A density barrier was also produced with the 70 MHz ICRF
resonance on the low
field side (LFS) at 5.4 T. Only 1.35 MW of ICRF power
from J-port was
necessary. ITB formation was again characterized by
peaking density profile
with ne_TS(0) = 6X10^20m^{-3} and ne(0) / ne(ave)
> 2. Formation of the
density
barrier at both 3.9T and 5.4T was preceeded by a transition to an
H-mode
phase, and the H-mode edge is maintained throughout the ITB evolution.
We
also attempted to heat the LFS density barrier mode at 5.4 T using the
available
80 MHz ICRF power from D & E ports. There was some evidence of
heating
within the barrier from the 80 MHz power (at the 0.5 MW level). The
neutron
rate more than tripled and the central density increase was arrested
at
ne_TS(0) = 3.5 X 10^20 m^{-3} during application of the on-axis ICRF
power.
These results are not conclusive, however, because there were two H->L
back
transitions which prevented the initial barrier mode from becoming well -
developed.
Operations
----------
The
runs on Thursday and part of Friday were devoted to extending the plasma
pulse
lengths in C-Mod. These experiments succeeded in extending the
end-of-flattop
(EOF) from 1.5sec to 3.0sec (3.2 sec for the TF), more than
doubling the
flattop time in these inductively-driven discharges. The
long-pulse
experiments were carried out at 5 tesla, with line-averaged density
in the
range 0.3e20<nebar<1.2e20 m^(-3), and plasma currents of 800kA. The
longest
discharges had current flattops of approximately two L/R times. ICRF
heating
was applied to several of these discharges, with power levels of ~1MW
sustained
for up to 2.4sec, using all three ICRF antennas run
sequentially. The
plasma pulse lengths were limited by administrative
restrictions on the
currents in the PF coils, especially the EF1 coils which
are mainly used
for X-point control. All power supplies functioned as
expected, and magnet
and bus heating during the long pulses was measured and
found to be consistent
with calculations carried out earlier. The maximum rise
in TF magnet
temperature was 39C. The between-shot cool-down time was only
slightly
longer than for normal length discharges, and in practice the
between-shot
time was dominated by setup time for the next shot and special
measurements
and inspections associated with monitoring the operation.
Evolution
of the wall particle inventory and recycling was investigated using
short
gas puffs during the long low-density flattop discharges. We showed that
the
wall does NOT saturate at a time <3 seconds, at least at the low
densities
run in this experiment. We also showed that the inventory in the
walls is not
so large that it will significantly affect our ability to run
at low density.
These results are relevant to planning for the C-Mod Lower
Hybrid/AT Physics
Program.
In the course of these experiments
we observed H-modes at nebar~0.65e20/m^3
with ICRF power of 1 MW. We also observed, for L-mode cases, a
(poorer)
confinement mode at these low densities, in which temperature
drops, sawtooth
amplitude decreases or disappears, and fueling efficiency
decreases (as
evidenced by the Ha increase). These poorer confinement
conditions tended to
appear between 1.2 and 1.6 seconds into the
discharge, and persisted for the
remainder of the pulse.
Divertor
heating was observed during the long pulses with the (LANL) IR camera
system.
We were able to puff N_2 via feedback in an attempt to reduce the heat
load
on the divertor; analysis of this aspect of the experiment remains to be
carried
out.
Ten shots on Friday morning were dedicated to high-field
plasma operation. We
operated at toroidal fields up to 8.0T for the first
time in several
years. All systems functioned nominally, and several 1MA
plasmas were
produced. ICRF heating (D-He3 minority heating scenario) was
applied
successfully.
ICRF Systems
------------
All
ICRH systems were operational this week. ICRF heating at 80MHz (D and
E-port
antennas, transmitters #1 and 2) and 70MHz (J-port antenna,
transmitters
#3 and 4) was applied in support of the physics
experiments. System
performance was good up to a total power of 3-4MW. At
higher power
injections from near the top of the J-port antenna, and also in
some cases
from the D antenna, were observed, often leading to discharge
termination.
Diagnostic
Neutral Beam System
------------------------------
The DNB
operated at full current and voltage throughout this last week of
C-Mod
operations. The thermal sensitivity of
the suppressor supply appears to
have been remedied. Reduced efficiency of the cryopumps due to
postponed
maintenance did not affect operation. The DNB will be shut down for
approximately 6 weeks to
faciliate tear down of C-Mod. Operation
will then
continue until approximately 8 weeks before the next operational
period.
Operation during the shutdown will be directed toward improvement
of
conditioning techniques and component mix. All of the DNB diagnostics were
operated. In particular, CXRS spectra were
consistently observed. The
diagnostics
will remain intact for calibration until the end of next week.
Then we
will assess the success of repairs and modifications of the internal
optics
which were made prior to the current campaign.
Inner Divertor
Fabrication Project
----------------------------------
Modified
C-Plate tile support plates and transition keys are now in-hand.
Several
mills of material had to be removed from each side of the tile support
plate
to make plates fit more loosely into the pockets. Modified plates were
assembled into the C-Plate and they fit
perfectly in between the ribs.
The
remainder of the Tile Support Plates (220 total) are being
modified in-house.
There will be 3 more probes at the bottom portion
of the C-Plate than
originally planned.
These probes are being fabricated, and the new probe box
is nearly
complete.
Mock-up building is moving forward. Several long (1.5") studs have
been
welded to the Mock-up wall. BP coils and other diagnostic cables have
been
added to the Mock-up Wall. Plastic Rear Plates, Inconel C-Plate, Tile
Support
Plates, Tiles, Transition Keys, and Old Limiter Backplates with
tiles have been
mounted on the Mock-up Wall as well. Everything fits perfectly in the
Mock-up,
and we are waiting for the real Rear Girdle Plates to finalize the
mock-up
assembly.
Travel and
Visitors
-------------------
Tom Fredian and Martin Greenwald
travelled to Chicago for a meeting
of the SciDAC funded national fusion
collaboratory. Participants
from
MIT, GA, PPPL, ANL and LBNL attended.
This was the first face
to face meeting of the group and
concentrated on developing a
detailed work plan for the first year of the
project. MIT work on
the
collaboratory will be focussed on the addition of a certificates
based
authentication scheme for MDSplus and SQL server access.
Jim Irby,
Monty Grimes, Dave Terry, Jim Zaks and Ron Parker attended the Lower
Hybrid
Coupler Final Design Review last Tuesday, the 31st of July, at
PPPL. The
Review was successful and we concluded that procurements and
fabrication
could proceed. While at PPPL for the LH launcher design review,
Terry met
with Nevell Greenough and discussed briefly the demodulator and
protection
circuitry they are using as well as the new phase feedback system
they are
implementing.
Parker stayed at PPPL Wednesday and Thursday to attend
the FESAC meeting.