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.
21, 1999
Plasma operations continued at Alcator C-Mod last
week. Four runs were
scheduled,
but Friday's run was re-scheduled due to hurricane Floyd. A total
of 66
plasmas were produced with a reliability of 97%.
A fresh
boronization was carried out on Monday. A total of 114 psi of diborane
was
used for an average deposition of 1667 Angstroms. Boronization was
followed by 4.25 hours of helium ECDC at
5e-5 Torr.
Tuesday's run was devoted to a survey of impurity
concentrations during EDA
H-modes (MP#241). The concentrations were measured using passive x-ray and
VUV
spectroscopy. This run was carried out immediately after a boronization
and
the plasmas were extremely clean relative to pre-boronization
levels.
Z_eff was 1.2-1.25. The E and D
antennas were used for auxiliary
heating.
RF powers varied from 0.9 to 2.2 MW.
EDA H-modes were routinely
achieved; the H-factor was typically
1.6. During the EDA H-mode phase, the
radiated power as measured by the
2pi foil bolometer typically increased from
400 to 950 kW, while the power
as measured by the 2pi diode bolometer
increased from 50 to 250 kW. (This
was for input powers of ~2.6 MW.) Ar was
puffed for diagnostic purposes on
all but two shots, and was found to account
for ~20% of the radiated power
increase during H-mode. If we take the foil and
diode bolometers at face
value, then ~300 kW (55%) of the power loss brought
about by EDA is from
neutrals, while 80-100 kW (20%) is from Ar. The rest
(100-150 kW) is
probably accounted for by B and F. This requires further
quantitative
analysis. The following impurities were measured:
Ar (x-ray and VUV)
B (H-like and Li-like)
F (recombination on fully stripped and
Li-like)
C (Li-like)
O (Li-like)
Mo (Na-like and Ne-like)
Fe (Na-like)
Wednesday's
run was in support of MP 239, (Hubbard, Carreras et al, "Critical
Exponents
for L-H transitions and Pedestal Widths"). The basic plan was to
measure
the input power dependence of the edge pedestal parameters,
particularly
the Te pedestal, but also density pedestal.
The main technique
was to use RF power ramps, of various peak
levels and durations, to measure
the response of pedestal parameters as a
function of net input power. In
addition,
shots with constant power (or long steps) and Bt ramps were employed
to
measure the Te pedestal width in more detail.
The nominal conditions were
5.4 T, 800 kA and nebar~1.5e20/m3. Some
shots were also produced to test the
part of the Carreras/Diamond model
which predicts a dependence of pedestal
widths (esp ne) on the ionization
rate. This involved power ramps at
both
higher and lower ne. Under
the nominal conditions, steady EDA with constant
Prad was obtained even at
the lowest power. On a pair of shots, we ramped the
power down. Strong hysteresis was found; EDA H-modes
were maintained down to
P_rf~150 kW. Temperature pedestals got very small
at the low powers (~ 200 eV
vs ~ 400 with high power). The widths increased at high power, but
the
dependence was less clear at low P.
In the second half of
the run, the target density was varied.
The highest
density shot, ne~3e20/m3, stayed in L-mode at 1.9
MW. A series of shots with
ne~2.4e20
(at the L-H transition) and 3-3.5e20 in EDA were studied. A power
rampdown shot was included in
this sequence, again showing big hysteresis.
Te
pedestals were measured at 2.0 and 1.0 MW. They were rather cold, and the
width
was not notably different than at lower ne.
The visible bremsstrahlung
emissivity widths did seem wider. The
final sequence of shots had
progressively lower starting ne, and power
ramps. Two of these started as
ELM-free, and then went to EDA at higher
power providing a comparison of the
two modes, including fluctuations,
widths etc. The low and high ne
transitions
should provide interesting data on neutral effects etc, once the
Lyman
alpha data are analysed.
Thursday's run was in support of MP#237,
"Current dependence of toroidal
rotation". The purpose of this
run was to determine the scaling of the rotation
velocity with plasma
current, for otherwise constant target plasmas,
for H-mode discharges. The
range of currents achieved was from 0.8 down to
.4 MA, however there was
only one good shot at .4 MA due to difficulty
coupling 2 MW of ICRF power.
The run was terminated at 3PM due to concerns
over the approaching storm;
completion of these experiments was re-scheduled
for this week.
Physics
and Analysis
--------------------
Toroidal rotation
measurements in the plasma periphery (Rtan = 87.69 cm)
using ambient boron
impurity emission are now routinely made.
A boron line
at 2821.68 angstroms is measured in 4th order with a 1
m spectrometer and
an array detector.
Data are collected on virtually every shot and compiled
into a
database. Preliminary analyses indicate
that the ions are rotating
in the co-current direction at roughly 10-25
km/sec. Entry into H-mode
correlates with the toroidal velocity increasing
and exit from H-mode
correlates with the velocity decreasing.
David
Winslow (U. Texas) continued analysis of divertor probe fluctuation
data,
looking at the relationship between floating potential fluctuations
and
swept probe biasing. In addition, preliminary experiments using a
driven
tile probe system were performed.
Computer systems:
-----------------
Live
streaming video and audio from the control room is now available during
C-Mod
operations. The URL is
http://www.psfc.mit.edu/cmod/control_room_rv1.html
.
ICRF Systems:
-------------
Post-boronization
conditioning of D and E-port antennas went well. EDA
H-modes were readily obtained with moderate power. A
maximum power of 2.5 MW
was obtained and at present the power is limited
by the screen power
dissipation.
This limit arises because the 2274 plate impedance is high at 80
MHz
and the anode voltage approaches the screen voltage during a cycle
resulting
in high screen current. After
implementing a cavity model based on
work by C. Brunkhorst (PPPL), FMIT#2
was retuned to lower its plate impedance.
The trend predicted by the model
was verified in testing into dummy load, and
high power was obtained with
lower screen current (about 400 kW).
Testing
into plasmas will begin next week.
After
disassembling and inspecting FMIT#3 coax for arcing damage (none found),
the
line was reassembled, pumped down, purged and backfilled with insulating
gas. A few shots were taken into plasma at power
levels of 0.5 MW per
transmitter at the end of the week with no indication
of problems. A concept
for the
J-port antenna coupling loop modifications to reverse alternate
current
strap phasing has been worked out, and circuit dimensioning and fine
tuning
is in progress.
Travel and Visitors:
--------------------
Bill
Dorland visited the PSFC on Wednesday through Friday. He worked
with Martin Greenwald in making initial
comparisons between experimental
data and recent simulations of ETG
turbulence. He also consulted
with
David Mikkelsen on runs of his (Dorland's) linear gyro-kinetic
stability
code GS2.