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 Highlihgts
May
29, 2000
Operations
----------
Plasma
operations continued at Alcator C-Mod last week with four run days
scheduled
and completed. A total of 69 plasma shots were produced, with a
startup
reliability of 80%. Experiments in support of four MiniProposals were
carried
out; we also produced a series of shots aimed at comparing ICRF-heated
H-mode
quality under present conditions with those produced in previous
campaigns.
Plasma
physics operations will continue this week. A fresh boronization is
scheduled
for Tuesday night.
Physics and Analysis
--------------------
This
week, experiments were carried out to continue the investigations into
the
scaling of the EDA/ELMfree boundary with the ultimate goal of
understanding
the physics of EDA. It has previously been determined that the
EDA phase
usually occurs within a specific range of the safety factor q95. In
this
run, we varied plasma elongation at fixed q95.
There were no obvious
effects of elongation on the EDA/ELMfree
boundary. The EDA or ELMfree
character
did not change markedly throughout the scans.
There were
differences due to impurity injections, RF power
changes, radiated power
build-up, etc, but nothing obviously systematic
with kappa. The scan covered a
limited range of elongation, but if the
dependence were as strong as that for
q, the effect would have been apparent.
Part
of one run day was devoted to a mini-proposal (MP#269) to test the
theoretical
prediction of Perkins and White that ICRF-induced plasma rotation
in C-Mod
should change direction from co- to counter-current as the ICRF
resonance
is moved from the low to high field side of the tokamak. The results
so
far are interesting but not conclusive.
The toroidal magnetic field was
first scanned down from 5.3T to
4.5T and then back up to 6.0T. The ICRF power
was kept constant at
approximately 2 MW. H-mode plasmas were obtained at all
fields between 4.5
and 5.3T. Baseline rotation measurements at 5.3 T (ICRF
resonance at 0.48
cm to the LFS) indicated V_tor = 50 km /s (from High
Resolution X-ray
Spectroscopy) with EDA H-modes and stored energies of about
95 kJ. The
plasma rotation was still observed to be in the co-current
direction at
4.9 T (ICRF resonance at 4.6 cm to the HFS).
The mean V_tor was
slightly lower than at 5.3 T, although the
stored energy was also lower (80
kJ). At 4.5 T the rotation velocity indeed
reversed sign to about -20 km/s.
However, these plasmas were quite
different from the plasmas at 4.9T and 5.3T
because the ICRF heating was
far off-axis (9.4 cm to the HFS). Evidence of a
hollow T_e profile was
seen and sawteeth were suppressed during the H-mode
phase. Evidence of an
internal transport barrier (ITB) was also seen in that
the central density
was rising during the H-mode phase to about 4 X
10^20. Difficulty coupling
RF to the high-field plasma prevented us from
getting comparable results
when the toroidal field was increased to 6.0 T,
moving the ICRF resonance
9.4 cm to the LFS. These experiments
will be
continued.
One run day was devoted to the start of a
systematic study of the scaling of
H-mode pedestal parameters (MP#271).
RF power was varied in a series of discrete
steps for target plasmas at
different currents. Data were obtained at 800kA and
600kA. An attempt to
go to high current (1.4MA, corresponding to q~2.7)
resulted in a
disruption, which unfortunately caused sufficient degradation of
machine
conditions that further experiments were not possible on this day;
overnight
discharge cleaning was required before good plasmas were again
obtained.
These pedestal scaling experiments will be continued.
Last Friday's
run was devoted to a test of predictions by Y. Nakamura (JAERI)
concerning
the existence and location of a "neutral point" which exhibits
enhanced
stability for vertical displacement events after a thermal quench. On
JT-60U,
and other tokamaks, it was found that when the plasma is operated with
the
magnetic axis height at this neutral point, the initial vertical drift
after
a thermal quench is significantly slower than usual, sometimes allowing
vertical
control to be re-established, or for ameliorative action to be
taken. In
comparison with JT60-U, C-Mod typically operates at higher
elongations,
closer to the ideal axisymmetric stability limit, and therefore
provides a
more demanding test of this effect. We
ran a series of standard
plasmas at a range of different heights (keeping
the elongation nearly
constant by moving the x-point along with the
center), and disrupting them
using double-barrel silver-doped lithium
pellet injection. For plasmas with
Z
< +3 cm (measured with the core x-ray arrays), the thermal quench led
to a
prompt loss of vertical stability, with the plasma moving
downwards. For
plasmas with Z >
+3.5 cm, the thermal quench was followed by a prompt upward
movement. However, with Z centroid set for +3.0 and
+3.5 cm, we observed
clear signs of enhanced vertical stability. In one case the plasma remained
near
the midplane for 8-9 ms before finally heading downward. The last shot of
the
day exhibited vertical stability for more than 20 ms after the pellet
injection,
but for this case we need to confirm whether the pellets did indeed
cause
a full thermal quench, not just a minor disruption.
Peaked density
profiles, similar to those seen during last year's campaign,
were observed
again in the Ohmic H-modes of the May 18 run.
Peaking factors
(n(0)/n_ped) > 3 were found at the end of the
H-mode. The peaking begins
early
in the H-mode and increases throughout.
The lower plasma current
discharges had less peaking than the
higher current ones, and the highest
peaking was observed in the discharge
that returned to 5 T rather than
remaining at lower field. These results indicate that the peaking is
affected
by either the absolute field and current or their rates of
ramping, and that
higher field and current increase the peaking at least
up to the values
attempted so far.
It is not yet clear whether there is an optimum edge q
value or
optimum edge dq/dt rate. A miniproposal
to further study this
spontaneous peaking phenomenon in H-mode (MP254) has
been approved and is
awaiting scheduling
ICRF Systems
---------------
Both D and E-port antennas were operated in support of physics
experiments
this past week. Into
EDA H-modes, 2.8 MW was coupled to the plasma, and we
are investigating
the absorption efficiency. If we assume
90% absorption
efficiency, the H-factors were up to ~1.8, consistent with
the C-Mod database,
so the efficiency is expected to be reasonably
high. We are still
experiencing
faulting during plasma operation that we are continuing to
investigate. The faulting may be a result of having high
voltage points in
new locations compared to previous campaigns, e.g. gas
barriers at high
voltage locations.
We may need to make further adjustments to the resonant
loops and
power feeds, which are located external to the vacuum vessel, to
improve
reliability. In discussions with R.
Wilson (PPPL), we have resolved
inconsistencies in the reported antenna
voltages and resistances. We are
beginning
to cross check the various power and voltage measurements to
complete the
diagnostic set and improve our understanding of the performance
of the
antenna systems.
Randy Wilson and Joel Hosea visited MIT to
participate in physics experiments
utilizing D and E-port antennas and to
discuss plans to modify the J-port
antenna. Before modifying the J-port antenna, we agreed that we will
investigate
its ability to operate at high target densities and to reconfigure
the
external plumbing to have straps 2 and 3 powered by one transmitter and
straps
1 and 4 to be shorted at what is normally their high voltage
point. During
the last campaign, J-port would not operate at high target
densities and
modifications were made to the antenna during the winter in an
attempt to
address this problem. Reconfiguring the
external loops will allow
us to test the idea that parasitic loading may be
resulting in lower heating
efficiency compared to D-port. The modifications are aimed at improving
the
shielding for the radial feeds.
We propose to add shields to protect the
strap leads and to prevent
their fields from interacting with the plasma.
We
would also replace the side tile mounting plate with a solid one
that also
covers the radial portion of the current strap, and mount BN
tiles instead of
Mo. The top and
bottom protection tiles will be replaced with BN tiles as
well.
DNB
Systems
-----------
The new booster transformer for the
filaments was installed and tested.
Arc
current scans versus filament voltage were completed. The suppressor and
gradient grid
voltage scans were continued for 2 days until a suppressor
supply problem
prevented further operation of the beam last week. Tests will
continue in
this week.
Travel and Visitors
-------------------
Ian
Hutchinson attended the FFCC meeting at PPPL on Friday.
Brian
LaBombard, Spencer Pitcher, and Bruce Lipschultz, Rejean Boivin, Daren
Stottler
(PPPL) and Sanjay Gangadhara presented papers at the Plasma Surface
Interactions
Conference in Rosenheim, Germany. Posters on C-Mod research
authored by
Chris Boswell and Robert Nachtreib were also presented.
As noted
above, Joel Hosea and Randy Wilson visited from Princeton.
Y.
Nakamura arrived from Japan for a two week visit to continue the
JAERI/MIT
collaboration on disruption research.
He served as
co-session-leader for the neutral point experiment
described above.