Newsgroups: sci.physics.plasma
From WOLFE@CMOD2.PFC.MIT.EDU Tue Jan 23 15:43:38 1996
From: WOLFE@CMOD2.PFC.MIT.EDU
Organization: MIT
Subject: Alcator C-MOD Weekly Highlights

Alcator C-MOD Weekly Highlights
Jan 22, 1996

Operation continued on Alcator C-MOD last week, with a total of four run days
scheduled and completed. Over 100 successful plasma shots were produced. This
week was a good demonstration of the versatility of the facility, with each
day featuring a different magnetic field, from 2.6 to 7.9 Tesla, and a
different RF heating scenario. All the runs benefitted from the boronization
carried out over the previous week.

As reported previously, the run on Tuesday was devoted to exploring H-mode
behavior with boronized walls. This run, at 5.3T employing D(H) ICRF, was
quite successful, producing our highest stored energies, neutron rates, and
H-factors to date. Beta-normal values up to 1.5 were also observed. H-factors
above 2 relative to ITER89-P and above 1.5 relative to ITER ELM-free H-mode
scaling were obtained in both ELM-free and ELMing discharges.

Wednesday's run explored D(He3) heating at 7.9 Tesla. This run produced much
better heating than had been achieved previously in this heating regime.
H-modes were readily produced, though not quite of the quality of those
achieved at 5.3 T. It appeared that we were close to the threshold power at
2-2.5 MW, with some delay between rf turnon and h-mode onset. Heating was
dependent on He3 fraction, with best heating at an intermediate fraction (not
yet absolutely calibrated, but estimated to be 5-10%), with no H-modes
achieved at higher fraction and weaker heating at lower fraction. A coarse
density scan was also carried out.

Thursday's run investigated second harmonic hydrogen minority heating, at a
field of 2.6 Tesla. Scans of density and minority (H) concentration were
carried out. Current was fixed at 0.55MA (q_psi~3.2) for these scans. Heating
was definitely observed, but it is not clear how efficient the heating was.
H-mode was observed except at the highest density. The stored energy typically
doubled in H-mode plasmas. H concentration was varied by changing the gas mix
in the plenum. The H fraction in the mix was varied in the following order:
20%, 40%, 10%, 0%, and 100%. Preliminary indication from H_alpha/D_alpha
measurements is that the H concentration in the plasma was not varying as much
as we had hoped. Isotope dependence of energy confinement was not clearly
observed.

Direct electron (mode conversion) heating was carried out at a field of 6.4T
in H(He3) plasmas on Friday. This run was a continuation of experiments begun
last June, and is in support of thesis research. The purpose was to use
modulated heating to study transport coefficients by perturbative techniques.
Results were obtained at currents of 0.6, 0.8, and 1.1MA. Good central heating
was obtained, although at somewhat lower He3 concentrations than expected.
Higher fractions produced off-axis heating.

In addition to the principal experiments, standard fiducial shots were run
throughout the week, and boron levels were monitored spectroscopically, to
track the evolution of the boronization. By the end of the week (over
100 high-power tokamak shots) the Molybdenum levels were noticeably increased
and boron levels were down, but neither had returned to pre-boronization
levels. Additional boronization is scheduled for this week.

John Rice attended the 10th APS Topical Conference on Atomic Processes in
Plasmas in San Francisco; he presented an invited paper on "Observations of
2l-nl' Transitions from Zr, Nb, Mo and Pd in Near Neonlike Charge States",
which featured experimental results from C-MOD. Ben Welch of the University of
Maryland presented an invited talk "Line Shape Measurements of Visible Light
Emission from the Alcator C-Mod Tokamak".

Roger Bengston of University of Texas is visiting Alcator this week to further
plans for collaborations between the FRC and C-MOD.