Alcator C-Mod Weekly Highlights June 26, 2000 Operations ---------- Plasma operations continued at Alcator C-Mod last week, with four run days scheduled and completed. A total of 75 plasmas were produced, with a reliability of over 80%. Experiments included a continuation of the study of the physics of density limits, tests of the J-port ICRF antenna with a modified feed configuration, a study of isotopic changeover rates, and a study of ICRF mode conversion physics in H-He3-D plasmas. Following completion of last week's runs, a fresh boronization was carried out over the weekend. Plasma operations are scheduled to continue this week. Physics and Analysis ---------------------- We continued our density limit experiments which emphasize the role of transport in determing the observed empirical limit. Successful fast scanning probe data was collected in divertor plasmas covering a density range 0.4 - 1.0 x limit. We now have the data to characterize the fluctuations and convective transport across this range. As reported previously, the fluctuation levels and autocorrelation times for fluctuations at the separatrix increase near the density limit. We also got good data from the PCI. It showed the growth of incoherent low frequency fluctuations (<100 kHz) as the limit was approached. There was also a clear shift toward lower frequencies. In order to design a particle control system for C-Mod, we need to understand the role of the wall in pumping and fueling. The wall dominates particle inventories, containing up to 1000 times the number of hydrogenic atoms as the plasma. Isotope changeover experiments were carried out to study these effects. We began with a machine in which virtually all the hydrogen isotope had been removed. Before the changeover the ratio R=H/(H+D) = 2.5% This rose to 30% on the first shot with H2 puffing. After 5 shots the ratio rose to 65%. The total H2 puffed on each shot was approximately 50 Torr liters. The wall inventory is estimated to be about 300 times larger, so a puff with 0.3% of wall inventory changes over 30% of plasma ions. The characteristic turnover time (R/dR/dt) was 3 seconds for the first shot then slowed down gradually for later shots, reaching about 30 seconds after 7 shots. Thus the changeover is not at all exponential. The advantages of EDA type H-modes have been made clear previously. Our studies emphasize the physics which underlies this mode, with the goal of extrapolating it to reactor like conditions. Resistive ballooning has been predicted to be a dominant mode in the plasma edge. In the resistive ballooning dispersion relation, m_i^{1/2}/q is a stabilizing term. This might explain the q dependence we see in the EDA/ELMfree boundary. High q would make the term smaller, destabilize resistive ballooning and lead to EDA. If this hypothesis is correct, hydrogen discharges, with m_i = 1 should stay in EDA down to lower q. We carried out a series of experiments looking for the presence of EDA in ohmically heated hydrogen H-modes. All H-mode discharges produced showed clear signs of EDA based on Halpha, radiation and the presence of the quasi-coherent mode on PCI, reflectometry and the A side probe. During this run, the Hydrogen fraction never rose much above 0.6 - thus meff dropped to 1.4. If the threshold depended on sqrt(meff) this would have dropped from 1.41 to 1.17, roughly a change of 20%. For a nominal q threshold in the range 3-4 for D, we would expect a drop to 2.5-3.3 for the mix in this experiment. On the surface, this would tend to confirm a dependence on m_i in the proposed direction. However, we need to check that the profiles were otherwise similar. Experiments at JET to try to access the "EDA" H-mode regime, as seen on Alcator C-Mod, were carried out last week. ICRF heating with over 9MW of power was employed. Upper and lower triangularities of about 0.4 were obtained, with q95 scanned from 4.2 to 5.9; operation was at relatively high density (5e19 at 1.7MA, and 3.5e19 at 1.2MA). No EDA transitions were obtained. With strong gas puffing during the H-mode in feed-forward, the background D alpha emission was raised somewhat, but the plasma still went into a clear Type I ELMy regime, although the ELMs were smaller than those seen in the usual NB case. ICRF mode conversion physics was investigated through detailed measurements of the electron power deposition profile, and observation of the mode-converted ion Bernstein wave (IBW) with the Phase Contrast Imaging (PCI) diagnostic. Limiter discharges with H-He3 plasmas were employed for this experiment. Mode-converted ion Bernstein waves (IBW) were observed in the C-Mod core plasma for the first time with the PCI sytem. The observed wave numbers, in the range of 4 to 6 cm^-1, are in agreement with the dispersion relation of an ion Bernstein wave located closely beyond the H-He3 ion-ion hybrid layer, where mode conversion is expected to take place. Signal at 5 to 7 times background noise level was seen for both D port and E port antennas (the PCI laser passes through E port top and bottom), although it was stronger for D port. Preliminary investigation of the GPC electron temperature diagnostic indicates off-axis electron heating due to the electron Landau damping of the ion Bernstein wave. ICRF System ------------ External connections to the PPPL ICRF antenna at J-Port were reconfigured over the weekend of 6/17-18. The center two straps were connected as a dipole to transmitter #3, while the outer two straps were shorted to ground to remove them from the circuit. This configuration is expected to reduce possible leakage coupling to plasma surface waves and operation in this mode should provide additional information on the reduced heating efficiency of this antenna. Careful measurements of antenna loading as a function of power and plasma-antenna separation as well as heating efficiency comparisons were made for all three antennas. Preliminary indications are that, as a dipole, J-port heats approximately as well as D and E according to central Te and neutrons. This suggests that properly grounding the antenna box and shielding the feed straps should improve the 4-strap configuration. Further analysis of these results is in progress. DNB System ----------- During the previous week, the beam was used to inject a hydrogen beam into a deuterium plasma. Data was acquired for the resulting H/D ratio and the impact of the beam on the discharge prefill. The BES system was used to acquire data for an estimate of the beam density to compare with the data from the DNB internal diagnostics. Analysis is now being completed. Travel and Visitors ------------------- Earl Marmar, Rejean Boivin, Jerry Hughes, Yijun Lin, Chris Boswell, Norton Bretz, Ned Eisner, Bill Rowan, and Ricky Maqueda attended the High Temperature Plasma Diagnostics meeting in Tucson last week, and presented papers on C-Mod work. Randy Wilson came to MIT 6/21-22 to participate in the ICRF measurements and to help review plans for further antenna modifications during the July vacuum opening. Ian Hutchinson and Joe Snipes are at JET, where they participated in the experiments to obtain "EDA" H-modes on that machine. Miklos Porkolab attended the 27th EPS Conference on Plasma Physics and Controlled Fusion in Budapest on June 12-16. He also represented APS on the EPS Board Meeting and on the Program Committee Meeting. He then attended the 3rd Europhysics Workshop on the "Role of Electric Fields in Plasma Confinement and Exhaust" on June 18. At the EPS meeting he presented the Poster "Initial Results from an Upgraded ICRF System on Alcator C-Mod", On his return from attendance at the PSI Meeting in Rosenheim and the International Workshop on Electrical Probes in Magnetized Plasmas in Berlin, David Winslow (UT-FRC) arrived at C-Mod last week to complete installation and begin experiments with his turbulence probe. After working through some torrvac problems last week and addressing some continuing problems with the probe position sensor and the stepping motor controller, David plans to continue taking data on C-Mod this week. Alan Lynn, a UT-FRC graduate student, joined the on-site contingent at C-Mod for an extended visit. He will divide his time between DNB and FRC ECE. Gary Hallock (UT-FRC) and his graduate student Ashley Shugart visited to assist with operation of the C-Mod PCI system.