Alcator C-Mod Weekly Highlights Feb. 9, 1998 Plasma operations resumed on Alcator C-Mod last week. Four run-days were scheduled and completed. 115 plasma discharges were successfully produced. On Friday, the toroidal field was increased to 7.9T for the first time in the present campaign. Tuesday's run was devoted to H-mode confinement scaling (MP#155). A scan of plasma current from 0.4 to 1.4MA was carried out, at a toroidal field of 5.6T with the density near the boundary between Elm-free and Enhanced D-alpha (EDA) operation. Stored energy scaled roughly linearly with Ip, though it was about 30% below our best results. The impurity particle confinement time, tauI, also seems to scale with current. The soft xray pedestal width scaled with 1/Ip, though with some ambiguous shots. We scanned Bt to resolve edge electron temperature profiles with the ECE grating polychromator (GPC). These didn't show much Ip scaling, suggesting that either the GPC and the edge xray array are looking at quantities with different widths, or that the GPC resolution is too large to see these differences. Fluctuations were seen with Bp probes and on the reflectometer - more analysis needs to be done. Some scanning probe data was obtained at the lower currents. Disruptions limited operation at the highest currents, with q95< 3.0. We got much stronger EDA's at low currents and ELMfree discharges at high currents. This suggests some sort of q dependence on the as yet unknown EDA mechanism. Confinement studies continued on Wednesday with a scan of triangularity (MP143B). We ran cases at 3 upper triangularities, [.16,.4,.62], and 3 lower triangularities, [.4,.5,.6]. Density scans were performed at each. At the end of the day continuous, i.e. single shot, dynamic triangularity scans were performed at fixed density. In the first, delta_l = .6->.4 and delta_u = .35->.17; for the second only delta_u was scanned = .5->.2 A wide variety of H-mode types were observed. There is apparently a great deal of operating space in between pure ELMfree and pure steady EDA. Using the appearance of a steady EDA as a measure, we find that it requires the highest densities and divertor pressures when the triangularity is low, and less for the moderate and higher triangularities. Xray pedestal widths seem to be a good metric for h-mode phase. In all cases it is narrower for ELMfree, wider for the mixed modes, and widest for EDA. The dynamic scans showed that the width also responds to triangularity independent of confinement mode and in the same sense. As the triangularity scanned down, the width decreases, all in EDA - later, as the confinement mode switches to ELMfree, the width continues to narrow. On Thursday, we revisited the question of impurity pumping by the prototype cryo-pump (MP#177). The previous experiment had produced ambiguous results, and it was suspected that the vessel walls were pumping more than usual due to the recent boronization. Ohmic plasmas with medium density and density ramps to divertor detachment and EDA H-mode plasmas were investigated. 'Steady-state' and reproducible conditions were targeted in order to investigate the utility of the cryopump. Argon or krypton and helium-3 was puffed into these types of plasmas with the cryopump on or off. The core impurity ion densities were monitored with x-ray and VUV spectrometers and the divertor impurity neutral densities with an RGA. We got good reproducible data with the pump cold and warm, during both H and L mode discharges.The first part of the run was devoted to quantifying the effects the cryopump had on both core and divertor parameters. Generally there are clear differences in the warm and cold cases. The divertor pressures and h-alpha signals are lower, source gas required higher, and densities measurably lower when the cryopump is cold. The divertor parameters are dramatically different in the warm and cold cases. With the pump cold the density and radiated power are down a factor or two or so and the electron temperature is up a similar amount. Unlike our last attempt running mp177, there is no doubt that the pump can have a large effect on divertor operation. In Ohmic plasmas with constant medium density, the core argon and krypton are "pumping out", i.e., the time history of the signal is decaying away. With the cryopump operational, the core argon density is slighlty lower than when the pump is warm. Krypton line brightnesses also show a slight decrease in the equilibrium value with the cryopump operational. When the density in these Ohmic plasmas is ramped to divertor detachment, the krypton line brightness does not change. The penetration factor for puffed krypton entering the core is lower after detachment with the cryopump on than it is after detachment with the cryopump off. This is similar to the results seen for argon in run 980121. Measurements of the neutral particle density in the divertor indicate that the krypton density is not much different with either the pump cold or warm. In EDA H-modes, the "pump out" effect is still evident on the core argon density and there is no difference between plasmas with and without the cryopump operational. Core krypton brightness measurements show no "pump out" behavior. There is a slight decrease in the core krypton brightness with the cryopump operational. There is a slight decrease, <15%, in the divertor neutral krypton with the cryopump operational. Also, there is less krypton in the divertor with an H-Mode plasma than an L-mode plasma. The analysis of the helium-3 data is ongoing. Friday was our first run of this campaign to call for toroidal field in the 8 tesla range. Voltage regulation on the alternator has been improved, resulting in higher voltage from the TF supplies, and a consequent reduction in the time required to reach high field, and in the magnet heating and between-shot cool-down times. Power systems behaved well during the high-field operation. The physics run on Friday was in support of MP#150A, on D(He3) heating. In the process of raising the toroidal field up to ~8T shot-to-shot, H-modes were obtained for D(H) minority heating with the H cyclotron resonance as far as 16.7 cm off-axis. The plasma current was also increased such that the scan was done at constant q. The H-factor (calculated assuming 90% absorbed RF power injected) progressively decreased as the resonance was moved. This result differs from JET's experience and reconfirms earlier C-Mod results. Furthermore, the inverse sawteeth phenomena observed on the inner wall b-dot probes vanished approximately when the H cyclotron resonance passed beyond the sawtooth inversion radius. The speculation is that the sawteeth redistibute the minority ions, which decreases the absorption. Another result is the bursting neutron phenomena often observed after a H->L transition. The H and 2nd harmonic D resonance is well outside the inversion radius, so 2nd harmonic D absorption cannot be the cause of the increased neutron rate. For fields above 7.1 T, where the H cyclotron resonance is approximately at the antenna position, He3 was added to systematically investigate the minority and mode conversion regimes. All of these high-field plasmas were in L-mode. The physics results are encouraging. A scan of He3 concentration was performed at Bt~7.9T. The RF was square-wave modulated at frequencies from 55Hz up to 200 Hz to permit spatial localization of the electron (mode-conversion) heating as the He3 concentration was varied. At low concentrations of He3, the bulk ion temperature increases by ~1.2 keV while the electron temperature increases by ~0.7 keV. For higher He3 concentrations, significant mode conversion was observed. The electron heating was observed to move further off-axis as the He3 fraction increased, in agreement with theoretical expectations. In a single shot, as the He3 concentration increases, the RF goes from predominantly minority to mode conversion (ion heater to electron heater). Over the weekend, the tuning of FMIT#3 was investigated. A faulty variac was found and is to be repaired and remounted. Retuning of FMIT#3 to 78 MHz will continue this week, time permitting. Ken Takase and Paul Bonoli have successfully ported the TORIC code (written by M. Brambilla) to a local workstation. This will allow analysis of C-Mod discharges on a more routine basis. Progress continued on the power supplies for the DNB. Refurbishment of the control system for the Accel supply continued. The protection crowbar for the Mod/Reg tube was installed. The cables and transformer oil for the Mod/Reg filament supply were installed in prepartion for oil conditioning using the techniques developed for the Accel supply. Miklos Porkolab attended Advanced Tokamak Workshop at GA last week, and presented the talk: "Alcator C-Mod Advanced Tokamak Research". He also attended the DIII-D Advisory Committee meeting Tuesday morning, to hear the DIII-D Program Overviews. This week will be a plasma operations week at Alcator C-Mod. A fresh boronization is planned prior to the scheduled physics runs.