Alcator C-Mod Weekly Highlights May 3, 1999 There were two days of plasma operations on Alcator C-Mod last week. Two runs originally scheduled were lost due to a fault in the alternator drive motor on Wednesday and problems with the ICRF systems on Friday. This week is a maintenance week, with no runs scheduled. Thursday's run completed the ohmic portion of MP#222, on impurity compression and enrichment. This proposal is an attempt to extend the compression and enrichment database to gases other than argon and investigate the effect of the divertor bypass on impurity behavior. Argon and krypton were puffed into Ohmic plasmas with nl04 ranging from 0.67 x 10^20 to 1.48 x 10^20 with Ip=0.8MA. The detachment threshold for these plasmas was determined to be ~1.25 x 10^20. The scaling of argon compression with density was evaluated; these compression values are reduced by a factor of ~2 from the previous run campaign. The krypton data are yet to be analyzed. The deuterium compression also scales with density in a way similar to that observed in the previous run campaign, with the value of the deuterium compression reduced by ~25%. Taken together with the argon compression data, these results seem to indicate that the divertor is not as "closed" as in the previous run campaign. The divertor bypass does indeed have an effect on the impurity behavior. The difference on the argon compression with the bypass open and bypass closed is a factor of about two. (The compression is higher with the bypass closed.) The deuterium compression is also about two times lower with the bypass open. Divertor detachment reduces the impurity compression greater than a factor of two. After detachment occurs, the deuterium compression remains constant at the pre-detachment level. Both of these observations are similar to those seen in the 1998 run campaign. The effect of the bypass on impurities is found to be a function of density. At very low density, the bypass does not appear to have any effect on the impurity levels in the core and the divertor, and thus the impurity compression remains constant. At medium density, nl04=0.84 x 10^20, opening the bypass during the shot decreases the impurity compression by ~2.5x. This is due both to an increase in the core concentration and a reduction in the divertor impurity concentration. At high density, nl04=1.1 x 10^20, opening the bypass causes the divertor plasma to detach. The combination of open bypass and detachment results in the argon compression being reduced by a factor of ~6. Physics and Analysis: --------------------- Carbon "plume" imaging continued during piggy-back experiments last week. In these experiments, trace amounts of ethylene gas are injected via the F-port vertical scanning probe. Calibrations taken at the end of the runs indicate that as few as 2 x 10^16 molecules were injected per probe scan. Exposure times for the PPPL and LANL Xybion intensified cameras were optimized for viewing CIII light. Optimization of the camera exposures for CII light is still required. An interesting result was observed during the dynamic flapper scans on successive shots. During these discharges, the flapper was closed at the beginning of the shot, and opened at 0.9 seconds, while the probe scan times were at 0.65, 0.85, and 1.05 seconds. On at least one shot a dramatic difference was seen in the CIII emission before and after the flapper opening, with a strong asymmetry in the "plume" with the flapper open (indicating a strong flow towards the divertor). Correlations with discharge parameters still need to be made, and further analysis is required. The density rise following the H-mode transition in C-Mod has been analyzed. It is found that the density rise for a number of shots is well-fit by an exponential. This implies that the ion source profile stays fairly constant across the H-mode transition and that there is a step change in transport. Furthermore, the density rise was compared across 2 shots that were essentially identical for core characteristics. However, the bypass was changed (open/closed) for these 2 shots leading to very different divertor gas pressures. Although the divertor pressure drop in both cases was more than enough to explain the core density rise, we cannot account for the neutrals from the divertor that do not go to the core. This implies an important role for the wall. Evidence for a resistive interchange mode during the current rise phase has emerged from detailed analysis of the ECE signals on a single shot, at a time when the pressure profile is slightly hollow. During the current ramp of Shot 990302013, highly localized fluctuations were observed on only one of the inner channels of GPC, which is very unusual. The pressure gradient of the inner q=5 surface, which lies near this ECE channel, was predicted to be marginally positive by the reconstructed pressure profile. Such a hollow pressure profile with q_min > 1 in a low beta plasma has been predicted to be unstable in terms of resistive interchange mode. As it is highly localized, a resistive interchange mode is quite challenging to observe within current C-Mod diagnostic capabilities. ICRF Systems ------------ The run on Tuesday was devoted to optimization of ICRF coupling by adjusting the target plasma conditions, including gaps and matching of the plasma shape to the antenna geometry. At issue is both maximizing the antenna loading and avoidance of fault indications. Progress was made in identifying target plasmas with satisfactory behavior. However, in the course of the run, both transmitters #1 and #2 gave indications of faulting, indicating possible problems with the final amplifier tubes (FPA). During subsequent testing into dummy loads, these faults persisted. During this testing transmitter #2 experienced a tube arc which resulted in damage to the FPA tube, rendering it inoperable. Examination of the transmitter #1 is continuing, and the status of its FPA tube is not yet known. The damage to the tube in transmitter #2 appears to indicate a failure of the protection circuitry which is intended to prevent damage in the event of a tube arc. An inspection and review of the protection circuitry has begun, involving engineers from PPPL as well as MIT. As a precaution, all ICRF operation has been suspended until satisfactory completion of this review and implementation of any recommended remedial actions has been accomplished. Diagnostic Neutral Beam ----------------------- All power and directly associated control systems for the DNB have now been operated and tested to full performance with the exception of the modulator/regulator for the acceleration voltage supply. The mod/reg construction is near completion. At present, some of the engineering support for the DNB is being diverted to the ICRF; however, we expect to test the mod/reg system prior to the completion of the present CMOD operational campaign and to be prepared to run the beam in the test cell during the scheduled outage beginning in the summer. Visits and Travel: ------------------- Rejean Boivin visited LHD in Nagoya, where he presented a paper "High Resolution Bolometry in Alcator C-Mod," by Boivin, Goetz, Marmar, Rice, and Terry. He also attended the ITER Diagnostic Expert Group meeting, where he presented an informal talk on Novel Diagnostics in Alcator C-Mod on behalf of the Alcator Group. Rejean Boivin, Martin Greenwald, Ian Hutchinson, Dmitri Mossessian, and Thomas Pedersen presented papers at the Transport Task Force Meeting in Portland. Miklos Porkolab attended the SEAB Review of the Fusion Program at PPPL on Thursday and Friday, April 28,29. He also had a meeting with Joel Hosea to discuss the Lower Hybrid plans for C-Mod. Dr. Ricky Maqueda from LANL was on-site last week. The LANL IR camera is operating routinely with a view of the outer divertor structures and the outer strike point. The most striking result is that no toroidally symmetric features of surface heating are observed. In particular, there is no toroidal band of surface heating at the nominal position of the outer strike point. Surface heating is typically observed on the edges of the divertor tiles, especially on the "nose" tiles. The heated edges are on the ion side (as defined by the plasma current direction). Surface temperatures are typically less than 50 deg. C, and the hottest spots are around 200 deg. C. Heating is also observed on specific divertor tiles. This is a result of poor thermal contact and/or because they are raised slightly. Dr. Maqueda is working on correlating some of the observed hot spots with the strike point position and on trying to observe any effects of divertor detachment. Charles Skinner of PPPL made preliminary measurements of the He II linewidth and shift using a channel from the tangential fiber array which is usually fed to the Kaiser spectrometer for analysis of the helium beam. The He II measurements may potentially allow the determination of ion temperatures and toroidal drift measurements near the separatrix. Analysis is underway. Dr. S. Zweben of PPPL visited C-Mod on Apr. 28-29 to work with Brian LaBombard and Jim Terry on the imaging of turbulent edge fluctuations. Clear filamentation of the edge H-alpha light emission was seen using the LANL camera at exposure times of 10 microsec. This turbulent structure had a few-cm scale in the poloidal direction and a meter-length scale in the toroidal or parallel direction (as seen previously). This structure seemed to disappear during H-modes, but this could just be due to the higher frequency associated with H-mode turbulence. The plunging probe was used to puff Helium and Deuterium near the separatrix, and clear visible light images were recorded with a 1 microsecond exposure time. However, these images have not yet shown any filamentation inside the 1-cm scale of the puff cloud, despite the large small-scale fluctuations seen by the nearby Langumir probes.