Alcator C-Mod Weekly Highlights August 6, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 12 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. On Monday we completed experiments in the reversed field configuration, and operation resumed on Wednesday with the field and current returned to the normal (ion grad-B drift down) orientation. Three days of research experiments were accomplished, along with approximately one half day for power systems testing following the bus work associated with the field reorientation. A total of 83 plasma discharges were produced with a reliability of 91%. Runs supported experiments in the Transport, Divertor/Edge, and Lower Hybrid Physics areas. Beam-into-gas tests (with no fields) were also carried out on Tuesday, and following several runs, in order to debug variability in the MSE system. Boronizations were performed on Sunday and on Thursday night in preparation for the experiments scheduled on the following days. On Saturday morning our power utility successfully completed a temporary reconnection of the 13.8 kV AC line which furnishes experimental power for major C-Mod subsystems to an alternate substation. This is the first step in the work to remedy the high impedance on this line which had been restricting C-Mod operations. Plasma operations are scheduled to continue at C-Mod this week. Program Planning ----------------- The C-Mod Experimental Program Committee met on Tuesday, July 31. The main agenda item was review of pending miniproposals. Ten MP's were approved, one was returned for revision, and a decision on another was deferred pending availability of needed resources. The committee also confirmed plans for ICRF frequency changes to 70 and 50 MHz for the J-port antenna system during the current campaign. Operation Details ----------------- Monday's run (MP#452a) continued to explore H-mode operation in lower single null topology with the direction of toroidal field and current reversed (ion grad-B drift directed upwards). The experimental goals included the diagnosis of pedestal structure, edge relaxation mechanisms and confinement properties, and comparison with H-mode properties found in the normal field case (ion grad-B drift down). We were able to establish long-lived H-modes at 800kA and 600kA, with limited scans of target density. Relatively low collisionalities were obtained in many of these discharges, with pedestal temperatures measured from Thomson scattering and CXRS reaching 1keV, and with central electron temperatures from 5 to 6 keV. Energy confinement was very good in these shots, with H_89>2. Edge quasicoherent modes were measured, generally with lower lab-frame frequencies than their normal field counterparts. Some ELMs, perhaps Type III, were also observed. Analysis of these discharges is ongoing. Tuesday was taken as a maintenance day, with the main activity being the reversal of the DC bus work to return to standard field and current orientation. In addition, a series of beam-into-gas shots without fields were taken to measure the Doppler shift with a spectrometer looking through the MSE optics to observe whether the Doppler shift changes shot-to-shot or within a shot. Such changes would imply that the MSE optics change their viewing angle with respect to the beam and would potentially explain some of the variability of MSE plasma measurements. No such spectral variation was observed. Power system testing and re-establishment of plasma operation were completed on Wednesday morning, after which we proceeded with the scheduled experiment, MP#429 "SOL turbulence with limited plasmas". The overall goal of this miniproposal is to obtain turbulence data suitable for comparison with gyrokinetic simulation codes, in particular the GEM code written by Bruce Scott (IPP, Garching). Stewart Zweben (PPPL) is the lead experimenter on this MP. Ohmically heated limiter plasmas have been selected for the first stage of these comparisons. The run on Wednesday completed a dataset comprising a scan of toroidal field 2.8 < B < 6.8 tesla, with instances at high, moderate, and low density for each field. Good data was obtained with the fast GPI diodes and Langmuir probes. This run completed the dataset required for the MP, and analysis of the turbulence data and simulation runs will be carried out. The run on Thursday was devoted to MP#515, "Effect of launched LH wave n_parallel spectrum on current profile". The goal was to operate the LH system over the phase range required to vary the launched n|| from 1.6 to 3.1, use ICRF heating to vary the electron temperature, and observe changes in the fast electron and current profiles using hard xray emission and MSE. We were unable to carry out the planned experiment because of two problems: the available LH power was restricted to <400kW due to local heating at the molybdenum limiter which protects the LH grill, and the ambient outdoor temperature caused the LH HV power supply to exceed its trip set point. The appearance of a hot spot on the guard limiter, which at higher power levels resulted in disruptions, had not been observed in previous experiments and may have been a consequence of the specific plasma shape employed. The phenomenon did not appear to be correlated to the value of n||. The thermal limit on the HV supply necessitated termination of the Lower Hybrid experiment in the afternoon, and the remainder of the run was transferred to discharge development for the experiment scheduled for Friday, MP#498 "Density Peaking at Low Collisionality". A new control technique for dynamically transitioning an H-mode plasma from LSN to a double null or marginally upper null configuration, in order to facilitate use of the upper divertor cryopump for control of the H-mode density, was successfully demonstrated, and used during Friday's experiments. The goal of Friday's experiment (MP#498) was to extend the data range, in the nu_eff, q95 plane, for particle transport experiments and to investigate density peaking at low collisionality. Previous work at C-Mod and elsewhere has shown clear evidence of density peaking in H-modes at low collisionality. Data from AUG, C-Mod and JET suggest that the density profile for a low collisionality device like ITER will have a peaking ratio, ne(0)/nebar ~ 1.4-1.6. This result has important implications for fusion gain on ITER and may also affect plasma stability through modifications in the pressure profile and bootstrap current. It may also impact the divertor behavior through the relative drop in edge density. The experiment on Friday was aimed in particular at investigating the density peaking effect at higher values of q95 than had been previously been studied in C-Mod, in order to extend the range of comparison with ASDEX and JET datasets. While the lowest target nu_eff values were not reached, a scan of q95 and beta suitable for comparison with the other facilities was obtained. ICRF System ----------- The J-port antenna was reconfigured to power straps 1 and 2 from FMIT#3 and straps 3 and 4 from FMIT#4. By running the strap phasing at [0,pi,pi,0], instead of the previous [0,pi,0,pi], more than 2MW of power was coupled to the plasma using the antenna in this configuration. Lower Hybrid System -------------------- The lower hybrid system was employed in support of the C-Mod experiment on Thursday of last week. Lower hybrid system operation had to be stopped due to a "container over temperature" indication from the high voltage power supply. Steps to remedy this problem have been implemented, and we do not expect a recurrence during operation this week. Travel and Visitors -------------------- Stewart Zweben (PPPL) visited MIT last week and served as Session Leader for the experiment on Wednesday. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly