Alcator C-Mod Weekly Highlights July 23, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 10.7 weeks Operations ---------- Research plasma operations continued at Alcator C-Mod last week. During the regularly scheduled maintenance day on Monday, the toroidal field and plasma current orientations were reversed from the standard configuration in preparation for a series of experiments. Power systems testing and establishment of a reliable startup scenario in the new orientation were accomplished by noon on Tuesday. Two and a quarter days of research operations were completed during the remainder of the week. A total of 62 plasma discharges were produced with a reliability of 84%. The runs supported miniproposals, from the Operations and H-mode Integrated Scenarios areas. One brief (five hour) boronization was carried out last week, following the startup activities on Tuesday. The upper divertor cryopump was employed in the experiments on Wednesday and Friday. Plasma operation is planned to continue next week with additional experiments in the reverse field and current orientation. Operation Details ----------------- The run on Tuesday was devoted to MP#506 "H-modes with cryopumping in upper single null topology". The goal of this experiment was to explore the effect of cryopumping in H-modes in USN with reversed field (ion grad-B drift upward), which was expected to exhibit strong pumping by the upper divertor cryopump and easy access to H-mode because of the favorable grad-B drift. However, while H-modes were readily obtained at several currents (0.40.75 at the lower (active) xpoint and small delta_u<0.2 at the top. By inverting this shape and operating with reverse field we hoped to produce similar ELM behavior while exploiting the improved diagnostic coverage afforded by the new probes located in the upper divertor plate tiles. Use of the cryopump to assist in density control was also contemplated, though in view of Tuesday's results this aspect was not emphasized in the run plan. In the event, a power supply limitation, tentatively identified as an instability in the circulating current regulation of the EF1L four-quadrant supply, prevented reliable operation in the specified equilibrium. Only three useful H-mode discharges were obtained, and while some ELM activity was observed, minimal progress was made toward the goals of the MP. Friday's experiment under MP#475 was devoted to further investigation of the reduced pumping efficiency observed in reverse field upper null discharges on Tuesday. A series of discharges, including upper, lower, and near-double null cases, designed to closely match the ones obtained with the forward field orientation during a previous run day were produced, and the resulting upper chamber pressure and pumping speed were determined. A density scan was performed in the upper null configuration and with a near-balanced double null. At higher density, nebar>1.5e20/m^3, both the pressure and pumping speeds were the same as in the normal field case. However, at lower density the upper chamber pressure was found to be reduced, with a significant reduction in pumping effectiveness. Interestingly, access to ohmic H-modes was enhanced in these lower density cases in which the pump was active, despite the reduced pumping. The pressure in the upper chamber and the corresponding pumping speed were found to be higher in a slightly downward biased double null configuration than in upper null, for target density nebar~1e20/m^3. This observation is consistent with the interpretation of the reduced upper chamber pressure being associated with the location of the high recycling region at the inner leg of single null discharges; for the double null case, the recycling at the magnetically isolated inner leg is greatly reduced and that on both upper and lower outer legs increases. Program Planning ----------------- On Monday, July 16, a meeting of the C-Mod Experimental Program Committee (EPC) was held to review miniproposals and discuss plans for the remainder of the FY07 experimental campaign. Seven new and revised MP's were approved. A number of others were deferred to a future meeting. Topical group leaders and coordinators reported briefly on the current status of experiments and plans in their areas, including consideration of machine and auxiliary system requirements. Based on the number of high-priority experiments involved, reverse-field operation is planned to continue through the end of this week. A decision on proposed changes of ICRF frequency and configuration (J-port antenna) was deferred pending more data from near-term operation. Lower Hybrid System -------------------- The lower hybrid system was not employed during last week's experiments. ICRF System ------------ The J-port antenna was reconfigured as a 4-strap to further investigate the reduction in voltage handling observed earlier in the campaign. J-port had been operated at high power as a 4-strap antenna at 78 MHz until the first boronization. After boronization, the antenna performance degraded and a significant oscillation was observed during vacuum conditioning. After an initial round of investigation it was reconfigured as a dipole, which limits us to 2 MW source instead of 4 MW. During last week's reconfiguration we have been able to vacuum condition strap pairs individually to 30 kV in vacuum but at 20 kV when the antenna is phased with all 4 straps powered. We also confirmed this during plasma operation, and further investigation will be carried out this week. Diagnostic Neutral Beam System ------------------------------ The long-pulse diagnostic neutral beam performed nominally in support of experiments last week. Typical parameters were 50kV and 6 amps of ion current. Options for helium gas recovery and recycling for the DNB cryosystem are being investigated. Diagnostics ----------- We have observed shot-to-shot variability in the pitch angles measured by the Motional Stark Effect diagnostic of order several degrees. In the frame of reference of the diagnostic itself, this corresponds to angular changes of about 1 degree. In at least some shots, the angle offsets in the MSE frame of reference are approximately equal in all of the ten spatial channels. This observation has given rise to the conjecture that some part of the MSE optical system is rotating or moving due to electromechanical or thermal forces. To look for possible motion of optical components external to the torus, we mounted a laser beam onto MSE's critical optical component, a photoelastic modulator (PEM). The beam was directed at a target about 50 cm away. The beam's position on the target was recorded in real time by a video camera. We observe motion of the laser spot of order 0.5-1.0 mm throughout an ensemble of about 20 plasmas including some disruptions. This corresponds to a maximum rotation of the external optical components of 0.1 degrees, i.e. less than 10% of the rotation that would explain the shot-to-shot variability. This indicates that, if indeed an optical component in the MSE optical system is moving during plasma shots, it can only be a component inside the torus. Meetings -------- A total of 44 Contributed abstracts based on C-Mod research were submitted for the 49th annual Meeting of the APS Division of Plasma Physics, to be held in Orlando, FL. The total includes 30 poster and 14 oral presentations. There will also be four invited talks featuring C-Mod results presented at this meeting. Travel and Visitors -------------------- Josh Stillerman and Tom Fredian participated in a USITER CODAC workshop at the USITER offices in Oak Ridge. The CODAC conceptual design documents being produced by the ITER CODAC working group were discussed and comments and ideas from this USITER working group will be communicated back to the central working group. Martin Greenwald was in Washington on Monday and Tuesday to attend a meeting of FESAC and present a status report from his planning panel. Miklos Porkolab also attended this meeting. Steve Scott visited PPPL on Monday and Tuesday for discussions regarding the PPPL/C-Mod collaboration. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly