Alcator C-Mod Weekly Highlights February 22, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 12.2 weeks Plasma Shots: 1447 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four research days were scheduled and three and a half were completed. The experiments supported research in the Advanced Scenarios, ICRF, Boundary Physics and Diagnostic Development areas. A total of 112 plasma discharges were produced with a reliability of 87%. Power systems tests to verify proper operation in the reversed field configuration were carried out on Monday, followed by three plasma discharges to confirm plasma control parameters and optimize startup settings. Extended (nine hour) run days were scheduled for Wednesday and Thursday, in order to accommodate multiple experiments on each day. However, an hour was lost at the end of Wednesday and and another hour Thursday morning resolving an issue with the EF1U power supply control signals. On Thursday evening, one of the ICRF transmitters (FMIT#3) experienced an arc between the Final Power Amplifier (FPA) tube and a cooling line which required taking the J-port antenna system out of service. The experiment underway was successfully completed using the D- and E-port antenna systems, but one of the two experiments scheduled for Friday had to be rescheduled as a result of the outage. Repairs are expected to be completed this week. This week will be a scheduled maintenance period at Alcator C-Mod. No plasma operations are planned. Operation Details ----------------- Tuesday's run was devoted to MP#654, "I-mode with lower single null and unfavorable grad-B topology". The experiment has several motivations and goals: a) exploit the better power handling capability of the lower divertor to further explore I-mode performance vs. shaping b) determine if the vertical target geometry in the lower divertor leads to consistently better confinement performance and c) document the I-mode power widths and peak heat flux using the better divertor diagnostic set in the lower divertor. This configuration resulted in a substantial increase in the power window for I-mode, relative to that obtained with upper null topology. With Ip=1MA and triangularity ~0.5, I-mode was obtained between 2.2 and 3.5 MW, with no H-mode transition. Shaping appears to play an important role in determining the difference between the L->I and I->H power threshold. Good absolute I-mode performance was obtained at 1MA, with H98~1.1 to 1.2 and pedestal Te >1 keV. I-modes were obtained with 0.8 < Ip < 1.25MA using typical C-Mod LSN equilibria. At higher triangularity (delta_L ~0.6), with the outer strike point off the vertical plate in the "slot" region, no I-mode transition was obtained; the discharge remained in L-mode despite having the same input (and net) power. This behavior contrasts with results in the USN configuration, where I-mode favors higher triangularity at the active X-point. Two experiments were carried out on Wednesday, employing similar plasma configurations with B~7.9T and D(He3) ICRF scenarios. The primary experiment, MP#649 "Scaling of parallel impurity transport with Mi, q95 and vi" is designed to investigate in-out asymmetry of impurity density on a flux surface, which theoretically arises from the effect of centrifugal force in a rotating plasma. Mode conversion flow drive (MCFD) is used to enhance the plasma rotation and to achieve high electron and ion temperatures in L-mode discharges. This MP comprises part of the thesis research of an MIT graduate student. Data were obtained at Ip=1 and 1.3MA with up to 4.5MW of ICRF and He3 fraction ~12%, in the mode conversion (MC) regime. Measurements of Mo asymmetry using both intentional laser blow-off (LBO) injections and incidental "natural" injections were obtained for analysis. Attempts to operate at higher current, up to 1.6MA, were not successful. The second experiment undertaken on Wednesday was MP#610, "Investigate Thomson/ECE Discrepancy in High Temperature Discharges". This experiment addresses a diagnostic issue identified in high temperature (Te>7 keV) discharges at TFTR and JET, which is currently the subject of an ITPA Joint Experiment (DIAG-03). This experiment takes advantage of the high temperatures obtained in these B~8T high power D(He3) L-mode discharges, particularly in the minority heating (MH) regime. On Wednesday we successfully obtained good data from both TS and ECE diagnostics up to Te~8keV, but insufficient data exists at the highest Te to compare the measurements in detail in the region of greatest interest. Further work on this MP will be required to improve the statistics and extend the range of the dataset. High power operation in reversed field LSN plasmas, on both Tuesday and Wednesday, was compromised by a high incidence of molybdenum injections resulting in frequent disruptions. The origin of these injections is not certain, but may be related to the high heat fluxes measured at the inner divertor target, which, in contrast to normal field operation, is typically in the attached divertor regime in the unfavorable grad-B drift configuration. Seeding with neon was found to be somewhat beneficial, but was not optimized during last week's experiments. Two experiments were scheduled for Thursday. The first, MP#646 "Development of scanning probe operation as an energetic ion loss detector", attempted to observe fast ion losses due to TAE activity using the A-port scanning probe (ASP) set to "dwell" outside the last closed flux surface and biased for ion collection. The idea was to observe modulation of the probe signal at the TAE frequency. The experiment was designed to use ICRF heating in the current rise and early flattop in low current (Ip~600kA) L-mode discharges, to increase the likelihood of exciting TAE activity. Due to the late start, only one hour was available for this exploratory experiment, and we were not successful in producing the required TAE signature. Based on the results, a revised plan for achieving the goals of this proposal was formulated, emphasizing operation at higher current and ICRF power, with power applied during the flattop phase. The second experiment on Thursday was MP#629 "Characterization of fast ions generated by ICRF: static and dynamic response", which comprises part of the thesis research of an MIT graduate student. This experiment employs the Compact Neutral Particle Analyzer (CNPA) to measure the fast ion distribution in ICRF heated plasmas for comparison with RF code simulations. For purposes of this experiment the CNPA sight-lines were adjusted for more discrete spatial resolution; in addition, the CXRS diagnostic was used to measure the boron density in the plasma, and PCI was used to locate the mode conversion layer to provide an estimate of the H minority concentration. Both modulated and steady ICRF heating waveforms were employed. Current scans from 0.6 to 1.2MA were carried out at Bt=5.4 and 5.1T. Initial (control room) analysis confirms previous observations of improved fast ion confinement at higher current, as evidenced by higher CNPA count rates. Higher count rates are especially pronounced for sight-lines farther away from the ICRF resonance location. Detailed analysis of these data is underway. On Friday we devoted half a run day to MP#635 "Effects of RF heating on the edge/SOL poloidal velocities". This experiment comprises part of the thesis research of an MIT graduate student. This was the second half-day devoted to this experiment, and the goal was to compare results in reversed field with the prior, normal field, observations. Friday's experiments were conducted using the D- and E-port antennas; additional data using the J-port antenna is still required for a complete comparison. A density scan from 4e19 < nebar < 1.5e20/m^3 was carried out in LSN configuration at q95=4.7 (Ip=0.8MA, B=5.4T). This was followed by a scan of q95 (varying current at fixed Bt) in order to change the mapping from each antenna to the gas puff imaging (GPI) location, at several target densities. A comparison was also made to an USN discharge. Initial analysis indicates that there is a clear effect on the SOL flow structure with different q's, implying that the mapping between antenna and GPI does matter. There is also a large effect of topology (USN vs LSN). There is no obvious effect of density on the phase velocity profiles over the range studied. In addition to the GPI data, structure in the floating potential in the SOL was also observed using the ASP on the same discharges. These data will be analyzed in detail. ICRF Systems ------------ On Thursday, FMIT#3 developed a high voltage fault external to the final power amplifier tetrode. Arc damage was found between the cavity walls and the PVC water cooling pipes. The pipes will have to be removed, cleaned, and replaced, and the aluminum cavity buffed clean. We expect this work to be completed this week. The TZM side tiles for the ARRA-funded rotated four strap antenna have gone into production using our in-house EDM facility. TZM blocks for all the antenna tiles have been pre-cut and some have been EDM'ed. The first batch of 38 Faraday rods have been received and inspected. Thirty-four out of the 38 have been accepted. R. Vieira and J. Zaks visited the vendor to review QA and fabrication processes for the back, side, top, and bottom plates. Further plating tests of brazed components were successfully completed. Alan Binus and Steve Wukitch had a teleconference with our vendor regarding the status of the FFT order. The first FFT's are expected at the end of September. We have begun preparing to ship a power supply and power amplifier to the vendor's facility for testing purposes. Lower Hybrid System -------------------- The LH system was not required for any of the C-Mod runs last week. Efforts were focused on preparations for upcoming LH system upgrades. The 4th cart top frame was laid out and blower assemblies were test fit in the cart. The fan mounting brackets and hose adapter parts have been machined and welding will be started soon. Voltage programmable phase shifter and attenuator modules have been delivered and are being tested. Travel and Visitors ------------------- Martin Greenwald was at General Atomics in San Diego for a meeting of the DIII-D PAC; Martin serves as chair of this committee. Greg Hanson (ORNL) visited with Cornwall Lau, Yijun Lin, and Steve Wukitch to discuss the present SOL reflectectometer, analysis routines, results, and future plans. Greg and Cornwall refined the analysis routines to improve the inversion accuracy. A set of upgrades have been discussed regarding measuring PDI, fluctuations, magnetic field, and a second microwave system with three frequency capability to measure both steep and relatively flat density profiles. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly