Alcator C-Mod Weekly Highlights July 2, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 9.1 weeks Plasma Shots: 1393 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and 2.5 were completed. A total of 63 plasma discharges were produced with a reliability of 88%. The runs supported experiments in the Diagnostic Development, Transport Physics, Edge Physics, and Lower Hybrid Physics topical areas; all of the experiments last week contributed to student thesis research. A vacuum incident resulted in a partial up-to-air on Thursday morning. An argon backfill was performed to remove a damaged gate valve, and the vessel was then pumped down and high temperature (120C) bakeout and ECDC was carried out overnight. Scheduled operation on Thursday was canceled, and startup on Friday was delayed. Plasma operations are planned to continue this week. Program Planning ---------------- A meeting of the Experimental Program Committee was held on Monday June 25. Priorities were reviewed and a process set up to compile and distribute progress on every high-priority experiment for the balance of the campaign. Six new experimental proposals were reviewed and approved. Physics ------- Last week, US patent 8217353 was issued for "Non-astigmatic x-ray imaging with matched pairs of spherically bent crystals" to Drs. Manfred Bitter, Ken Hill, Steve Scott and Frank Jones of PPPL, John Rice (PSFC), Jinseok Ko (KSTAR, former MIT graduate student), Alexander Ince-Cushman (McKinsey & Co., former MIT graduate student). This plasma diagnostic was first implemented on Alcator C-Mod several years ago to provide time- and space-resolved measurements of ion temperature, rotation speed, and impurity density. It has subsequently been implemented on several other fusion experiments world-wide, including EAST, KSTAR, LHD, and NSTX, and it is under consideration to be installed on ITER. Mr. Paul Geelen, a Master's student at Einhoven University in the Netherlands, completed a highly productive 4-month visit to the PSFC this week. He adapted an MSE simulation code (MSESIM), that was originally developed to simulate the MSE spectrum on MAST, to work also for Alcator C-Mod. The MSESIM code includes a number of physical effects that are not present in the PERF code (which was written by Nick Hawkes for the design of MSE on JET and which was adapted some years ago for use on C-Mod), including: 3D emission volume, beam divergence, numerical treatment of finite size of collection optics, Paschen Back effect, and non-statistical population of the energy levels. When MSESIM is operated in a reduced mode so that its physical model approximates the simplified physics incorporated into the PERF code, it yields very similar MSE spectra and polarization angles that agree with those computed by PERF to typically ~0.02 degrees (maximum difference < 0.04 degrees). In the process of benchmarking MSESIM against PERF, a factor ~10 error in PERF's signal intensity was discovered (caused by an addition to the PERF code since its arrival at the PSFC; the original code was correct). When MSESIM is operated with all of the 'non-ideal' physics terms turned on, the result is that the polarization angle that will be measured by MSESIM (for a fixed actual pitch angle in the plasma) is a function of the center wavelength of the MSE bandpass filter; mis-positioning of the CWL by 0.1 nanometer generates an error of approximately 0.1 degree. Operation Details ----------------- The run on Tuesday morning was dedicated to MP#696, "Continued investigations of dilution effects on turbulence and transport in low density ohmic plasma". This experiment comprises part of the thesis research of an MIT graduate student. The goal of this run was to determine the propagation direction of modes observed using the Phase Contrast Imaging (PCI) diagnostic. Discharges above and below the Linear Ohmic Continent (LOC) to Saturated Ohmic Confinement (SOC) transition range, with and without N2 puffing to modify the dilution, were repeated with the PCI diagnostic masked to observe propagation in the CW or CCW direction. The dataset is being analyzed. The second half of Tuesday's run was devoted to MP#703 "Commissioning of the Correlation Electron Cyclotron Emission Diagnostic". The purpose of this run was to measure electron temperature fluctuations using the CECE diagnostic. A series of discharges were obtained varying the toroidal field from 5.3 to 5.6T, followed by a sequence including LOC, SOC, and ICRF-heated L-mode conditions. Fluctuation data from these regimes are being analyzed. On Wednesday we carried out part of MP#667 "Commissioning of new SOL probes". This work comprises part of the thesis research of an MIT graduate student. The purpose of this run was to get ion temperature profiles in the SOL with the new ASP Retarding Field Analyzer (RFA). These will be compared with a future run with the new Ion Sensitive Probe (ISP). Initially, good electron and ion temperature profiles were obtained in low densities. These will make for a good comparison with the ISP. However, later in the day the probe became unreliable. Preliminary analysis indicated that the slit was shorting to the head due to inertial effects during the scan. Following the run, the RFA probe head was removed and a redesign to address this issue is in progress. Following a series of reconditioning shots, the run on Friday was devoted to MP#664 "Study of spectral broadening of LH waves". This experiment contributes to the thesis research of an MIT graduate student. Spectral measurements were obtained of LH waves, using multiple probes, and spectral measurements were made of low frequency ion cyclotron modes using the inner wall probes. SOL density and temperature profiles were obtained with both inner-wall (WASP) and FSP scanning probes. Fluctuation measurements were made using an X-mode reflectometer in front of the LH launcher, and also using the Gas Puff Imaging (GPI) diagnostic. As the density was raised, the amount of LH power collected by the inboard probes appears to decrease, and that by the divertor probes increases. Multiple effects (PDIs, scattering, collisional absorption) may be needed to explain these observations. The Parametric Decay Instability (PDI) sideband was observed to become significant at LH net power above 500kW. In inner-wall limited discharges, the LH power reaching the inner wall probes was observed to increase. ICRF Systems ------------ The ICRF system operated in support of experiments last week, and for evaluation purposes following the vacuum incident. The systems recovered to 4 MW, 0.2s pulses, with the pulse length restricted by programmatic constraints. The H/H+D was rising to ~20% at the end of 0.2 s pulses. We found that monopole phasing of the four strap, field aligned antenna again resulted in higher hydrogen evolution per MW injected. The GPI and emissive probe data suggest that there are larger enhanced potentials and greater penetration for monopole phasing compared to dipole phasing. We have disassembled the fast ferrite tuner (FFT) that experienced arcing last month. We noted this time that arcing had occured between the ferrite and the strip line to which it is attached; likely an air gap is forming which then arcs and destroys the ferrite tile. We will be sending pictures of the assembly to the manufacturer and discuss options for repair in the near future. Lower Hybrid System ------------------- The LH low power microwave control upgrade was tested last week with simultaneous amplitude and phase feedback control. Amplitude control to < 0.1 dB was achieved at minimum attenuation (-15 to -25 dB) and to < 0.5 dB at higher attenuation (-25 to -35 dB). Phase control to < 0.5 deg was observed at all attenuation levels. The LH system was used for the half day run on Friday afternoon. A fast body current fault on klystron 10 occurred during HV turn-on in a test shot prior to the run. The body current fault persisted even at low beam voltage (30 kV). The symptoms suggest a possible faulty body current sensor. The LH system operated satisfactorily without klystron 10 for the duration of the run. Travel and Visitors -------------------- Jay Kung, an RF engineer from PPPL, was at MIT on Tuesday, 6/28 to troubleshoot the high frequency reflectometry channels with Seung Gyou Baek. Two fixed frequency channels (112 GHz and 140 GHz) are now back online, while one of the Gunn diodes of the swept frequency channel needs to be replaced. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly