Alcator C-Mod Weekly Highlights July 23, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 11.1 weeks Plasma Shots: 1646 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and 3.3 were completed. A total of 98 plasma discharges were produced with a reliability of 95%. The runs supported research in the Transport Physics and H-mode Scenarios topical areas. Run time was lost due to an alternator over-temperature alarm on Tuesday, and computer outages on Friday. Fresh boronizations were carried out over Monday and Wednesday nights in preparation for the experiments. Plasma operations are planned to continue this week. Operation Details ----------------- The runs on Tuesday and on Thursday morning were devoted to MP#698, "ITER-like discharge studies at reduced Bt". This experiment contributes to ITPA Joint Experiment IOS-1.1 "ITER demo at q95=3, betaN=1.8, and n/nG< 0.85". Last week's experiment extended work done previously in ITER-like EDA H-mode plasmas at 2.7 T, to higher values of n/nG and higher net power. ICRF heating at the second harmonic of the hydrogen gyrofrequency was employed. We successfully obtained high performance (beta_N up to 2.1) plasmas with n/nG approaching or exceeding the ITER target value of 0.85. RF power levels up to 4.5MW were successfully coupled, increasing the range of P/Pthreshold at high density by nearly a factor of two relative to the previous dataset. Neon seeding was found to improve RF operation at high power. Confinement quality, expressed as H98y2, appeared to be reduced at higher n/nG. These data are being analyzed. The run on Wednesday was dedicated to MP#653b, "Hidden variables in Neo-Alcator scaling". Additional data for this MP were taken on Friday. This experiment, which contributes to the OFES FY12 Joint Research Target on Core Transport, aims to decouple hidden variables Te/Ti and collisionality from density, to understand the mechanism for the density scaling of confinement in the neo-Alcator regime. Density does not appear in the dimensionless delta-f gyrokinetic Maxwell equations, so it should not affect core confinement directly, except through collisionality. Likely parameters would be Te/Ti, Zeff, and collisionality, as well as R/Ln, all of which exhibit strong covariances with density. Density scans, power scans at constant density, and density scans at constant Te/Ti were employed to help unravel this scaling. Complete fluctuation data, including CECE, Gas Puff Imaging with D2, PCI, Reflectometry (in core as well as edge), fast TCI, Mirnov loops, and polarimetry, were obtained and will be compared with gyrokinetic simulations. Run time on Thursday afternoon and Friday morning supported MP#691, "Development of TEM in high performance ITBs". The primary purpose of this experiment is to study the formation of the trapped electron mode (TEM) instability that is associated with ITB control and saturation in Alcator C-Mod. The instability should be driven by steep density gradients in hot plasmas resulting from high power off-axis ICRF heating. Identifying this instability relies on observing the characteristic fluctuations with PCI, using additional techniques to determine the regions of origin, including sawtooth analysis and ICRF power modulation. Interpretation of the data depends on obtaining well resolved ion temperature and toroidal rotation profiles. Discharges with Internal Transport Barriers (ITB) were produced by running EDA H-modes with off-axis ICRF heating; RF power at f~80MHz was employed with Bt0=6.3 T, placing the minority proton resonance on the low-field side near r/a=0.5. The RF power was successfully scanned from 3MW to 5MW. This experiment contributes to the FY12 Joint Research Target. Two hours on Friday were devoted to the completion of MP#702, "Cold pulse modulation experiments for the study of non-local heat transport and correlation with rotation reversals". This work comprises part of the thesis research of an MIT graduate student. "Cold pulses" were generated using Laser Blow Off (LBO) injections of CaF2 to cool the edge of ohmically heated discharges, in both the Linear and Saturated Ohmic Confinement (LOC and SOC) regimes. The injections were repeated at 10Hz in order to increase the statistics and improve modeling of the transport phenomena. On this run day, data were obtained in the LOC regime at Ip=800kA, and in both LOC and SOC regimes at 1.1MA. These data are being analyzed, along with datasets obtained in earlier experiments run under the same MP. ICRF Systems ------------ The ICRF system was successfully operated in support of experiments last week. The second harmonic H minority heating scenario was successfully utilized for 2.7 T discharges, and may have significant direct fast wave absorption in the high density H-mode phases with betaN ~2. We plan to analyze these discharges carefully with a full wave solver. We had a conference call with our vendor regarding the first pair of ARRA-funded fast ferrite tuners and their latest test data. The phase shift for 60 < f < 80 MHz exceeds the minimum specification requirements (>100 deg) and the two elements have nearly identical response. However, at 50 MHz, the maximum achievable phase shift is ~60 deg, and so does not meet specifications. Since the first pair will be used in the fixed frequency, 80 MHz systems, we can accept these tunersm and we expect the vendor to ship them shortly. The vendor will investigate modifying the next four tuners to improve the response at 50 MHz. The current plan in this regard is to reduce the permanent magnetic field to shift the center frequency down, using magnetic shunts. If this works we expect to implement the same changes to the first two tuners. If it proves unworkable, we may be able to use the shunts to tune the center frequency, adding them when we want to operate at 50 MHz. This would add some complication to switching to 50 MHz, but would allow for optimal performance at both 80 MHz and 50 MHz. Lower Hybrid System ------------------- Progress was made on construction of the LH3 launcher. Ceramic vacuum windows were delivered by the manufacturer with 45 of the 48 delivered windows meeting dimensional specifications. The first 8-way splitter assembly was machined and is ready for plating. Design specs for the LH3 control system to Digital Plasma Control System (DPCS) have been determined. The LH3 controller will accept total forward power and waveguide-to-waveguide phasing as inputs from DPCS, and will output calculated net power to DPCS. An interface board block diagram has been developed. Feedback control of amplitude and phase for individual klystrons will be contained within the LH3 control system. Diagnostics ----------- Preparations for installation of the accelerator-based first wall diagnostic ("AGNOSTIC") on C-Mod are underway. The RF Quadrupole (RFQ) accelerator is to be installed at the B-Horizontal port. The mounting stand for the accelerator is now in place at B-port. The accelerator itself is still in an auxiliary setup lab, where it is routinely producing ~900 keV deuteron beams for several hours each day. The AGNOSTIC data acquisition system (including detectors and fast digitizers for pulse processing) is being operated routinely with the RFQ in the lab. Detectors and DAQ are performing well. Several beam targets, including boron-coated C-Mod molybdenum tiles, have been irradiated by the RFQ in the lab and data acquired on them. Data processing is underway. Travel and Visitors -------------------- Miklos Porkolab and Earl Marmar attended the FESAC Sub panel on MFE Priorities Meeting in Washington, DC, on July 18 and made presentations on near term priorities. Chuck Kessel and Francesca Poli (PPPL) visited MIT on Tuesday through Thursday last week. Chuck served as Session Leader for the experiments on Tuesday and on Thursday morning. In addition Chuck and Francesca worked with Yunxing Ma, Nathan Howard, and Catherine Fiore on running TRANSP on the ITER-like discharges. Amanda Hubbard spent last week at IPP Garching, in order to carry out I-mode matching experiments with ASDEX Upgrade. This was part of ITPA joint experiment TC-18. Devoted discharges were run as scheduled on Tuesday 7/17 and Thursday 7/19. However, general difficulties with reversed field operation led to instabilities and disruptions above 800 kA, so that the target parameters, and I-mode, could not be obtained. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly