Alcator C-Mod Weekly Highlights May 27, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 15.7 weeks Operations ---------- Research operations continued last week at Alcator C-Mod. Four run days were scheduled and completed, supporting experiments from Rotation Physics, Lower Hybrid Physics, MHD, Operations and Control, ICRF Physics, and Diagnostics research areas. A total of 93 plasma discharges were produced with a startup reliability of 88%. Seven beam-into-gas shots with toroidal and vertical fields were also produced for MSE calibration. One overnight boronization was carried out prior to Thursday's experiment. The C-Mod experimental campaign for fiscal year 2008 is now completed. A total of 15.7 weeks (62.9 days) of research operations were accomplished, producing a total of 1888 plasma discharges. No further plasma operations are planned this year. The tokamak is being warmed up to room temperature in preparation for inspection and maintenance. Operation Details ------------------ Tuesday's experiment was devoted to MP#550 "Locked Mode Rotation Studies". The purpose of the experiment was to use locked modes as a means of perturbing the toroidal rotation profile in support of momentum transport studies. The non-axisymmetric coils (A-coils) were used to induce locked modes, braking the intrinsic rotation, and then unlock the mode so that the dynamics of the "spin-up" can be studied. Locked modes were readily obtained in discharges with initial rotation of up to 30km/sec. Considerable development time was required to determine conditions under which the mode would unlock. Time-resolved profiles of the subsequent rotation evolution were obtained, and are being analyzed. This work comprises part of the thesis research of an MIT graduate student, and also contributes to the 2008 OFES performance (JOULE) target to "evaluate the generation of plasma rotation and momentum transport." Two experiments were carried out on Wednesday. The first half-day was devoted to MP#552 "Test of DPCS Multi-processor Capabilities with an Anti-Saturation Adaptive Control Routine". This experiment employed for the first time a new multi-processor computer in the C-Mod digital plasma control system (DPCS). An adaptive routine running asynchronously on a "slave" process on a second cpu was used to sense impending power supply saturation and generate an interpolated transition to a pre-loaded alternate "safe" equilibrium program. The adaptive response successfully prevented loss of control and potential disruption due to actuator saturation. The implementation of real-time multiprocessor control, employing communication through shared memory coordinated by semaphores (locks), is prototypical of a number of planned developments in the C-Mod control system requiring additional processing resources and multiple time scales. This work was carried out as part of the thesis research of an MIT graduate student. The second half of Wednesday's run was dedicated to MP#530 "Disruption runaway studies". The focus of this half-day run was to try some additional ideas to generate relativistic runaways during disruptions, in order to model the expected runaway generation in ITER. The basic idea is to use LHCD to produce a seed population of fast electrons (100-200 keV) prior to the disruption. Unlike the previous experiment on this subject, the LH power was maintained until the time of the disruption, and the experiments were conducted at low density (nebar~5e19/m^3). We were successful in using the LHCD to reliably produce plasmas with relativistic runaways prior to disruption, as indicated by observation of synchrotron emission. However, there were no clear examples of relativistic runaways during the current quench. In addition, we were unable to reliably trigger prompt, reproducible disruptions under conditions that are amenable to relativistic runaways. Use of lithium pellets was not fully satisfactory in these experiments, because of shallow pellet penetration in the presence of the non-thermal electrons. The first part of Thursday's run was devoted to MP#378 "Mode Conversion Flow Drive Study in D(3He) Plasmas Using J-antenna at 50 MHz". The experinent was designed to follow up on observations of enhanced toroidal rotation during ICRF mode conversion experiments using current drive phasing. A comparison of rotation profiles was made between discharges with minority heating (He3 minority at 50MHz, H minority at 80MHz), and in the mode conversion regime (He3, 50MHz). In the minority heating regimes the rotation was smaller and had the usual dependence on stored energy, and T_i < T_e. In the mode conversion regime with the MC layer near the axis, enhanced plasma rotation, by about a factor of 3, was observed, along with T_i > T_e. The second experiment on Thursday was primarily in support of MP#515 "Effect of launched LH wave n_parallel spectrum on current profile". Good current drive was obtained at n_parallel = 1.6 and n_parallel=1.9, with target plasmas at B=6.2T, Ip=800kA, nebar<4e19/m^3. In one case, zero loop voltage was maintained for ~200msec, although the plasma appeared to be affected by a locked mode. MSE data were obtained for many of these discharges, and the current profiles are being analyzed. One shot was obtained at B=7T with a ramped density program, in support of MP#551 on "Penetration of accessible LH waves". A similar shot, at higher density, was also taken on Friday, extending the dataset for this MP to this higher field. Friday's run was primarily devoted to MP#554, "MSE Calibrations - torus temperature effects and background issues". The experiment included a number of conditions and goals related to the MSE diagnostic. Goals included: verification that the shot-to-shot drift in the MSE diagnostic can be reduced by maintaining the torus temperature constant; a test of the effect of the first lens heating on the MSE diagnostic within a shot; exploration of the effect of combined LH and ICRF on MSE background; and exploration of the effect of ICRF on MSE background in D plasma. To address the first point, the tokamak was maintained at operating temperature overnight, with no ECDC, and the run began with a series of beam-into-gas calibrations shots with TF and vertical field over a period of several hours. The observed drift in apparent angle was substantially reduced relative to prior runs during which the temperatures measured on the MSE in-vessel optics cannister varied as a result of machine cool-down at the start of the run. A final calibration shot taken at 17:00 after a number of plasma discharges, which modified the MSE temperature, showed a significant change in the apparent pitch angle. The effects of thermal stress-induced birefringence due to heating of the surface plasma facing lens was addressed by observing the change in pitch angle in the ohmic phases before and after high power ICRF-heating in plasmas with an extended current flattop. To first order, no changes were observed, indicating the rapid heating during plasma operation does not result in significant errors in the pitch angle measurement by MSE. Experiments were carried out to determine whether rapid changes in the MSE background (polarized light not associated with the DNB) correlated with ICRF and LH turn-on and turn-off are due to line spectra or specific locations in the MSE field of view. No impurity lines associated with the background were observed, and the tentative conclusion is that the background in question is due to particular bright spots within the field of view. A similar conclusion was obtained with respect to the background during ICRF H-mode plasmas. DNB Systems ----------- A refurbished cathode assembly was installed, and the DNB was operational in support of experiments on Thursday and Friday last week. ICRF Systems ------------- FMIT #3 and #4 transmitters were retuned from 70 MHz to 50 MHz, and the J-port antenna matching network was returned to the 50 MHz configuration, for use in the mode conversion experiment on Thursday. After the main part of the experiment had been accomplished, the final power amplifier (FPA) in FMIT#3 suffered an apparent fault in the filament. We are continuing our efforts, along with the vendor, to understand the mechanism for the reduced lifetime we have observed for this generation of FPA tubes. Lower Hybrid System -------------------- LH was operated in support of experiments on three days last week. One klystron exhibited body current in excess of alarm setpoints during the run on Thursday. This tube was disabled (filament turned off) and operation continued. For Friday's run the filament current in this klystron was reduced, resulting in decreased body current readings, and the klystron was returned to service. Klystron 117, which had been refurbished, but developed a vacuum leak upon delivery to MIT, has been returned to the vendor and will be undergoing evaluation. Travel and Visitors -------------------- Robert Granetz gave an invited seminar on fusion energy to the IEEE Nuclear and Plasma Sciences Society and the IEEE North Shore Subsection. The talk was presented at the U. Mass campus in Lowell, MA on Thursday evening, 22 May, and was well received by the audience. Ian Hutchinson attended the International Conference on the Physics of Dusty Plasmas at Ponta Delgada (Azores). He presented a plenary talk entitled "Direct Calculation of Plasma-Dust Interactions". Randy Wilson (PPPL) was at MIT on Thursday to participate in the lower hybrid experiments. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly