Alcator C-Mod Weekly Highlights July 5, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 11.5 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, with four run days scheduled and completed. A total of 118 plasma discharges were produced with an overall reliability of 89%. Experiments carried out last week supported research in the Transport and Operations areas, the Burning Plasma thrust, and contributed to the C-Mod Level 1 (JOULE) target "Measure plasma behavior with high-Z antenna guards and input power greater than 3.5 MW". Two boronizations were carried out, over Tuesday and Thursday nights. Plasma Operations are scheduled to continue this week. Run Summaries -------------- Tuesday's run was devoted to MP#426, "Structure and Stability of Large Type I ELMs", which aims to study access conditions to the large ELM regime in C-Mod and to study their spatial structure in order to increase physical understanding of the responsible instability and connect to results on other experiments. During this run the JFT2-M shape proved more conducive to production of discrete ELMs than the "inner nose limited" shape, which was also investigated. A complicated ELM structure is observed by the fast diode array as well as in the outboard edge video images. In the SOL the ELM looks like a large blob with fine structure. Fast diodes detect a coherent ELM precursor on views inboard of the separatrix. No large scale spatial structure, i.e. filaments, was observed with the wide-angle-view movies. Nonetheless, changes in emission from the lower inner-wall/divertor region on a ~2 microsec timescale were observed in this view. The ELM character was substantially modified as the triangularity was scanned, with the duration of the ELM varying from ~60usec to 4msec as the X-point radius was moved over a range of about 4 cm. ELMs were most apparent when the density was just above the low-density H-mode threshold, nebar~8e19/m3. Pedestal temperatures as observed on Thomson scattering were up to 1keV early in the H-mode phase, decreasing as the density rose. The outermost ECE channel observed a 15% temperature perturbation due to the ELMs. On Tuesday night a fresh boronization was carried out with the ECDC resonance scanned from 50 to 80cm, which covers the divertor region but not the inner wall or outboard structures. Wednesday's run was devoted to evaluation of this boronization, continuing our investigation of the effect of boronization with all metal walls and plasma-facing components (MP#417). Steady H-modes were obtained with confinement enhancement over ITER_H89 approaching 2. ICRF reconditioning was carried out in upper null discharges, and progressed more quickly than after other recent boronizations. A ramp on the RF power waveform seemed to be beneficial in terms of reducing impurity influx and in obtaining full RF power with minimal faulting. Slightly smaller outer gaps and higher target density helped increase RF reliability by lowering the antenna voltage for fixed RF power. The 20% higher target density (nebar~ 2e20/m^3) also led to a dramatic drop in Mo radiation as well as the total radiation. Further increases in target density had no deleterious effects and may have been beneficial. QC modes were observed on all H-mode discharges with target nebar>1.6e20/m^3. Operation with the strike point on the floor (slot divertor configuration) was not as good in terms of confinement or impurities as with the strike on the outer vertical plate. H-modes obtained in upper null discharges earlier in the day appeared to be less EDA-like, but had confinement enhancement over L-mode above 2 in some cases. The run scheduled for Thursday was in support of MP#421 "Obtaining Steady, Low-density H-modes", which proposes to investigate a technique under which the L-H transition is obtained at reduced density in a low-current plasma and then ramping the current and RF power up to their final values with no additional gas puffing. This run was not successful due to excess radiated power at the lower densities. This effect may have been due to degradation of the boronization or to a small air leak between 7:00 and 10:00 PM on Wednesday night. The experiment was discontinued after 10 shots, and will be rescheduled. The remainder of Thursday's run was devoted to MP#348a, "Initial tests of the boron injector", which is intended to establish a technique for prolonging or improving boronization by introducing boron powder directly into tokamak discharges. The boron injector has been modified since the last run of this experiment to allow larger granules (90-180 um) and to inject as much as twice as much boron per pulse. Following a fiducial shot, the injector was enabled with a long pulse train beginning early in the discharge. This program resulted in measurable boron levels in the plasma but also led to early disruptions. Following recovery from these disruptions, the number of pulses from the injector was reduced, and subsequently increased shot-by-shot until another disruption ensued. Long pulse trains were again tried later in the run with timing adjusted to begin the injections after the plasma reached current flattop. The equilibrium was also modified to an outboard limited configuration to see if this resulted in deposition of the boron on more critical surfaces. The total radiated power remained constant throughout the day at about 50% of the input power, although the Mo radiation was reduced by the boron injection. Boron was observed spectroscopically, and boron hydride signatures were seen on the RGA, with spectra and amounts similar to those observed following disruptions under well-boronized conditions. These data are being analyzed. There was no obvious beneficial effect of the boron injector on plasma performance during this run. Following a full boronization (coverage extended to include the inner wall) on Thursday night, Friday's run was devoted to MP#422 "ITB formation -- Ti critical gradient dependence". The purpose of this experiment is to understand triggering mechanism for producing off-axis ICRF internal transport barriers. The question of whether ITB formation can be explained in the framework of suppressing ITG turbulence via increasing ion temperature scale length to the critical scale length is addressed by varying Bt and thus shifting the RF resonance location on shot-to-shot basis while monitoring the gradient scale length. A detailed scan of the ICRF resonance position was performed, increasing the field from 5.3 to 6.4T in small increments while maintaining constant q. However, ITB conditions were not obtained in this sequence, and therefore the goals of the MP were not completed during this run. This experiment will be continued at a later date. Lower Hybrid System ------------------- Work continued last week in preparation for prototype braze tests. Fabrication of new stainless steel couplers is on shedule as is delivery of new alumina windows. Work continues on hardware and software upgrades to the phase and amplitude calibration system. ICRF System ----------- Work is underway in preparation for the scheduled change in frequency of the FMIT#3 and 4 transmitters and J-port antenna from 78 MHz to 50 MHz. Calibration of the forward and reflected power monitors for the new coax length has been completed. Testing of the stub tuner has also been carried out. Diagnostic Neutral Beam System ------------------------------- We continued conditioning of the DNB last week, and have reached beam currents up to 5.5 A while pulsing for a one second total pulse length at 50% duty cycle (modulated beam 100ms on/ 100ms off). Higher currents still produce some beam faults. Travel and Visitors -------------------- Glen Wurden and Ivo Furno made a three day visit, from June 26-29, in order to get the IR camera remote digital acquisition interface working. They reinstalled the EDT capture card hardware, updated its software, increased the control computer memory, and debugged the LVDS trigger system. After rebuilding the trigger interface hardware box, and adding a trigger transformer (to break a persistent triggering ground loop), the system is now working. One hundred twenty frames of 12-bit image data is automatically written to the MDS tree. Ivo Furno is transferring his collaboration back to Glen, as Ivo is heading back to Europe next week, to the TCV tokamak. Amanda Hubbard and Brian LaBombard attended the EPS Meeting in Taragona, Spain last week. LaBombard presented an invited talk entitled "Transport-driven scrape-off layer flows, the role of the X-point and connections to the L-H power threshold in Alcator C-Mod". Olaf Grulke (MPI Greifswald) presented a paper "Turbulence imaging of spatiotemporal fluctuation structures in the scrape-off layer of Alcator C-Mod". Martha Redi (PPPL) submitted a poster and four page paper entitled "Testing Gyrokinetics on C-Mod and NSTX". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly