Alcator C-Mod Weekly Highlights April 11, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 4.5 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 88 plasma discharges were produced with a startup reliability of 77%. Experiments were carried out in support of five principal MiniProposals. Two of these experiments were co-led by collaborators. Plasma operations are planned to continue this week. The runs on Tuesday and Wednesday continued the work on MP#417 "Plasma performance, boronization and ICRF heating effectiveness with high-Z antenna guards and all metal surface", which directly supports our level 1 (JOULE) research target. A series of limiter discharges were run to evaluate L-mode confinement in the un-boronized state. A density scan between 1.5 < nebar < 3.1e20 m^{-3} was carried out at 1MA, 5.4T with a nominal 3MW of ICRF heating. The trend of degradation of heating and/or confinement at high target density, previously observed in a boronized machine, was recovered in these pre-boronization experiments. Next a series of highly radiative plasmas were produced using injected krypton in limiter and upper null discharges. These discharges provide a cross-check on calibration of the bolometers, as well as providing data on the effects of impurity radiation and radiation profiles on plasma performance. The MP#417 experiments continued on Wednesday with a density scan in lower single null discharges at constant ICRF power, as we continued the evaluation of plasma performance in pre-boronization H-mode discharges. In the density scan the stored energy increased somewhat at the higher target density cases, and radiated power decreased. Puffing additional deuterium into the H-mode phase succeeded in raising the H-mode density and resulted in marginally higher stored energy. However, increasing the puff rate also raised the midplane neutral pressure, and led to increased faulting of the J-port ICRF antenna. Puffing neon in order to cool the edge did not reduce the Mo source rate, and did not help the H-mode performance. Wall pumping efficiency and hydrogen removal were evaluated by leaving the torus pumping valves closed during and after a series of shots with a range of densities at constant 1MA current. Pressure in the machine after the plasma varied from 3-10 mTorr. Relative hydrogen content in the evolved gas, indicated by the mass 3/4 ratio on the RGA, was lowest for the high density discharges. These data are being analyzed. The run on Thursday was dedicated to MP#397 "Comparison of C-Mod EDA regime with JFT2-M HRS regime", and was led by Naoyuki Oyama and Kensaku Kamiya of JAERI, along with Amanda Hubbard of MIT. On both C-Mod and JFT2-M, boronization has operationally been found to be a necessary condition for steady EDA or HRS regimes. However, the corresponding physics reasons are not yet well understood, since both machines have for several years mostly been running with boronized walls. For this run, we revisited the plasma shape and bulk parameters of a previous successful comparison with JFT2M at q_95=3.4 on 4/16/2004. While not part of the original mini-proposal, this run day exploits the current all-metal machine conditions and should contribute to the JOULE target to study the effects of removing boron from C-Mod. We carried out density and power scans with JFT2-M like shape at IP/BT~0.9MA/5.4T (q95~3.4). As previously observed before boronization, the global character was that of standard ELM-free H-modes, with increasing density and radiated power which are terminated by H-L transitions, sometimes preceded by small Type-III-like ELMs. The pedestal electron pressure, Pe_ped, was limited at almost constant Pe_ped over a wide range of collisionality, nu*e_ped, (~0.2-5.0). The pedestal density and line averaged density overlap well the ranges of the previous boronized plasmas in 2004. An interesting result was that the 'QC-mode', a key signature of Enhanced D_alpha H-mode, was seen on the Phase Contrast Imaging diagnostic at f~150 kHz, only at higher nu*e_ped during 'ELM-free' periods. Its amplitude increases with density, particularly during the shots with gas puffing. Its amplitude was rather weak and apparently not sufficient to stabilize the density rise, although it was noted that dne/dt decreased. At lower target density, the H-modes consisted of very short 'dithers', triggered by, and lasting only for the duration of, large sawtooth heat pulses. There were strong buildups of edge radiation, expelled at the back-transitions. However, in contrast to prior runs, this seemed to give a 'quasi-steady' state with near-constant density and radiation, with the back-transitions filling the usual role of ELMs or QC-mode (which was not present). Energy confinement was better than previously seen in the current campaign, with H factors up to 1.5 transiently and 1.3 for long periods, which is as high as we have observed in these unboronized conditions. Two experiments were carried out on Friday. During the first half of the day we revisited MP#412a, " Improved hydrogen-to-deuterium changeover using radiative terminations and disruptions", which had a previous run early in this campaign, while H/(H+D) ratios were high, ~50%. The intent of this run was to evaluate the H/D recovery rate by planned disruptions now that H/(H+D) has fallen to ~5%, and to provide fiducial shots on H/D recovery for comparison after our first boronization. In addition, we wished to document the effectiveness of using RF heated discharges, with larger stored energy, for the intentional disruption program. Dennis Whyte (U. Wisc.) served as co-session leader for this experiment, working remotely from Wisconsin. The gas recovery values had H/(H+D)~10%, in contrast to 40-70% seen during the earlier experiment. The H/(H+D) ratios in the recovered gas is observed to be linearly correlated with the fraction observed in the plasma, with a coefficient of ~1.8. This is consistent with wall fueling dominating plasma content, and indicates that disruption cleaning samples the wall H/D ratio in a similar manner to quiescent plasmas. Most of the recovered H was in the form of HD, rather than H2, confirming the important role that isotope exchange plays in setting the H/(H+D) ratio. The amount of gas released from the wall was less than seen on the first run day, for similar planned disruption events, consistent with the idea that conditioning reduces the total near-surface hydrogenic concentration. A strong correlation was observed between the plasma stored energy (and/or Te) and the amount of gas recovered. Fiducial shots with no disruptions indicated strong wall pumping in the unboronized all-metal machine. For standard 0.8MA discharges, roughly half of the injected D2 is retained in the wall. The reasons for this wall pumping, which does not appear to saturate, remain under investigation. The second experiment on Friday was in support of MP#365 "Inner wall screening of impurities and deuterium". The equilibrium was varied from lower null to upper null during the shots, while N2 was injected using the NINJA capillary on the inner wall. Nitrogen levels in the core were monitored using the McPherson VUV spectrograph. Simultaneously, we monitored both the Mo source rate and the core molybdenum levels. The core N intensity did not seem to vary with SSEP (the signed midplane mapped distance between the upper and lower separatrices). The Mo intensity also was not affected by SSEP, but did decrease with time, probably due to a reduction in the source rate associated with the N2 puff. Spectroscopic measurements did not show any Mo source at the inner wall during these discharges. Physics ------- At the weekly C-Mod staff meeting, Joseph Snipes described results to be presented by Geoff Cordey at the upcoming ITPA meeting in Kyoto, Japan in mid- April on identity experiments between C-Mod and JET to determine whether the collisionality or the Greenwald density limit fraction is more appropriate for dimensionless scaling of energy confinement. Comparisons were made between a high performance discharge on C-Mod and similarity experiments done on JET where the collisionality was matched and in other discharges where the density limit was matched, in addition to shape, beta, and q. TRANSP calculations were performed on the matching discharges and comparisons were also given of the calculated average thermal transport coefficient, chi_effective. While there was not precise agreement, the transport coefficient profiles agreed within error bars, particularly in the mid-radius confinement region. The results indicate that the collisionality match better fits the global confinement data than the density limit match. Further experiments are planned to scan the collisionality in C-Mod at this relatively high beta value (betaN = 1.7) to see if the matching holds at different collisionality. Lower Hybrid System ------------------- The lower hybrid system was operated in piggy-back mode during the run on Wednesday. Different klystrons were run one at a time to verify that the signal paths are correct and to validate the probe signals. Several calibration issues related to the CPS (Coupler Protection System) have been dealt with, and several signal path issues have been resolved. Long Pulse Diagnostic Neutral Beam ----------------------------------- Alignment of the beam to the machine is underway, in preparation for vacuum connection of the DNB to C-Mod. The beam duct is complete and ready for installation. Travel and Visitors -------------------- Martin Greenwald, Miklos Porkolab, and Earl Marmar attended the FESAC meeting in Gaithersburg last week. Gerd Schilling was at MIT 4/4-5 for discussions on operational progress. Ian Hutchinson and Robert Granetz attended the workshop on Dust in Fusion Plasmas, Tuesday 5 Apr 2005, as well as the first day of the TTF meeting. Granetz presented a talk on observations of dust in Alcator C-Mod. At the conclusion of the meeting, Hutchinson helped to draft an outline of the importance of dust studies Catherine Fiore, Joe Snipes, Kirill Zhuorvich, Jim Terry, John Rice, Perry Phillips, Steve Scott and Martha Redi participated in the Transport Task Force meeting in Napa, CA last week and presented results based on C-Mod experiments. Drs. Naoyuki Oyama and Kensaku Kamiya, of JAERI, Japan, completed their visit to C-Mod last week. They served as co-session leaders for Thursday's experiments on comparisons of the JFT2M High Recycling Steady (HRS) H-Mode regime and the C-Mod EDA regime. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly