Alcator C-Mod Weekly Highlights September 4, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 14.7 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four and a half run days were completed, supporting experiments in the Lower Hybrid and Transport Physics areas, the Integrated Scenarios thrusts, and the Diagnostic Development effort. A total of 118 plasma discharges were produced with a startup reliability of 88%; ten additional beam-into-gas shots with vertical and toroidal fields were produced in support of MSE calibration. Boronizations were carried out on Tuesday, Wednesday and Thursday nights. The cryopump was used during the experiments on Wednesday and Thursday. There was no recurrence of the ground-fault condition on the toroidal field magnet. With the completion of last week's experiments, C-Mod has achieved 98% of our operational target of 15 research weeks, satisfying our part of the OFES major facility operation JOULE Milestone for FY07, "Average achieved operation time of the major national fusion facilities as a percentage of the total planned operation time in FY 2007 - >90%". The tokamak is now warming up in preparation for in-vessel maintenance. No further operations are planned during this fiscal year. Operation Details ----------------- A half-day run on Monday was devoted to the completion of the dataset for MP#498 "LH-driven fast electron diffusion". X-ray emissivity profile data were obtained using modulated lower hybrid power with 90 degree phasing for three conditions: nebar=8e19/m^3 and 1.25e20/m^3 at 500kA, and nebar=1.6e20/m^3 at 800kA. This experiment comprises part of the doctoral research of an MIT graduate student. Tuesday's run continued the experiments devoted to MP#515 "Effect of launched LH wave n_parallel spectrum on current profile". The purpose of the run was to drive current with LH waves launched at several values of n_par, and to measure the resulting current profile, to the extent possible, with MSE. Hard X-Ray profiless were also monitored and will be compared with the j-profile changes inferred from MSE. LHCD was applied to both 800kA and 1MA discharges. The maximum coupled power was ~700kW, resulting in a decrease of ~50% in loop voltage. The driven current may have been somewhat closer to the edge than expected, judging from the rapid response of the loop voltage. With different values of phasing we observed both stabilization and destabilization of tearing modes. In the case of the largest drop in loop voltage the sawtooth inversion radius was significantly reduced; complete sawtooth stabilization was not observed on any shots in this run. The run on Wednesday supported MP#496 "High performance operation at Ip>1.2MA". The purpose of these experiments is to explore the confinement, transport, pedestal scaling and divertor behavior as plasma current is scanned above 1.2 MA with BT~5.3T, with plasma shaping similar to the standard ITER equilibrium (kappa~1.85, triangularity~0.5). These experiments will contribute to one of the C-Mod plain English goals for FY2008, Confinement at High Plasma Current. Extrapolation to these results to ITER parameters will be a major goal of these studies. Most of the day's experiments were carried out at Ip=1.35MA (kappa~1.8, q95~3.4). Plasmas with higher current (up to 1.5MA) typically ended in VDE disruptions, usually occurring after an H->L transition. Stored energies above 200kJ and volume average plasma pressures above 1.4 atmospheres were readily obtained with total input power above 4MW. Confinement enhancement (relative to the ITER-89P L-mode scaling) typically reached about 1.4 in H-mode.The best performance was obtained at higher plasma density (nebar~5e20/m^3). The first half of the run on Thursday was devoted to MP#504 "Slow L-H transitions", which comprises part of the thesis research of an MIT graduate student. The purpose of this experiment was to obtain data in the normal (ion grad-B drift down) field orientation with matching plasma conditions to the reversed field experiments carried out in July. Data were obtained with both lower and upper null equilibria. The second half of Thursday's run supported calibration of the new high resolution soft x-ray profile diagnostic HIREX_SR. This diagnostic, a collaboration with PPPL, is used to obtain profiles of ion temperature and velocity from hydrogen- and helium-like argon emission. Strong argon puffing into ICRF-heated high density upper null L-mode plasmas was used to produce highly radiative discharges dominated by argon emission. The cryo-pump was used to produce steady radiated power with a constant argon puff during the discharge; the pressure in the argon plenum was adjusted to vary the radiated power level from shot to shot. The argon count rate was increased above 0.5e5/sec using this technique. At this signal level, the integration time of the diagnostic could be reduced sufficiently to resolve sawteeth with good signal to noise ratios. The beam-based charge exchange recombination signals were also observed from He-like argon by the VISPEC diagnostic and also with the University of Texas CXRS system. Additionally, a calibration of the rotation velocity (zero point) measured by the HIREX_SR diagnostic was obtained by inducing a locked mode in a pair of 800kA ohmic discharges. These shots also contributed to testing of a real-time locked mode detection algorithm implemented through the plasma control system. This experiment contributed to the research of several C-Mod graduate students. The majority of Friday's run was devoted to MP#501 "Fluctuations in ITB plasmas". Off-axis ICRF heating was used to produce internal transport barrier (ITB) plasmas. The PCI (Phase Contrast Imaging) diagnostic was employed to monitor the fluctuation spectra. Masking on successive identical discharges permitted isolation of the upper and lower portions of the plasma, allowing determination of the localization and propagation direction of the instabilities. Ion temperature profiles were obtained for the core using HIREX_SR and further out in the profile by beam-based CXRS. Additional field cycles (no plasma) were run on Friday to continue the investigation of variability in the field angles measured by the MSE diagnostic, using the beam-into-gas calibration technique. These shots were produced at the start and end of the day. A significant variation (~ 1 to 2 degrees) was observed between the two datasets. Lower Hybrid System ------------------- The lower hybrid system was employed in support of experiments on Monday and Tuesday last week. Up to 700kW of LH power was successfully coupled to the plasma. Digital Plasma Control System ------------------------------ A real-time procedure for identifying locked modes, based on statistics of central electron cyclotron emission in normally sawtoothing discharges, was tested in DPCS during Thursday's run. The procedure successfully identified locked modes in both plasmas for which they were present. One false positive was recorded during 14 non-locked mode plasmas, which is consistent with the error rate observed during off-line tests of this algorithm. The real-time procedure operated successfully within the 100 usec cycle time normally used for C-Mod plasma control; the time allocated for operation of the locked mode detection routine was about 10 usec. This work comprises part of the thesis research of an MIT graduate student. Travel and Visitors -------------------- Randy Wilson (PPPL) visited MIT on Tuesday and participated as co-session leader in the Lower Hybrid experiment that day. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly