Alcator C-Mod Weekly Highlights March 3, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 6.7 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, with four run days scheduled and completed. A total of 114 plasma discharges were produced with a reliability of 86%. The runs supported primary experiments by the Lower Hybrid Physics, Advanced Tokamak Integrated Scenarios, and MHD groups. Piggyback experiments also supported research in the Transport and Rotation topics. An overnight boronization was performed prior to the plasma run on Thursday. The cryopump was successfully employed for density and particle control during several runs. On Sunday, Mar 2, a D2 backfill of the vessel was performed in order to carry out calibration of the edge Thomson Scattering diagnostic. All tokamak subsystems performed nominally last week, with minimal unscheduled delays or interruptions in the experimental schedule. Plasma operations are planned to continue this week. Operation Details ----------------- All of the adjustable phase shifters in the Lower Hybrid transmission lines have been replaced with fixed phase shifters, and the first portion of Tuesday's run was devoted to testing the power handling capability of the modified system. These tests were successful, with sustained net power levels over 1MW coupled to the plasma for the first time. In this experiment, power levels of approximately 1.2 MW were obtained, which is near the limit expected from available source power and the plasma coupling efficiency. The pulse width was limited by faulting in an Alcator C vintage circulator installed while repairs are being made to the circulator intended for operation in the current system. Run time on Tuesday and Thursday afternoon was devoted to MP#500 "Development of Diverted AT Target Plasma with ICRF, LHCD and Current Ramp". The goals of these experiments were to obtain early divertor operation (before 200 msec) and apply ICRH and LHCD to modify and sustain off-axis current density. The phase contrast imaging (PCI) and magnetics diagnostics were used to monitor RSAE activity, which are indicators of reverse shear profiles. We were successful in developing current rise programs with early diversion (~180msec), with up to 3.5MW of ICRF heating beginning as early as 0.1sec, resulting in Te0~4keV by 0.2sec, and LHCD as early as 0.12sec. Most of the LHCD cases used a phasing of 90 degrees, but the best example of sustained RSAE activity was produced with a LH phasing of 120 degrees. The primary experiment on Wednesday was devoted to MP#494 "Fast electron driven Alfven eigenmodes". The purpose of this experiment was to investigate the parametric dependences of the high frequency bursting modes observed in the very early current rise (0.02 - 0.035 s) of discharges with LHCD, specifically to determine the dependence of the mode frequency on density and toroidal field to see if it scales as the Alfven velocity B/sqrt(n). The dependence of the mode amplitude on input power, LH phasing, and current ramp rate was also investigated. The results did not fully support the hypothesis that the modes are Alfvenic in nature. In particular, the observed frequency did not appear to be linear in the toroidal field over the range 4.8 < Bt < 6.3. Variation in the toroidal mode numbers could account for this discrepancy, and the spectral analysis to resolve this issue is in progress. Both the amplitude and the frequency of the modes was observed to vary with LH power. Variation of the LH launched phase (60, 90, 120 degrees) affected the excitation of the modes, with the 90 degree phasing giving the most robust activity. Hard xray data are being analyzed to determine the fast electron distribution for these cases. Since the primary experiment on Wednesday was concerned only with phenomena in the very early portion of the current rise, a piggyback experiment was carried out during the flattop portion of the plasma to provide discharge development in support of MP#537 "Momentum impulse from rapid SSEP changes". This effort was successful in developing rapid transitions from lower to upper null equilibria employing vertical plasma motion with the X-points locations held fixed. Vertical velocities up to 1.6m/sec with total programmed excursions of up to 16mm were found to be achievable within stability limits, and with acceptably small overshoots of the programmed position and subsequent ringing. The corresponding change in the magnetic balance parameter SSEP for these excursions was approximately -5mm to +5mm. These transitions appear to be adequate for the purposes of the Miniproposal, and initial data based on the results obtained during the piggyback development experiment are being analyzed. On Thursday morning we devoted a half-day to MP#478 "Joint Experiments with JET on non-resonant n=2 magnetic braking", which supports ITPA Joint Experiment MDC-12. This experiment was led by David Howell (UKAEA) from the JET site using remote videoconferencing tools. This run continued C-Mod experiments begun in June 2007, looking for evidence of magnetic braking from non-resonant n=2 perturbations applied using the non-axisymmetric coilset (A-coil). In the earlier experiments we observed no braking of the intrinsic rotation in EDA H-modes with ICRH power of 2MW and plasma current of 800kA (q95~5). This contrasted with results from JET which had observed braking in NBI discharges, which was interpreted as being consistent with theoretical expectations based on neoclassical toroidal viscosity (NTV). Since the theory predicts stronger braking at higher ion temperature, these experiments were carried out with twice the auxiliary power (4MW). We obtained data at 800kA, for comparison with the previous experiment, and at 1MA (q95~4), which was closer to the q95 value in the JET experiments. No magnetic braking was apparent at either plasma current, implying that a straightforward extrapolation of the JET result based on the NTV theory is not consistent with the results. If the NTV theory is valid, the discrepancy might be due to differences in the plasma regimes (e.g. collisionality) or related to different response of the intrinsic rotation in C-Mod compared to the NBI-driven rotation in JET. The run on Friday was devoted to MP#505c "Study of sawteeth in low-density plasmas and associated RSAEs". The goal of this run was to excite RSAEs during the sawtooth cycle using ICRF. These modes were formerly observed in plasmas of rather low density with moderate to high ICRF power (3-5 MW). Many cases of RSAEs during sawteeth were found, with the best excitation at 5.7 T and 700-800 kA. The day was concluded by a series of secondary current ramps on the flattop portion of the discharge which resulted, unintentionally, in density excursions which will be useful for characterizing the Alfvenic nature of the modes. Additionally, a few RSAEs were excited during the current ramp. This work comprises part of the thesis research of an MIT graduate student. ICRF Systems ------------- Interchanging the E-port antenna fast ferrite tuner units to put the FFT with lower voltage standoff in the less demanding location successfully increased the overall power capability of the system. Subsequent to the change, we coupled 1.85 MW through the E antenna into an H-mode discharge, with the maximum voltage in the matching network reaching 37 kV without faulting. This voltage will be sufficient to cover the expected plasma conditions and is similar to the voltage range of the system as a whole. Lower Hybrid System ------------------- High power testing of the five new lower hybrid circulators was completed last Saturday afternoon. Four of the circulators met specifications and successfully operated under 100% reflected power conditions at high power for 500 ms without faulting. One of the circulators faulted after 100 ms. We are discussing with the vendor how to bring this circulator into spec. Four of these circulators will be needed for the fourth lower hybrid cart which will increase the lower hybrid source power from 3 MW to 4 MW. Diagnostics ------------ The outer 16 channels of the FRCECE electron cyclotron emission diagnostic have been repaired and were operational through the week. The RF unit for the inner 16 channels is under repair by the manufacturer. The frequency doubler is being tested to determine if it can generate sufficient local oscillator power, or if it will have to be replaced. Travel and Visitors -------------------- Dr. Yoshimura from the National Institute for Fusion Science (NIFS), Toki, Japan, has begun a ten week visit at the PSFC. His area of interest is primarily in plasma flows and he is planning to perform flow experiments in the edge of the Large Helical Device (LHD) using Langmuir probes. He will be participating in experiments on Alcator C-Mod, working with Brian LaBombard and Noah Smick on the use of Gundestrup-type scanning-Langmuir probes. Amanda Hubbard and Jerry Hughes visited PPPL Feb. 28--29 in order to collaborate on two NSTX experiments, both of which contribute to ITPA joint experiments (PEP-6 and PEP-16), and in which C-Mod is a joint participant. Thursday's experiment examined the dependence of ELM characteristics on magnetic balance, in particular the sensitivity of ELM type to proximity to double null. Friday morning constituted an NSTX contribution to a comparison study of small ELM and EDA regimes in C-Mod, MAST and NSTX. A companion experiment is ongoing on C-Mod. S. Wukitch, Randy Wilson, Paul Bonoli, John Wright and Steve Richardson (MIT PSFC Theory Division) presented papers at the US-Japan workshop held at PPPL Feb 27-28. Bill Rowan presented a talk at the UT Friday Plasma Seminar entitled "Light Impurity Transport at an Internal Transport Barrier in Alcator C-Mod." Dennis Whyte traveled to Princeton to present the PPPL colloquium "The Challenges of Plasma-Surface Interactions in ITER & Beyond" Bruce Lipschultz attended the one day IAEA paper selection committee meeting in Gaithersburg MD on February 28. US papers will now be revised before submitting to IAEA. Bruce also presented the MIT PSFC Seminar on Friday, Feb 29, entitled "Characteristics of D Retention in a High-Z Tokamak with Implications for ITER and Beyond." C-Mod engineers Jon Savage, Atma Kanojia, and Bill Burke attended a class on CPLD programming in Texas last week. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly