Alcator C-Mod Weekly Highlights June 14, 1999 Last week was a maintenance week at Alcator C-Mod. No plasma operations were scheduled. This week will also be a maintenance week. The tokamak is presently in a maintanance/standby mode, with LN2 cooling and vessel heaters turned off. All components are now at or close to room temperature. We will remain in this state until the ICRF system is ready to resume operation. A backfill in D2 and/or H2 for Thomson calibration will be scheduled during this interval. ECDC and a fresh boronization will also be scheduled prior to resumption of tokamak operation. Physics and Analysis -------------------- The behavior of RF antenna phase balance faults (an imbalance in the current phase in the top and bottom parts of the antenna straps) vs. plasma shape has been investigated for the last seven RF run days. There seems to be a trend of an increasing fault rate for larger up- down asymmetry, defined as the difference in distance from the top of the RF limiter to the plasma and the bottom to the plasma. Questions still remain regarding the effect of overall distance between plasma and limiter (the right gap). Further investigation, including looking at older data, is continuing to see what impact this may have on plasma shape and position consistent with reliable RF operation. The Rydberg series up to n=14 of helium-like chlorine, argon and sulphur have been observed in Alcator C-Mod plasmas. High n satellites to these lines of the form 1s$^2$2s -- 1s2snp and 1s$^2$2p -- 1s2pnp with 3 $\le$ n $\le$ 12 have also been seen for chlorine and argon. Accurate wavelengths of these satellites have been obtained and comparison has been made with code calculations. Line intensities have also been compared with collisional radiative modeling that includes the contribution from dielectronic recombination and inner shell excitation rates to each line's emission. Analysis of the high resolution visible continuum array data has been extended to generate profiles of the quantity n_e*sqrt(Z_eff), by taking the square root of the emissivity profiles of the free-free bremsstrahlung, after taking out the weak electron temperature dependence of this emission. This signal will now be routinely available after each discharge. The time history of Z_eff in the core of the plasma, based on this signal in combination with the cenral density measured with the core Thomson scattering system, is also being calculated and written to the tree. Analysis of a shot which exhibited an ohmic H-mode phase, followed by an enhanced neutron rate L-mode phase, clearly shows the peaking of the density profile which results from the prompt loss of electron density in the outer half of the discharge following the back-transition. This, in turn, appears to lead to the spontaneous formation of a core transport barrier. The effects of subsequent sawteeth, which lead to a return to the normal L-mode profiles, are also clearly observed. ICRF Systems ------------ Final testing (wire test) of the protection circuits on the DC power systems of transmitters #3 and # 4 was successfully completed. Initial RF tests into dummy load were performed on both of these units. FMIT#4 produced 1.4 MW into dummy load for 0.5 sec; from FMIT#3, 0.4 MW was injected into the dummy load. The maximum power output was limited by the reliability of the RF control system, which is now being improved, along with other diagnostic instrumentation. Preliminary testing of the screen crowbars and grid bias regulators for FMIT#1 and #2 was completed. Installation of these devices is necessary before beginning testing of FMIT#1's and #2's crowbar circuits. Travel and Visitors ------------------- Chris Brunkhorst (PPPL RF engineer) was onsite at C-Mod all last week, helping with the ICRF work. Gabriele Manduchi from the RFX experiment in Padua visited Tom Fredian and Josh Stillerman to discuss future directions of the MDSplus data handling system. Dr. Manduchi, a member of the original MDSplus development team, has been developing java based tools to extend the graphical interfaces of MDSplus to other non-X-window/Motif platforms. Dr. Rejean Boivin visited Chalmers University of Technology, Goteborg, where he presented a talk on "Neutral Particle Physics in Alcator C-Mod Plasmas". He also spent time at W7-AS where he worked with Dr. Louis Giannone on AXUV diodes for bolometry. Ron Bravenec, U.Tx., has been at C-Mod since Thursday calibrating the BES filter/spectrometer for D-alpha profile measurements. This was done by shining a hydrogen lamp into one end of a test fiber and progressively moving the other end channel-to-channel. For each channel the bandpass filter was tuned to maximize the signal. The result: a relative calibration of the detector electronics, bandpass-filter attenuations, neutral-density filter attenuations, etc. Since the system has already been absolutely calibrated at 660 nm, and we know the variation of bandpass-filter attenuation with wavelength, the system will be absolutely calibrated at D-alpha as well. Ron will leave this Wednesday, and plans to take data remotely during the first few days following resumption of plasma operations . The D-alpha profiles will be compared with a model which uses density and temperature profiles from the TRANSP database. He will return afterwards, install new neutral-density filters into the spectometer, and move all nine fibers to look at the edge plasma. We will then take D-alpha fluctuation data, hoping to detect poloidal propagation of the fluctuations (among other things) which would give us a measure of the radial electric field. Martin Greenwald travelled to Brookhaven National Lab to attend a meeting of the ESnet Steering Committee. Topics discussed included network requirements for the DOE-NGI which looks likely to be funded for FY99-00, and DOE-SSI which faces a more uncertain future. A good deal of time was devoted to computer security issues, which have been raised to very high visibility in recent months. Bob Fink briefed the group on implementation plans for IPV6 - a new version of the internet protocol which has the alleviation of the address space crunch as its principal aim. Also discussed were upgrades to connections to Internet2 and international interconnection points, which provide connectivity to universities and international sites respectively.