Alcator C-Mod is about to begin its scheduled run campaign for 1997. The primary activities at Alcator C-Mod during the second quarter of FY97 were re-assembly of the tokamak, installation of new and old diagnostics, continued work on major systems upgrades (the RF transmitters and the Diagnostic Neutral Beam), continued analysis of C-Mod data from previous run campaigns, preparation of the Field Work Proposal for the fiscal years 1998 and 1999, and continued participation in the critical assessment of fusion science. A more detailed account of these activities follows.

Scientific Results

Analysis of H-mode regime transition from ELM-free to enhanced D_a is proceeding. The enhanced D_a regime is of great interest primarily because a promising de-coupling of energy and particle confinement is observed. The energy confinement remains at a level close to that of ELM-free H-mode, while the particle confinement decreases significantly. In some shots, the large decreases in impurity particle confinement (compared to ELM-free H-mode) were well correlated with increases in edge density fluctuations, which increased almost to the L-mode level. The effect is shown in Fig. 1.

Toroidal rotation velocities greater than 1×10⁷ cm/sec (1.5×10⁵ rad/sec) have been observed in the highest H-factor H-mode plasmas (ICRF only), in the co-current direction. When I_p is reversed, the rotation also reverses direction. Spatial scans indicated the rotation velocity is highly peaked near the magnetic axis. These measurements were made by observing the Doppler shift in x-ray line radiation from highly stripped argon. The argon toroidal rotation radial velocity profile has been used to determine the radial electric field during H-mode and enhanced D_a H-mode discharges. At r/a = .3, the field is 300 V/cm, about a factor of 4 larger than the ion diamagnetic term. A toroidal rotation velocity profile, constructed from the Ar line shift measurements and the rotation of the m=1 sawtooth precursors, is shown in Fig 2. It is important to remember that these rotations are observed in discharges which have no external momentum source.

C-Mod has contributed five shots to the ITER profile database. These shots include examples of ELM free H-modes, ELMy/EDA H-modes, reversed field L-modes (so called enhanced L-modes) and a normal L-modes. These are in addition to the hundreds of shots contributed to the ITER H-mode confinement, L-mode confinement, and H-mode threshold databases.

A numerical study of the effect on the neutral behavior of reducing the size of the inner divertor ``nose'' has been performed in collaboration with Daren Stotler at PPPL using the new Monte Carlo code, DEGAS2. The code used input plasma conditions taken from C-Mod probe measurements. The most important finding of this study is: Because of the divertor plasma's opacity to recycling neutrals, the ``nose-job'' should make little difference to either the amount of neutral ionization in the confined plasma or the resulting neutral pressure in the divertor.

Progress is continuing on the development of EDGEFIT, a 2-D ``interpretive'' (TRANSP-like) edge transport code (graduate student, Maxim Umansky). 2-D T_e solutions to a reduced Braginskii fluid model of the edge plasma and scrape-off layer are computed using density and temperature measurements from probes. These fitted solutions can be readily computed for any discharge for which good plasma measurements and EFIT reconstructions are available. The goal is to determine c_^ values (or profiles) that best fit the experimental data for a wide variety of operating conditions. From a large ensemble of fits, it is hoped that empirical transport scaling laws may be identified. Analysis is presently focused on the sensitivity of inferred c_^ values to errors in the input data set.

Key results from a graduate student's PhD dissertation (A. Niemczewski) were recently published in Nuclear Fusion. The article entitled, ``Neutral Particle Dynamics in the Alcator C-Mode Tokamak'', presents data and analysis that demonstrate the role of a cold plasma layer in plugging the divertor throat to neutrals in C-Mod. The observed high divertor/midplane neutral pressure ratios, particularly in detached plasmas, are shown to be consistent with the neutrals' scattering off a cold plasma layer and remaining trapped in the divertor volume.

Dr. Marco Brambilla from IPP Garching visited MIT and collaborated with Paul Bonoli on development and testing of the full-wave toroidal ICRF code TORIC. The code was tested on (H) and (³He) minority heating scenarios in C-Mod as well as on mode conversion electron heating (MCEH) cases. TORIC was found to work well in the cases of H(³He) and D(³He) MCEH when compared with the experimentally measured electron heating profiles. The code utilizes a numerical broadening of the ion-ion hybrid layer in order to ensure a well-behaved electric field solution at that location. The code was also tested with and without the order reduction algorithm (ORA) implemented. It was found that results were acceptable in all cases with the ORA turned on (i.e., the results were the same as solving the full fourth order wave equation with the ion Bernstein wave included).

Based on the ICRF code testing done at MIT it was decided that a version of TORIC should be made available with ORA only. This will be done in collaboration with Marco Brambilla. This numerical model could then be implemented in FPPRF, in place of the SPRUCE full-wave code, which has been shown to have numerical problems. The version of TORIC with ORA only was found to execute rapidly on a CRAY J-90 (90 cpu sec.). A comparable cpu requirement would be expected on a VAX ALPHA workstation or a UNIX workstation. The modified FPPRF code would then be used in TRANSP for analysis of ICRF heating experiments. We also plan to couple TORIC to an ICRF current drive package in order to more carefully assess the current drive capability (via mode converted IBW) of the 40 MHz rf system.

Operations - Diagnostic and Systems Improvements

An large amount of work was done over the last three months to re-assemble the machine. After the scheduled inspection and maintenance, the magnets were re-assembled around the vacuum vessel, and the supporting super-structure was built up around the magnets and vessel. The history of this assembly can be viewed at the PSFC WEB site as a time-lapsed series of images of the Alcator experimental cell. The ``movie'' was compiled from what were live images. The live images of the cell and machine, updated every few seconds, are available on the WEB at

http://www.psfc.mit.edu/cmod/live.html

When we are running the machine, a live view of the control room, available for our off-site collaborators and other interested persons, will be available at

http://www.psfc.mit.edu/cmod/controlroom.html.

Video showing a wide angle, invessel view of the most recent plasma discharge in D_a light will be available on the Web as well. (These quarterly reports can be found on the WEB at http://www.psfc.mit.edu/server-java/CModQuarterlies.) During the machine maintenance and re-assembly, invessel work was occurring almost continuously. This work included both maintenance of invessel components and installation of new diagnostics.

a) New Invessel Diagnostic Installations

A new optical fiber viewing system has been installed. This system is unique in that fibers enter the torus through a novel vacuum feedthrough to viewing optics attached within the vessel. The advantages of this system are that viewing windows are not needed and the optics can be placed close to the diagnosed regions. Views include radial arrays at the fast scanning probe and limiter gas puff locations (for impurity screening and transport analysis), and spatially integrating views of RF antennas and limiters. These latter views will help in the diagnosis of molybdenum source rates.

The Omegatron probe, a Z/M ion spectrum analyzer, has been installed on J-port top (graduate student, Rob Nachtrieb). The first plasma operation of the probe (in gridded energy analyzer mode) will be attempted in ECDC conditioning discharges. During tokamak plasmas, this mode of operation will allow a measurement of the ion temperature in the scrape-off layer, in addition to the Z/M impurity spectrum.

Modifications to the F-bottom fast-scanning probe-drive to inject trace amounts of impurity gas deep into the scrape-off layer (up to the separatrix) have been completed. The flexible capillary delivery system has demonstrated vacuum integrity for up to 20,000 cycles. It is expected that the actual system will be used for less than 1000 cycles per run campaign. Tests show that the inertial valve embedded in the probe head is also performing to specification. External plumbing to the torus gas delivery system remains to be connected. Design, fabrication, and installation of two optical imaging diagnostics to record light from impurity line-emission ``plumes'' emanating from the fast-scanning probe have been nearly completed. The first system, a toroidally-viewing telescope, has been installed inside the vacuum chamber. A toroidal view of the injection through the tip of the scanning probe (in the SOL) will be used to obtain parallel flow velocities by measuring doppler shifts of selected impurity lines (U. Md. and PPPL collaboration). The measured flow velocities will be compared to Mach probe measurements from the probe as well as from analysis of the injection plume as seen by a second viewing system, a vertically viewing CCD camera/filter/periscope. The optical fiber bundle and camera/filter system is assembled, and awaits installation and checkout (graduate student, Sanjay Gangadhara). The latter system will be used to deduce local cross-field and parallel transport by tracking the dispersal of impurity emission plumes (in part with U. Toronto collaboration). Combined with information from core spectroscopy diagnostics and the Omegatron Z/M spectrometer, the gas delivery system will also be used to assess directly the screening of the SOL for extrinsic, non-recycling impurities.

One of the x-ray imaging arrays has been reconfigured to provide a very high resolution view of the plasma edge and scrape-off region (1 mm spatial resolution), with data sampling rates up to 100 kHz. This upgraded diagnostic will be used to study the magnitude and scaling of the H-mode edge pedestal width, as well as ELM's and other edge instabilities.

Our collaborators from the Johns Hopkins University, Sean Regan, Mark May, and Vlad Soukhanovskii, have installed the ``XUV Invessel Monochromator'' in the machine. This diagnostic uses curved multilayer mirror coated optics as disperion and imaging elements. It was designed, constructed, and photometrically calibrated at JHU. Its view is of the x-point region, along the same line as the x-point Thomson scattering laser beam, and as such does not look through the main plasma. It will monitor a resonance line from N⁺⁴ (162.5 Å) during nitrogen gas puffing experiments.

The contribution of the CX-neutrals flux to the power balance is of great interest on C-Mod, and a number of diagnostic changes have been made in order to quantify it. Sets of absolutely calibrated silicon diodes (AXUV) have been installed. Normal bolometers measure radiation but are also sensitive to power carried by neutrals. The new diodes are mostly insensitive to neutrals and have a very good time resolution. Two arrays (19 and 16 channels) were added inside the vacuum vessel. The first one looks at the edge over a 4 cm span at the separatrix (midplane) with a chordal resolution of 2mm. This should also give us invaluable information on the pedestal formation during H modes. The second array covers the low field side of the plasma to provide emissivity profiles with very good time resolution. Finally, 2 detectors have been installed with a broad view of the plasma (2p), one being a normal bolometer and one being an XUV silicon diode. Since they are sharing the same view, comparisons between the 2 detectors should yield additional information on the neutral component of the power loss. Finally, a gas capillary has been repositioned to allow gas to be fed into the x-point bolometer array box. This change was made to allow the effects of neutrals and radiation to be separated in the bolometer data.

New shutters for protecting (from boronization) the tangential interferometer mirror and the re-entrant window for the wide angle TV view were tested and installed.

b) External Diagnostics

The new, neutral particle time-of-flight diagnostic (originally used at LANL) for measurement of low energy neutrals has been installed.

A new 95mg Cf₂₅₂ source was purchased from Oak Ridge, replacing the old source which had decayed to the 18 mg level. The new source allowed us to achieve good signal-to-noise levels during our neutron calibrations in much less time.

In order to measure the D₀ Lyman alpha line, molecular deuterium lines, and the strong carbon and nitrogen lines above 1200 Å, the long wavelength end of the spectral range of our VUV spectrograph is being extended from 1100 to 2200 Å. In addition, an in situ calibration facility for the spectrograph has been developed, installed, and is being tested.

The Divertor RGA is in the process of being calibrated off line on a test stand. This diagnostic is used to follow the concentration of impurity gases in the divertor for impurity transport analysis. John Goetz and Bruce Lipschultz have developed a method of monitoring the gain of the RGA amplifier on a day-to-day basis. They are in the process of calibrating the sensitivity to impurity gases (Ar, Ne and He) in the presence of a substantial D₂ background.

Many calibrations, including neutron, D_a arrays, and visible bremsstrahlung, and alignments including tangential interferometer, bolometer, core and X-point Thomson scattering, and fiber view registrations have been completed. Magnetics, x-ray tomography, fast thermocouple, divertor probes, and MHD coils, among other diagnostics, were connected and tested for proper operation.

An extensive set of pumping tests on the prototype cryopump has been carried out. The pump easily handles gas loads larger than those expected on C-Mod. Pumping speeds of 1000 l/s have been measured during deuterium gas puffs with peak pressures of 25 mTorr. The prototype cryopump is now installed behind a divertor module.

c) Systems Improvements

With PPPL collaborators (Dr. Gerd Schilling and John Gumbas), the new 40-80 MHz transmitter (FMIT#3 and #4) and associated transmission line system have been assembled. For both FMIT#3 and 4, the Driver, IPA, and FPA have been assembled, and the HV switchgear has been tested and operated remotely. The trigger circuits have been installed and tested. Safety interlocks and bias and filament power supplies have been tested on both FMIT#3 and 4. All non-HV transmitter tests have been completed and all non-HV power supplies have been tested. The associated transmission line, resonant loops, and decoupler loop have been assembled.

Preparations for the addition of a major new diagnostic, the Diagnostic Neutral Beam, acquired as part of our collaboration with U of Tx - FRC, continue. The DNB is scheduled for installation for the 1998 run campaign. Installation and commissioning of the DNB power supplies continued. Reassembly of the Mod/Reg has begun, and we began to develop the design changes that are required to incorporate the new ACCEL supply. Control system design on the fast interface between the main control system, the ACCEL supply, and the crowbar continued.

FWP Preparation and Presentation

A significant effort went into the preparation of the Field Work Proposal, covering the years 1998-1999. Ian Hutchinson presented our research plans at the FWP meeting in Germantown in early April. He was accompanied by Steve Wolfe, Miklos Porkolab, and Earl Marmar. Part of the executive summary is excerpted here because it clearly states C-Mod's position in the fusion science effort:

Alcator C-Mod is the high-field, high-density divertor tokamak in the world fusion program. It is one of five divertor experiments capable of plasma currents exceeding one ma. Of these, JET and JT60-U, are the flagships of their national programs, and DIIID and ASDEX Upgrade are intermediate size and field devices. The current density in Alcator C-Mod, for example, is about eight times that in DIIID and JET. This positioning provides an overall theme to the Alcator program:

Compact high-performance divertor tokamak research to establish the plasma physics and plasma engineering necessary for an ignited tokamak experiment and for attractive fusion reactors.

Because of its compact dimensions, Alcator C-Mod investigates an essential area in parameter space, which complements the world's larger experiments, in establishing the tokamak physics database.

Three key areas of investigation have been called out in which Alcator C-Mod has a vital role to play.

·

Divertor research on C-Mod takes advantage of the advanced divertor shaping, the very high scrape-off-layer power density, unique abilities in impurity diagnosis, and the High-Z metal wall, to advance the physics understanding of this critical topic.

·

In transport studies, C-Mod is making critical tests of both empirical scalings and theoretically based interpretations of tokamak transport, at dimensional parameters that are unique but dimensionless parameters often comparable to those in much larger experiments.

·

In the area of Advanced Tokamak research, so important to concept optimization, the high-field design of the device also provides long pulse length, compared to resistive skin time, which provides an outstanding opportunity to investigate the extent to which enhanced confinement and stability can be sustained in steady-state, using active profile control

Collaborations and Participation in the Fusion Science Community

Our collaborators from UT-FRC have been concentrating on preparations for the spring campaign. Roger Bengtson (U. Texas) has made progress assembling components for a horizontal fast scanning probe, optimized for fluctuation studies. The probe is currently being shipped to MIT and will be installed on K-port horizontal. The probe should be operational for the latter half of the spring 1997 run campaign. Piggy-back experiments using some combination of F-bottom (MIT), K-side (U.Texas), A-side (MIT) fast-scanning probes, and the F-port divertor probes are being planned for the run period. The goal is to characterize both intrinsic and driven SOL fluctuations (using a technique developed on TEXT) by correlating probe measurements on the same field line but separated by a distance of 5 to 10 meters.

The ECE radiometer which will be used by the Auburn/MIT/UT collaboration for electron temperature profiles and fluctuations is now at MIT, also with all its required CAMAC equipment.

Testing of components for two DNB diagnostics, BES and CXRS, is planned. The equipment for both of these is in the final stages of preparation at UT.

PPPL collaborators Joe Bartolick, Boris Grek, and Dave Johnson successfully completed the re-installation and alignment of the X-point Thomson scattering diagnostic. The Raman calibration of the system has been completed as well.

Work continued on the repair and upgrade to the hardware used in the high resolution, visible light measurements made by our U. of Md. collaborators (Ben Welch and Jim Weaver). An optical system to provide a wavelength calibration for the spectrograph was completed and tested. This system will be used in the upcoming run period as an absolute wavelength standard for Doppler shift measurements. The spectrograph was also modified so that both the 2400 lines/mm grating and the 600 lines/mm grating could be used. This will take advantage of the higher sensitivity and blaze (5000 Å) of the 600 lines/mm grating to view the less intense (boron) impurity lines. Jim Weaver has relocated to the MIT area and will remain at MIT for the duration of the summer 1997 run period.

A new IR imaging system is being installed at C-Mod by Los Alamos National Laboratory collaborators Ricky Maqueda and Glen Wurden. This system is based on an Amber Radiance 1 IR camera on loan from Los Alamos and a periscope utilizing ZnSe optics. Bench tests have show good imaging capabilities for objects placed in a 35 cm x 35 cm field-of-view, located ~ 90 cm away from the first optical elements (simulation of C-Mod's divertor region). Good field depth was demonstrated as well. At present this system is partially installed on C-Mod and will be fully installed and operational soon after the igloo on top of C-Mod is installed before the upcoming run.

The KODAK fast framing (1000 frames per second) camera (on loan from Los Alamos National Laboratory) used on C-Mod during the previous run period is being re-installed on C-Mod after being used on the last run period of TFTR. A new tangential view is available for this run campaign.

Martin Greenwald, Joe Snipes, and Steve Wolfe participated in the April ITER Confinement and Database Expert Group Meeting in San Diego. Joe Snipes presented results of regression fits to data in the ITER H-mode threshold database from eight different tokamaks. The regression to all of the ITER appropriate H-mode threshold data ( 2.3 < q₉₅ < 4, 1.4 < k< 1.8) gave a result very close to that presented by Takizuka at the 1996 IAEA meeting ( P_threshold = 0.6 n^0.75 B^1.0 R², yielding a P_threshold for ITER just over 100 MW). Using a suggestion by F. W. Perkins to choose only the lowest threshold points reduced the scatter in the data from an RMSE = 30% to an RMSE = 21.5% and lowered the predicted ITER threshold to about 50 MW. This should be considered an optimistic projection to ITER since it uses only the lowest threshold points in existing tokamaks assuming ITER will be able to achieve similarly low threshold power levels.

Considerable effort was spent on ITER divertor database work. Spencer Pitcher put together a new set of up-to-date data used for Brian LaBombard's transport analysis. These data were packaged in a format suitable for ITER and shipped to them. In addition, Bruce Lipschultz organized participants at all the major laboratories in a discussion of a common method of extracting scrapeoff lengths from edge data. A technique was agreed upon (integral) and tried out on C-Mod, JT-60U and ASDEX-UG data. The results were discussed at a three day workshop in San Diego (March 10-12) which Lipschultz attended. The technique was refined there, and the participants agreed to try again to put together a dataset from each machine and deliver it to ITER and Lipschultz. C-Mod also contributed edge pedestal data (A. Hubbard) to this group as well. Other main topics were ELM's and separatrix location and its effect on edge measurements.

Jim Terry spent two weeks at JT-60U studying the JT-60U divertor operation and observing the overall machine operation. One of the purposes of the visit was to present to the Japanese the techniques we have developed at MIT to measure experimentally the rate of volume recombination occurring in the divertor. Some time was also spent analyzing spectroscopic data from the JT-60U divertor experiments, looking for signatures of recombination in the JT-60U radiative/detached divertor.

Members of the C-Mod RF group (P.T. Bonoli, C. Rost, Y. Takase, and S. Wukitch) attended the Twelfth Topical Conference on RF Power in Plasmas, held in Savannah, GA on April 1-3, 1997. Yuichi Takase gave an invited talk on ICRF heating experiments in Alcator C-Mod and the other conference attendees from MIT gave posters on related topics.

Two students received their Ph.D.'s, Paul Stek, now at JPL, and Peter O'Shea, now an MIT Post-doc located at GA.

Near Term Plans

Our near term plans are focussed on the up-coming run campaign. (The schedule can be found on the WEB at http://www.psfc.mit.edu/cmod/run_schedule.html.) Mini-proposals for experiments are being solicited from the community. (Approved MP's can be found at http://www.psfc.mit.edu/server-java/MiniProposals.) The broad experimental areas under investigation at C-Mod are organized as follows: Divertor Research, RF Physics and Heating Research, Transport Studies, MHD Studies, and Advanced Tokamak Scenarios. Soon the approved experiments will be scheduled. The experimental campaign will be executed as soon as the machine and diagnostics are fully operational.

Meetings to plan Alcator C-Mod's role in addressing the major issues in fusion science have begun and will continue in preparation for the joint National Tokamak Workshop, scheduled to occur at GA in July.

``Experimental Divertor Physics'', C.S. Pitcher and P.C. Stangeby, an invited review for Plas. Phys. Contr. Fusion, in press.

``Heat Transport at the Boundary of ASDEX-Upgrade'', C.S. Pitcher et al., Plas. Phys. Contr. Fusion, in press.

``The Relation of Edge Confinement to Global Confinement in ASDEX Upgrade'', C.S. Pitcher et al, Phys. Plasmas, in press.

``Radiative and 3-Body Recombination in Alcator C-Mod'', D. Lumma, J.L. Terry, and B. Lipschultz, Phys. of Plasmas, in press.

``Neutral Particle Dynamics in the Alcator C-Mode Tokamak'', A. Niemczewski, et al., Nucl. Fus. 37 (1997) 151.

``The Measurement of the Intrinsic Impurities of Molybdenum and Carbon in the Alcator C-Mod Tokamak Plasma using Low Resolution Spectroscopy'', M.J. May, et al., Nucl. Fus., in press.

``H Mode Confinement in Alcator C-Mod", M. Greenwald, et al., accepted for publication in Nuclear Fusion.

``Remote Control of Alcator C-Mod from LLNL", S. Horne, et al., accepted for publication in Fusion Technology.

``X-ray Observations of Central Toroidal Rotation in Ohmic Alcator C-Mod Plasmas", Rice, J.E., Marmar, E.S., Bombarda, F., Qu, L., Nuclear Fusion, vol.37, no.3, p. 421-6.

``X-ray Observations of Up-Down Impurity Density Asymmetries in Alcator C-Mod Plasmas", Rice, J.E., Terry, J.L., Marmar, E.S., Bombarda, F., Nuclear Fusion, vol.37, no.2, p. 241-9.

``Negative Magnetic Shear Modes of Operation in the Alcator C-Mod Tokamak Near the Beta Limit", Bonoli, P.T., Porkolab, M., Ramos, J.J., Nevins, W., Kessel, C., Plasma Physics and Controlled Fusion, vol.39, no.2, p. 223.

``The High Resolution Video Capture System on the Alcator C-Mod Tokamak", Allen, A.J., Terry, J.L., Garnier, D., Stillerman, J.A., Wurden, G.A., Rev. Sci. Instrum., vol.68, no.1, pt.2, p. 947.

``Data Acquisition System for Alcator C-Mod", Fredian, T.W., Stillerman, J.A., Greenwald, M., Rev. Sci. Instrum., vol.68, no.1, pt.2, p. 935.

``A Compact Thomson Scattering System", Dimock, D., Grek, B., Johnson, D., LaBombard, B., Lipschultz, B., McCracken, G., Rev. Sci. Instrum., vol.68, no.1, pt.2, p. 700.

``Time-of-flight Neutral Particle Analyzer for Alcator C-Mod'', Boivin, R.L., Koltonyuk, M., Munson, C.P., Mayo, R.M., Rev. Sci. Instrum., vol.68, no.1, pt.2, p. 982.

``Neutral Beam Diagnostics for Alcator C-Mod", Rowan, W.L., et al., Rev. Sci. Instrum., vol.68, no.1, pt.2, p. 300.

``Diagnostic Neutral Beam and Active Spectroscopy Requirements for the Alcator C-Mod tokamak", Marmar, E.S., Terry, J.L., Rowan, W.L., Wootton, A.J., Rev. Sci. Instrum., vol.68, no.1, pt.2, p. 265.

``ITER Physics Program and Implications for Plasma Measurements", Mukhovatov, V. ..., Marmar, E., ...,et al., Rev. Sci. Instrum., vol.68, no.1, pt.2, p. 1250.

Please note the following report is for informational use. It may not be cited as a scientific reference nor may it be quoted in publications without permission. Contact Jim Terry (terry@psfc.mit.edu) with comments or questions.