The primary activities at Alcator C-Mod during the third quarter of FY99 were: analysis of data from the present run campaign; review, repair and maintenance of the ICRF auxiliary heating systems; and continued participation in the critical assessment of fusion science. A more detailed account of these activities follows.

Scientific Results

Core Confinement and Edge Pedestal Results

The study of H-mode regimes, pedestal conditions and confinement is a focus of research on Alcator C-Mod. A good correlation is found on C-Mod between pedestal temperature and both core T_e gradients and global confinement times, across all L- and H-mode regimes, including the ``Enhanced D-alpha (EDA) H-mode'' regime. Similar trends were also seen on ASDEX Upgrade [C. S. Pitcher et al., Physics of Plasmas 4 (7), p. 2577 (1997)]. This is suggestive of a `stiff' type of transport regime, in which core transport depends strongly on boundary conditions and temperature profiles tend to exhibit a self-similar behavior. For example, recent modelling of some C-Mod discharges using the IFS-PPPL model [M. Kotchenreuter et al., Physics of Plasmas 2, p. 2381 (1995)] predicts that core T_i increases with boundary temperature. However, the model tends to underestimate seriously the experimentally observed gradients in T_i profiles, which are close to T_e profiles, and to overestimate the central T_e.

The correlation between edge and core parameters does not in itself prove a causal relationship; it could be that some change in turbulence is affecting both the edge and core transport. In order to study the relationship in a more controlled way, an experiment was carried out in which impurity gases were puffed into steady-state EDA H-mode discharges. This increased the radiation, primarily in the outer half of the plasma, and thus lowered the pedestal temperature by a variable degree. A short puff of neon or krypton increased the radiation by ³ 100%, with only a slight effect on the edge or average density (up to 14% ). For both gases, the incremental emissivity peaks at r/a ~ 0.85, just inside the pedestal, with 80% of the integrated power outside r/a of 0.7. The core confinement in these radiative discharges also drops as T_e,ped is lowered. There is a 35% decrease in dT/dR at y=0.5, despite the fact that the net power flux across this surface varies by only 0.430 MW, or ~ 17%. t_E and the H_ITER-89P factor decrease similarly, to near L-mode levels. Any further increase in radiation causes an H-L transition. It thus appears that an independently imposed change in pedestal temperature and pressure does indeed cause a change in global transport, strengthening the argument that core confinement is sensitive to the plasma boundary conditions.

In a separate experiment, absorbed RF power was varied from 0.63 to 2.75 MW in a series of EDA H-modes at constant n_e. This resulted in a variation in T_e (y=0.85) from 0.4 to 0.63 keV. The H factor and the T_e gradient at y=0.5 increased with edge T_e, as shown in the top two panels in Fig. 1. However, the temperature gradient scale length at y=0.5 does not change significantly, as shown in the bottom panel. Although in this case it is the centrally deposited input power which varies, the correlation between edge T_e and both the core T_e gradient and H factor mirrors that in the impurity scan. At these power levels we have not observed a clear saturation in any of these quantities with P_total, though the trends are non-linear. It will be interesting to see the behavior as RF power is doubled in the near future. Dynamic experiments to test the dependence of the pedestal widths on input power are also planned.

Edge Pedestals and Impurity Transport

Observations of pedestal positions from the edge x-ray arrays and the visible continuum array indicate that the location of the outboard midplane x-ray pedestal foot (position plus half-width) is closely correlated with the location of the visible bremsstrahlung plateau (position minus half-width). This may be evidence for the effects of neoclassical impurity transport on the outgoing fully stripped fluorine ions that are the dominant contributor to the edge x-ray signals (due to the radiative recombination of these ions). Fluorine transport modeling, which uses the bremsstrahlung profiles to construct density profiles and includes the neoclassical inward pinch terms, yields profile shapes which are in excellent agreement with those seen experimentally near the outboard midplane. The neoclassical inward pinch magnitude is dominated by the strong edge density gradient. This model cannot explain the x-ray profiles which are observed near the top edge of the plasma, where the emissivity pedestal is typically located much closer to the last closed flux surface. Vertical impurity drift, as has been documented for highly stripped argon ions [Rice, et al., ``X Ray Observations of Up-Down Impurity Density Asymmetries in Alcator C-Mod Plasmas'', Nuclear Fusion 37 (1997) 241], is a likely mechanism influencing the location of the recombining fully stripped fluorine at the top of the plasma. Two dimensional impurity transport modeling will be required to sort out these effects quantitatively.

Divertor and Edge Results

New results have been obtained in the study of the role played by neutral particles in the evolution of the H-mode transport barrier near the edge of the plasma. Neutrals can affect the edge dynamics through the particle, momentum and energy balance. A number of new diagnostics were installed on C-Mod for the purpose of measuring the edge midplane profiles of radiated power, of total (radiation plus neutral) power, and of Lyman_a emission. Standard Abel inversion techniques are used to derive local emissivities from these measured brightness profiles. From the Lyman_a emissivity, in conjunction with electron density and temperature, the local ionization rate and neutral density profiles are inferred. In addition, by Abel inverting the difference between the two power measurements, the local power emissivity carried by neutrals is inferred. In Fig. 2 the measured radial ionization profiles (derived from the Lyman_a profiles) are shown for L and H-mode phases of the same discharge, in which 1 MW of ICRF heating was applied. Although the electron density is increasing sharply at the top of the pedestal (up to 3-4 ×10²⁰ m^-3) during H-mode, the Lyman_a emissivity drops across the whole profile. With the assumption of poloidal and toroidal symmetry, the ionization rate inside the separatrix has been estimated, and is found to be larger in H-mode than in L-mode. This change in ionization rate inside the separatrix can explain, in part, the increase in density observed during H-mode. However, the scaling of H-mode density rise with change in ionization rate requires more careful analysis, since a poloidal variation and/or any uncertainty in the location of the separatrix can introduce large uncertainty in the fueling rate.

The neutral density profile is also obtained from the Lyman_a emissivity profile. As expected, the inferred neutral density drops strongly at and inside the separatrix, with a scale-length as small as 3 mm, which is comparable to the measured electron density pedestal width. However, the neutral density just inside the separatrix is still surprisingly large, up to 0.1% of the local plasma density. This large fraction is simply the consequence of the dominance of multiple charge-exchange events over ionization, and by the large neutral pressure found in the main chamber. The relatively large neutral density also means that neutrals carry out a significant amount of power, which can be measured using the two different edge bolometer arrays. It is found that almost all the neutral power emissivity originates from the edge of the plasma, although the detailed profile and the peak edge neutral emissivity are limited by the ~ 2 cm radial resolution of the bolometer arrays. These results have been modelled using a 1-D Monte-Carlo neutral transport code which takes into account charge-exchange and ionization, and uses as inputs the neutral pressure at the wall, density and temperature profiles in the main plasma and scrape-off layer. Good agreement (within 25%) in local power emissivity versus the measured quantity is found. The localization of these losses is also important. The simulations indicate that nearly 50% of the power carried by neutrals should originate from outside the top of the pedestal. However, since the pedestal width found in Alcator C-Mod is only a few millimeters wide, it is presently difficult to verify this directly with the bolometers.

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As has been reported previously, Alcator C-Mod now has ten sets of mechanical flaps which, when open, allow gas conduction from the private flux region of the C-Mod divertor to the region outside the main plasma. The open-or-closed state of these bypass flaps can be changed as desired in ~ 50 ms. Experiments using these bypass flaps have been motivated by suggestions that tight divertor baffling can improve the performance of tokamak discharges, for example, in the achievement of high energy confinement times [e.g. S M Kaye et al, J Nucl Mat 121 (1984) 115]. It was further believed that tight baffling could result in better neutral compression/helium exhaust and in improved impurity screening. All of these would ultimately improve the performance of an energy-producing tokamak reactor. Over the intervening years, virtually every divertor tokamak has attempted to optimize baffling by arranging the mechanical divertor structure to restrict the leakage of gas out of the region. One problem with this activity, and one of the reasons that definitive answers on many aspects of the subject have yet to be obtained, is that mechanical changes typically take years to institute and thus, before and after comparisons are made across years, under varying machine conditions, magnetic geometry and diagnostic calibrations. These bypass flaps are being used routinely, and experiments performed with them have provided clear answers to many of these speculations. We have found no discernable effect of the divertor bypass (within an accuracy of ~ 10%) on energy confinement. This includes Ohmic, L-mode and Enhanced D-Alpha (EDA) H-mode discharges. For example, the stored energies of two EDA H-mode discharges, each with 1.5 MW of ICRF power, one with the bypass open and the other with it closed, are nearly identical. This is the case, even though the divertor pressure is a factor of ~ 2 lower with the bypass open,indicating a quite strong effect on neutrals in the divertor.

However, the bypass flaps do affect the divertor retention and screening of impurities, as detailed in the last quarterly report. Argon and krypton compression (i.e. the ratio of impurity atoms in the divertor to impurity ions in the main plasma) with the bypass flaps closed is a factor of about two greater than the compression with the bypass open.

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Results from the (LANL) IR imaging system have been analyzed for recent shots. The surface heating in the divertor region below the outer nose is dominated by hot spots with typical sizes of 1-2 cm² and discontinuous toroidal bands. The position of these hot spots are consistent with the strike points as determined from EFIT equilibria reconstructions. Surface temperatures of the hot spots are observed to reach ~ 500 deg. C, corresponding to an energy flux of ~ 10 MW/m². Other surfaces reach typically the 150-250 deg. C range. Data analysis continues, as well as preparations for the oncoming runs.

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Quite low divertor electron temperatures (less than 1 eV) are sometimes observed in Alcator C-Mod divertor plasmas. The temperatures are determined from spectroscopic measurements which utilize emission from atomic deuterium or emission resulting from the formation of atomic deuterium, i.e. photo-recombination. Accurate measurement of divertor electron temperatures in this low range is important for several reasons. In this range the balance between ions and neutrals varies strongly with T_e; the experimental evaluations of the plasma source (ionization) and one of the plasma sinks (volume recombination) depend critically on the local temperature; finally, it facilitates a more realistic comparison with the modelling of these plasmas. A critical analysis of four spectroscopic techniques used on Alcator C-Mod to measure T_e in the divertor was carried out. The results demonstrated, at least on Alcator C-Mod, the capability of T_e measurement in the » 0.4-1.5 eV range with an accuracy of approximately ±0.25 eV. Two of the techniques were found to yield temperatures with this accuracy, while the other two overestimated the temperatures. The reliable techniques are the ``continuum slope'' method (which relates the slope of the photo-recombination continuum emission, at wavelengths less than 920 Å, to the local T<sub>e</sub>) and the ``Saha T_e'' method (which uses the fact that above some level, n^*, the population densities of the excited levels of atomic deuterium are described by a Saha distribution i.e. µ e^{(IP(n > n*)/T_e)}). The other two spectroscopic methods, which assume that the visible continuum emission is produced only by free-free and free-bound bremsstrahlung, were shown to overestimate the temperature, most probably because of emission from some other source. The ultimate goal of this work is to study the measured T_es and the implications of these T_es in understanding the dissipative divertor. A better understanding is possible if these chordal measurements can be used to give localized temperatures. Using the fact that the T_e measurements are weighted to the local emissivity along the chord, the temperatures can be made more local by measuring the 2-d distribution of recombination-associated emission.

RF Research

Central to the theme of tokamak concept improvement is the class of tokamak operating modes characterized by high energy confinement time, high b_N [b_N = b_t / (I_p /a B₀)], and high bootstrap current fraction (f_bs). One key tool for accessing these regimes of advanced tokamak operation is flexible current density profile control by heating and current drive. Part of C-Mod's program after CY 2002 will stress these issues by utilizing an extensive Lower Hybrid current drive system which is presently scheduled to come on-line in the fall of 2002. At the present time there is an extensive modelling program. Time dependent calculations of lower hybrid (LH) current profile control in Alcator C-Mod have been done using the TRANSP, FPPRF, and LSC codes. In these calculations, up to 3 MW of LH current drive power was applied in plasmas with high power ICRF minority heating (P_ICH = 1.8-3 MW) and fast current ramp up. Using experimentally measured target temperature profiles, off-axis current generation resulted in nonmonotonic q-profiles with q_min @ 1.6. Self-consistent effects of off-axis electron heating by the LH power were also included in the analysis and significant broadening of the electron temperature profile was found with q_min \gtapprox 2 and a larger shear reversal radius. The present work demonstrates (through time dependent simulations) that off-axis LHCD can be used to maintain reversed shear current density profiles with q_min \gtapprox 2 as the plasma evolves from start up. The calculated current profile and its components are shown in Fig. 3 for two of the modeled cases.

(3a)

(3b)

Figure 3. TRANSP/LSC results for current profile control in C-Mod [B₀ = 5.3 T, P_LH = 2.7 MW]. (a) Current densities (A / cm²) vs. (r/a) for simulation with experimental temperature profiles. (b) Current densities (A / cm²) vs. (r/a) for simulation with predicted electron temperature profile. Solid curve is total current density, chain-dot curve is lower hybrid current density, dashed curve is bootstrap current density, and dotted curve is ohmic current density.

In Fig. 3a are the profiles when the experimental T_e profiles are used, while Fig. 3b shows the cases when the LH heating is included in the modelled electron temperature profiles.

MHD Research

Closely spaced poloidal field pick-up coils mounted to the inner wall and two outboard limiters sampled at 1 - 2 MHz have been used to measure high frequency (150 - 900 kHz) modes during hydrogen minority ICRF heating at B_T=5.4 T. Such high frequency modes are observed only under two different conditions: with low or reversed shear (1/q dq/dr) during strong heating in the current rise, and in steady-state Enhanced D_a (EDA) H-mode. These modes are not observed in any ohmic plasmas or in ELM-free H-modes. In the low shear current rise cases, there are often multiple modes with frequencies that can increase or decrease rapidly from 150 - 450 kHz in times as short as 15 - 20 ms. Calculations of both the fast particle distribution and the TAE gap structure support the hypothesis that the current rise modes are TAE's. In the EDA cases, there is usually only one high frequency mode present, with nearly constant frequency between 500 - 900 kHz, persisting for up to hundreds of ms. The current rise modes rotate in the ion diamagnetic drift direction with frequencies in the Toroidal Alfven Eigenmode (TAE) range. The mode frequencies in the EDA H-mode cases are also in the TAE range, but these modes rotate in the electron diamagnetic drift direction. This suggests that these modes may be edge modes, but fast particles are not expected in the edge, so the driving mechanism is uncertain.

Operations and Diagnostics

The present Alcator C-Mod run campaign, which began in late January, was interrupted in late April due to problems with the ICRF transmitters powering two of the three RF antennas. This has resulted in a large effort to understand the causes of the transmitter failures, and to review the RF power delivery systems. This effort was conducted in close collaboration with PPPL scientists and engineers. It is believed that one of the transmitter tube failures was due to an incorrectly set spark gap on the crowbar protection circuit. The other tube failure was probably due to a problem caused by debris from the internals of a check valve in the tube cooling system. Improvements to the systems, made as a result of these studies, have included installation of a second soft start resistor, addition of a screen crowbar and low energy storage grid bias regulator, reduction of a 150 ohm resistance to 40 ohms, addition of an ignitron holding anode power supply, and additional instrumentation. The transmitter cooling system for all four transmitters was also improved. An additional screen filter has been installed on the water inlet and the suspect check valve removed. At present, after upgrading the ICRF power system's safety, reliability, and diagnostics, two of the transmitters (FMIT #3 and #4) have operated at powers > 1.5 MW each into dummy loads and at 150 kW each to the J port antenna into machine vacuum. (The lower power is due to low loading resistance when coupling into vacuum.)

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Essentially all of the personnel formerly working on the DNB were shifted during this quarter to reviewing, repairing, and upgrading the ICRF power delivery systems. As a result, the DNB work has been delayed by approximately two months. However, we believe that the DNB will be ready for installation for the CY 2000 run campaign, presently scheduled to begin in January or February of 2000.

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The UT-FRC heterodyne radiometer (FRCECE) is now measuring electron temperature profile data. The 32 channel system has a radial resolution of approximately 0.6 cm. At 5.4 T, the spatial coverage is from the plasma core to beyond the separatrix. Using small toroidal field ramps, a relative channel-to-channel calibration was performed. An absolute calibration was derived from comparison with other electron temperature diagnostics. The FRCECE temperature profile agrees quite well the profiles from two other ECE diagnostics and the profile measured by the Thomson scattering system. The system will be in regular operation for the remainder of this campaign. Profile data will be available to support other C-Mod experiments. Initially, the FRC will use the excellent spatial resolution to explore the dynamics of small structures in the gradient region; that is, between the sawtooth inversion radius and the pedestal.

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The laser ablation impurity injection system is fully operational and running reliably. CaF₂ is now the material of choice, with several central and edge charge states available in the VUV and x-ray spectral regions. The MIST impurity transport code has been updated to include calcium, and modelling of x-ray spectra shows good agreement with the observations. Time histories of VUV and x-ray lines are also in good agreement with simulations for L-mode discharges.

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Images of carbon `plumes' have been recorded. In these experiments, trace amounts of ethylene gas ( » 10¹⁷ molecules) are injected over a 10 msec period through the F-port vertical scanning probe. Two fast-gated (0.2 msec exposure) CCD cameras (on loan from PPPL and LANL) were used to view the resulting emission patterns of CII or CIII light. One camera views the plume from the top (along -Z direction) while the other views from the side (along -R). Video images were captured using the newly installed Matrox color frame grabber system and IDL-based widget interface. Based on the success of this frame grabber system, another one is being put together for divertor imaging.

Plumes of both CII and CIII emission appear to be extended along the magnetic field line in both directions. Asymmetries in the pattern suggest a reversed plasma flow (flow out of the divertor) for injections near the separatrix. The plumes also indicate a strong cross-field plasma drift near the separatrix in a region where the ExB poloidal drift is expected to be high. These preliminary experiments show that the ethylene injection does not perturb the discharges in any detectable way. A comparison of CII signals before and after a series of discharges with ethylene injection showed no change. Consequently, these experiments may be continued in a `piggy-back' mode of operation.

Plans for further experiments with ethylene include a reduction and optimization of ethylene puff and camera exposure. Progress is underway towards a full 3-D reconstruction of the emission plume from the two camera images.

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Ion impurities in the upper divertor scrape off layer plasma have been observed using the Omegatron probe. The Omegatron probe combines a gridded energy analyzer and an ExB ion mass spectrometer. Ion species with mass to charge ratios M/Z=2, 4, 6, 10, and 11 have been observed, with the M/Z=2 intensity always dominant in deuterium plasmas. The intensities of the M/Z=6, 10, and 11 species are larger after a limited plasma discharge than they are after a series of diverted plasma discharges. When present, the intensities of the M/Z=10 and 11 species appear in proportions that match the natural isotopic abundances of boron-10 and boron-11. An increase in the intensity of the M/Z=4 species is observed after puffing helium into a tokamak discharge, either through an inner wall capillary or through the helium probe at the midplane of an outer limiter. During a plasma discharge the evolution of the M/Z=4 intensity follows the inventory of helium injected; this is expected since helium is a recycling impurity. Impurity resonances can be identified unambiguously at intensities down to a hundred nanoamperes, which corresponds to approximately 0.5% of the total current in the collection cavity. Efforts to improve resolution further are underway.

Collaborations and Participation in the Fusion Science Community
Since mid-May the C-Mod weekly staff meeting has been available for remote access and conferencing. Access to the speaker's slides is provided over the web using a new digital overhead projector. Audio is available using the ES-Net audio bridge. Presentations from remote sites may also be given using these facilities. This new capability allows easier participation by C-Mod collaborators in meetings at the PSFC. The hardware for this has been acquired as a result of the PSFC's participation in the ``Fusion Collaboration Information Technology Project''.

This remote conferencing technology was also used in the C-Mod Quarterly Review held and ``broadcast'' at MIT on July 2. Dr. Rostom Dagazian and Ms. Mona Bradford (DoE Contract Specialist) were present from DoE. Dr. David Mikkelsen of PPPL gave his presentation from Princeton using the digital overhead projector and the audio bridge, constituting a ``proof of principle'' for this method of conferencing.

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Dr. Ricky Maqueda from LANL was on-site in early April and in late June. The LANL IR camera is operating routinely with a view of the outer divertor structures and the outer strike point. (See the Scientific Results section.)

Dr. S. Zweben of PPPL visited C-Mod on Apr. 28-29 to work with Brian LaBombard and Jim Terry on the imaging of turbulent edge fluctuations. Clear filamentation of the edge H-alpha light emission was seen using the LANL camera at exposure times of 10 microsec. This turbulent structure had a few-cm scale in the poloidal direction and a meter-length scale in the toroidal or parallel direction (as seen previously). This structure seemed to disappear during H-modes, but this could just be due to the higher frequency associated with H-mode turbulence. The fast-scanning probe was used to puff helium and deuterium near the separatrix, and clear visible light images were recorded with a 1 microsecond exposure time. However, these images have not yet shown any filamentation inside the 1-cm scale of the puff cloud, despite the intense small-scale fluctuations seen by the nearby Langumir probes.

Marco Brambilla visited the PSFC and worked with the Alcator C-Mod group from April 19 through May 14. He worked mostly with Paul Bonoli on the implementation of a new version of the toroidal ICRF code TORIC on the local workstation cluster. The improved TORIC module now computes the electron damping of ion Bernstein waves using a new formulation which relies far less on a numerical broadening (ad hoc) of the ion-ion hybird layer as did the previous version. The code has also been coupled to an ICRF current drive module that was developed at MIT. The new code is being tested and compared with the old code as as well as with experimental data from mode conversion electron heating in C-Mod. Some effort has also been devoted to the implementation of an IDL post-processor for TORIC that can be used both at Garching and MIT. This will facilitate the transfer of future versions of the ICRF code between MIT and the US under the ASDEX Upgrade - US collaboration agreement.

Perry Phillips (U. Texas) was at MIT for last two weeks of April to work on the high resolution ECE system. He was also at MIT for two weeks in late June. He is developing software for calibration of the profile data generated by the new 32-channel ECE system (FRCECE) as well as developing software for profile analysis. (See the Operations and Diagnostics section.)

Prof. Ken Gentle of the University of Texas visited C-Mod in mid-May to direct the cold-pulse experiments. The objective was to inject sufficient quantities of carbon by laser ablation to drive significant, fast reductions in edge temperatures and cause prompt core temperature rises. The effect has been widely observed at lower densities on other tokamaks; the operating space in C-Mod was to be explored. Low power, low density target plasmas were chosen to begin. Both diverted and limited configurations were run, with no significant differences. Weak, but clear evidence of prompt interior heating was obtained. The effect disappears at higher density and is strongest at low currents (low power.) More conclusive experiments will require larger carbon injections: prompt peak radiated power on the ``2pi'' bolometer greater than the 300 kW maximum in this series.

Phyllis Rhoney, Lew Randerson, and Bill Davis from PPPL visited the PSFC May 26-27 to speak with MDSplus users and developers to gain some insights that will help them with the use of MDSplus on the NSTX experiment. Tom Fredian and Josh Stillerman discussed implementation details of MDSplus. Steve Wolfe, Martin Greenwald, Bob Granetz, Howard Yuh, Rejean Boivin and other C-Mod scientists discussed their use of MDSplus for plasma shape control, diagnostic setup, data acquisition and integration of large analysis codes into the MDSplus data handling system.

Ben Carreras from ORNL was on site for the week of June 6th. He is currently analyzing T_e fluctuations measured by GPC2 (the PPPL polychromator) in recent ohmic discharges, and will also look at signals from other diagnostics.

Several students from the University of Wurzburg are visiting the PSFC for the summer as part of an exchange program. They started June 1. Two students are working at C-Mod. They are Dominique Huebner, working under Bruce Lipschultz's supervision on spectroscopic problems, and Stefan Krotz, working for Steve Wukitch on ICRF problems.

Chris Brunkhorst (PPPL RF engineer) was onsite at C-Mod June 7-12, 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.

Ron Bravenec, U.Tx., was at C-Mod in mid-June calibrating the BES filter/spectrometer for D_a profile measurements. The D_a profiles will be compared with a model which uses density and temperature profiles from the TRANSP database. Later this summer he will install new neutral-density filters into the spectrometer, and move all nine fibers to look at the edge plasma. From these D_a fluctuation data he hopes to detect poloidal propagation of the fluctuations (among other things) which would yield a measure of the radial electric field.

Mike Roberts and Ray Schwartz of the DoE Office of Fusion Energy Science visited C-Mod on June 24-25. They discussed facility operations issues with Ian Hutchinson, Earl Marmar, Jim Irby, Steve Wolfe, Catherine Fiore, and Martin Greenwald.

Selected Domestic Travel

Rejean Boivin, Martin Greenwald, Ian Hutchinson, Dmitri Mossessian, and Thomas Pedersen presented papers at the Transport Task Force Meeting in Portland.

Miklos Porkolab attended the SEAB Review of the Fusion Program at PPPL on Thursday and Friday, April 28,29. He also had a meeting with Joel Hosea to discuss the Lower Hybrid plans for C-Mod.

Miklos Porkolab attended the National Academy of Sciences Review in San Diego on May 16-18 and made a presentation,``Wave Propagation and Interaction with Particles in Magnetized Plasmas:RF Heating and Current Drive''. He also attended the FESAC meeting at PPPL on May 20,21, where the PPPL Program review was given as well as a review of the three Proof of Principle experiments. Earl Marmar was also at the NRC-NAS review; he gave a presentation titled ``Diagnostics and Facilities Needs''.

Martin Greenwald travelled to PPPL in mid-May to deliver a seminar to their theory group on C-Mod transport experiments. He took the opportunity to consult with the local staff on various physics issues, software and data sharing, and ``Snowmass'' planning.

Tom Fredian attended a NTCC (National Transport Code Collaboratory) working group meeting at Lehigh University on May 25th. Tom is providing assistance in interfacing the physics codes to experimental data via an MDSplus data access gateway.

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.

Seven members of FESAC visited MIT on June 24th. FESAC was welcomed by Professor Lawrence Bacow, MIT's Chancellor, who emphasized the importance of the PSFC to MIT's research and educational activities. There were presentations on the activities of each of the PSFC's 5 Divisions, consisting of overview talks of recent activities, longer range plans, and issues. The Alcator program was featured prominently. Presentations were also given on LDX, Plasma Technology Spinoffs, and Magnet and Gyrotron Technology Programs. The basic experimental plasma physics program, as well as progress in theory research, relationship to astrophysics, and compact ignition experiments (IGNITOR) were also discussed. The presentations were followed up by laboratory site visits by committee members.

International Travel

Rejean Boivin visited LHD in Nagoya in late April, where he presented a paper ``High Resolution Bolometry in Alcator C-Mod'', by Boivin, Goetz, Marmar, Rice, and Terry. He also attended the ITER Diagnostic Expert Group meeting, where he presented an informal talk on Novel Diagnostics in Alcator C-Mod on behalf of the Alcator Group.

Bruce Lipschultz went to JET for several days in mid-May for discussions about divertor/boundary physics and attempts on JET to achieve EDA H-mode plasmas. One subject of discussion was the wall-core recycling analysis done at C-Mod. JET also sees similar shaped edge profiles leading to a diffusion coefficient increasing across the SOL, although their error bars for the diffusivity are large. Results regarding flow profiles in the SOL were also compared. There are reversed flows in the JET SOL extending farther out into the SOL than at C-Mod with higher Mach numbers (0.3-0.4). The JET Team have started analysis of these data and believe there are Pfirsch-Schluter flows that account for most of this effect. They do see a flow reversal when the magnetic field is reversed. Considerable time was spent reviewing the recent JET attempts to make EDA plasmas. Following the C-Mod practice, they aimed for high q, high triangularity plasmas with ICRF only. There were no obvious signatures to indicate they had obtained EDA H-modes in these experiments. They did obtain good confinement with little or no ELMs. It was noted that the confinement improved as the triangularity was increased prior to turning on the RF heating. The JET group are hoping to continue this work in future runs.

In mid June, before attending the '99 EPS meeting, 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.

Several C-Mod physicists attended the EPS Conference on Controlled Fusion and Plasma Physics, in Maastricht, the Netherlands, held June 14-19. Amanda Hubbard presented a talk on ``Pedestals and Confinement in Alcator C-mod H-modes''. Poster presentations were given by Joe Snipes, Rejean Boivin and Spencer Pitcher, and on behalf of Jim Terry (who didn't attend). See the ``Publications'' section for the EPS paper titles.

Miklos Porkolab also attended the EPS Conference in Maastricht. In his capacity as APS DPP Chair, he also attended their Board meeting and Program Committee Meeting for the 2000 EPS meeting, to be held in Budapest.

In late June, Rejean Boivin attended a Workshop on radiative mantles, radiative modes, and impurity seeding at Culham Laboratory (UK). He presented a paper ``Sources and Radiation Profiles, and their Effects on H-mode plasmas in Alcator C-Mod''.

Also at JET/Culham during this time was Spencer Pitcher. He also attended the Culham Workshop. In addition he had discussions with Culham staff on edge physics on Compass and MAST, and edge diagnostics (e.g. the Compass/MAST helium beam). With respect to the helium beam, Compass has been experiencing similar problems as C-Mod with regard to helium background light levels. Various solutions were discussed, including chopping the beams. There was some discussion about informal collaboration in the future.

Robert Granetz was at JET and Culham June 21-26, attending a tripartite workshop on issues of steady-state operation with internal transport barriers (ITB's), as well as a workshop on MHD, Disruptions, and Control. There is considerable overlap between the two areas because the high b_N and steep gradients at the ITB in reversed-shear plasmas tend to give rise to double tearing modes around the q_min radius, as well as neo-classical tearing modes when the steep pressure gradient is near the q=3/2 or q=2 radius. These problems are found to be less of a limitation in the flat (or ``optimised'') shear regimes (still with enhanced confinement), but it is expected that further increases in b_N will give rise to problems with resistive wall modes.

Near Term Plans

Our near term plans are focussed on resuming the current run campaign, which is scheduled to run through September. The C-Mod Run Schedule on the Web has been updated. Overall plans for the 1999 Campaign can be found at http://www.psfc.mit.edu/cmod/run_schedule.html

Daily operations information can be found on the C-Mod Operations Calendar at

http://www.psfc.mit.edu/cgi/calendars/cmod

which is updated as information becomes available. Those with access to the online OPS bulletin board are advised to check there for the most up-to-date schedule information.

Considerable effort has been expended in preparation for the so-called ``Snowmass'' meeting on ``Opportunities and Directions in Fusion Energy Science for the Next Decade'', occurring July 11-23 in Snowmass, Colorado. Seven of the C-Mod staff will participate.

An Alcator C-Mod 1999 Ideas Forum has been preliminarily scheduled for fall 1999.

J.E. Rice, et al., ``Central Impurity Toroidal Rotation in ICRF Heated Alcator C-Mod Plasmas'', accepted for publication in Nuclear Fusion.

R. Boivin, et al., ``Study of the effects of neutrals in Alcator C-Mod plasmas'', Proceedings of the 26th EPS Conference on Controlled Fusion and Plasma Physics, Maastrich, NL (1999), on the WEB at http://epsppl.epfl.ch/, soon to be published on CD-ROM.

A.E. Hubbard, et al., ``Pedestals and confinement in Alcator C-Mod H-modes'', Proceedings of the 26th EPS Conference on Controlled Fusion and Plasma Physics, Maastrich, NL (1999), on the WEB at http://epsppl.epfl.ch/, soon to be published on CD-ROM.

C.S. Pitcher, et al., ``Divertor Bypass Experiments on Alcator C-Mod'', Proceedings of the 26th EPS Conference on Controlled Fusion and Plasma Physics, Maastrich, NL (1999), on the WEB at http://epsppl.epfl.ch/, soon to be published on CD-ROM.

J.A. Snipes, et al., ``Fast Particle Driven Modes in Alcator C-Mod'', Proceedings of the 26th EPS Conference on Controlled Fusion and Plasma Physics, Maastrich, NL (1999), on the WEB at http://epsppl.epfl.ch/, soon to be published on CD-ROM.

J.L. Terry, et al., ``T_e Measurements in the Cold Regions of Alcator C-Mod Divertor Plasmas'', Proceedings of the 26th EPS Conference on Controlled Fusion and Plasma Physics, Maastrich, NL (1999), on the WEB at http://epsppl.epfl.ch/, soon to be published on CD-ROM.