J. Stillerman

20 years of research on the Alcator C-Mod tokamak

The object of this review is to summarize the achievements of research on the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994) and Marmar, Fusion Sci. Technol. 51, 261 (2007)] and to place that research in the context of the quest for practical fusion energy. C-Mod is a compact, high-field tokamak, whose unique design and operating parameters have produced a wealth of new and important results since it began operation in 1993, contributing data that extends tests of critical physical models into new parameter ranges and into new regimes. Using only high-power radio frequency (RF) waves for heating and current drive with innovative launching structures, C-Mod operates routinely at reactor level power densities and achieves plasma pressures higher than any other toroidal confinement device. C-Mod spearheaded the development of the vertical-target divertor and has always operated with high-Z metal plasma facing components—approaches subsequently adopted for ITER. C-Mod has made ground-breaking discoveries in divertor physics and plasma-material interactions at reactor-like power and particle fluxes and elucidated the critical role of cross-field transport in divertor operation, edge flows and the tokamak density limit. C-Mod developed the I-mode and the Enhanced Dα H-mode regimes, which have high performance without large edge localized modes and with pedestal transport self-regulated by short-wavelength electromagnetic waves. C-Mod has carried out pioneering studies of intrinsic rotation and demonstrated that self-generated flow shear can be strong enough in some cases to significantly modify transport. C-Mod made the first quantitative link between the pedestal temperature and the H-modes performance, showing that the observed self-similar temperature profiles were consistent with critical-gradient-length theories and followed up with quantitative tests of nonlinear gyrokinetic models. RF research highlights include direct experimental observation of ion cyclotron range of frequency (ICRF) mode-conversion, ICRF flow drive, demonstration of lower-hybrid current drive at ITER-like densities and fields and, using a set of novel diagnostics, extensive validation of advanced RF codes. Disruption studies on C-Mod provided the first observation of non-axisymmetric halo currents and non-axisymmetric radiation in mitigated disruptions. A summary of important achievements and discoveries are included.

MDSplus remote collaboration support-internet and world wide web

Abstract The consolidation of fusion research into fewer major experiments has motivated the fusion community to study methods of improving the efficiency in which these experiments produce scientific results. One mechanism for increasing the scientific output of these facilities is through remote collaboration. To make the best use of existing facilities, scientists must be able to participate in the day to day operation of these facilities from anywhere in the world as easily and effectively as if they were present in the control room of the experiment. It is not sufficient to just make the scientific measurements available to more scientists. Remote collaborators must be able to provide input and communicate closely with all members of the experimental team. The rapid improvements in network technology are beginning to make this goal achievable. The increase in network bandwidth along with platform-independent web based software technologies such as Java, are providing the infrastructure for truly networked fusion research. This paper will discuss some of the networked applications developed or employed at MlT s Plasma Science and Fusion center to improve the research environment for remote collaborators who work as part of the Alcator C-Mod experimental team.

Remote control of alcator C-mod from Lawrence Livermore National Laboratory

Operation ofa tokamak from a remote site has been demonstrated for the first time. The Alcator C-Mod tokamak, located at the Massachusetts Institute of Technology, was operated over the Internet from a remote control room set up at Lawrence Livermore National Laboratory in California. Prescription of the physics parameters such as plasma current, density, shape, heating power, and active diagnostics was accomplished entirely from the remote site using the same interface as when operating from the C-Mod control room. Engineering control of subsystems (e.g., vacuum, cooling, and power supply limits) remained under local control, providing appropriate equipment and personnel security. Although the principal purpose for running this experiment from a distance was to demonstrate the remote operation, it was planned as a productive physics run. The operation was highly successful; important new physics data were obtained, and valuable insight was gained into the potential of remote operation as well as its limitations.

X-windows-based user interface for data acquisition and display

A Macintosh‐like user interface for the MDS‐Plus data acquisition system is being implemented using the DECwindows MIT/X interface. MDS‐Plus is a model driven general purpose data acquisition system being developed collaboratively by the CMOD group at MIT Plasma Fusion Center, the RFX group at IGI‐Padua, and the ZTH group at Los Alamos National Laboratory. The model is a hierarchical description of an experiment, including all of the tasks to be performed and the results of having performed them. The inherent complexity of this experimental model requires the users to specify fairly complicated descriptions of what they want the system to do. A ‘‘Point and Click’’ interface simplifies this by presenting to the user a coherent set of choices which are valid in the current context. We are implementing a set of tools for data acquisition and data analysis which use DECwindows to this end. They include a data displayer (Scope Replacement), an experiment model editor (Tree Editor), a timing system, and a wavef...

Applications of visible CCD cameras on the Alcator C-Mod tokamak

Five 7 mm diameter remote-head visible charge-coupled device (CCD) cameras are being used on Alcator C-Mod for several different diagnostic purposes. All of the cameras’ detectors and optics are placed inside a magnetic field of up to 4 T. Images of the cameras are recorded simultaneously using two three-channel color framegrabber cards. Two CCD cameras are used typically to generate two-dimensional emissivity profiles of deuterium line radiation from the divertor. Interference filters are used to select the spectral line to be measured. The local emissivity is obtained by inverting the measured brightnesses assuming toroidal symmetry of the emission. Another use of the cameras is the identification and localization of impurity sources generated by the ion cyclotron radio frequency (ICRF) antennas, which supply the auxiliary heating on Alcator C-Mod. The impurities generated by the antennas are identified by correlating in time the injections seen at the cameras with measurements made with core diagnostic...

The new infrared imaging system on Alcator C-Mod

A new infrared imaging system has been installed on Alcator C-Mod. This system uses an Amber Radiance 1 IR video camera (filtered to the 4.2–4.4 μm band) to view a 30 cm×30 cm region of the lower divertor from above by means of a re-entrant 5-m long ZnSe based periscope. Capture of the standard 30 Hz video frames (8-bit) and camera control are performed remotely over fiber optic links by a Windows 95 PC, using a MuTech MV-1000 video grabber board. Plans are under way to directly capture the 60 Hz, 12-bit, 256×256 pixel images using a digital video camera interface with a fiber optic link from EDT (Beaverton, Oregon). Preliminary results show that during nondisruptive discharges no substantial surface temperature increase is observed on the upper sections of the divertor, with the exception of “hot spots,” although occasionally, increased heating in toroidal bands is seen. Bands can also be observed after disruptions that result in a downward movement of the plasma.

Data acquisition system for Alcator C-Mod

The Alcator C-Mod experiment requires an efficient data handling system to acquire and distribute the large number of measurements recorded during each pulse of the tokamak. Over 2000 signals amounting to over 80 megabytes of data are stored and distributed to 40 workstations for display and analysis. With machine pulses occurring every 10 to 15 min, most of the information must be distributed within 2 to 3 min after the pulse to enable the experimentalist to adjust the control settings for the next pulse. In addition to the local control room data distribution requirements, the system must also provide data access to remote sites which monitor diagnostics installed on the Alcator C-Mod experiment. This article will describe the hardware and software used to accomplish data handling tasks.

The high resolution video capture system on the alcator C-Mod tokamak

A new system for routine digitization of video images is presently operating on the Alcator C-Mod tokamak. The PC-based system features high resolution video capture, storage, and retrieval. The captured images are stored temporarily on the PC, but are eventually written to CD. Video is captured from one of five filtered RS-170 CCD cameras at 30 frames per second (fps) with 640×480 pixel resolution. In addition, the system can digitize the output from a filtered Kodak Ektapro EM Digital Camera which captures images at 1000 fps with 239×192 resolution. Present views of this set of cameras include a wide angle and a tangential view of the plasma, two high resolution views of gas puff capillaries embedded in the plasma facing components, and a view of ablating, high speed Li pellets. The system is being used to study (1) the structure and location of visible emissions (including MARFEs) from the main plasma and divertor, (2) asymmetries in gas puff plumes due to flows in the scrape-off layer (SOL), and (3) t...

Performance of the C-Mod shape control system

The poloidal field system for Alcator C-Mod (a high-field compact tokamak) can generate elongated, single or double null diverted plasmas. The shape control system consists of hardware (hybrid multiplier, real-time control computer, and associated electronics), the user interface (a set of X-Windows applications), and the underlying algorithms used by the interface to calculate the control matrices. The shape control computer generates demands to the power supplies, based on programmed waveforms and feedback from the diagnostics. Beginning in April 1993 we have used the shape control computer to program and/or feed-back control coil currents, power supply voltages, magnetic field quantities and gas pressure during plasma breakdown, and to tailor the fields during the current ramp. Representative data from the 1993 run and an evaluation of the overall performance of the shape control computer will be presented.

The design of the user interface for a large physics experiment

Abstract The paper describes the experience in developing and supporting the user interface of RFX, one of the large nuclear fusion experiments of the co-ordinated nuclear fusion experiment programme of the European Community. The aim of this work is to present the problems and some possible solutions when developing user interfaces in a scientific environment, especially in large physics experiments. An overview of the current state of interface technology in such an environment is first provided. The control and data acquisition system of the RFX experiment is then introduced and its user interface described in greater detail. Finally, our experience both in maintaining the system interface and in training its users is described.