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Toroidal plasmas created with negative magnetic shear in the core region offer advantages in terms of MHD stability properties. These plasmas, transiently created in several tokamaks, have exhibited high-performance as measured by normalized stored energy and neutron production rates. A critical issue with extending the duration of these plasmas is the need to maintain the off-axis-peaked current distribution required to support the minimum in the safety factor q at large radii. We present equilibrium and transport simulations that explore the use of electron cyclotron heating and current drive to maintain this negative shear configuration. Using parameters consistent with DIII-D tokamak operation (Strait E et al 1995 Phys. Rev. Lett. 75 4421, Rice B W et al 1996 Nucl. Fusion 36 1271), we find that with sufficiently high injected power, it is possible to achieve steady-state conditions employing well aligned electron cyclotron and bootstrap current drive in fully non-inductively current-driven configurations.

External impurity injection into L mode edge discharges in DIII-D has produced clear confinement improvement (a factor of 2 in energy confinement and neutron emission), reduction in all transport channels (particularly ion thermal diffusivity to the neoclassical level), and simultaneous reduction of long wavelength turbulence. Suppression of the long wavelength turbulence and transport reduction are attributed to synergistic effects of impurity induced enhancement of E × B shearing rate and reduction of toroidal drift wave turbulence growth rate. A prompt reduction of density fluctuations and local transport at the beginning of impurity injection appears to result from an increased gradient of toroidal rotation enhancing the E × B shearing. Transport simulations carried out using the National Transport Code Collaboration demonstration code with a gyro-Landau fluid model, GLF23, indicate that E × B shearing suppression is the dominant transport suppression mechanism.

Major fusion experiments and modeling efforts rely on joint research of scientists from several locations around the world. A variety of software tools are in use to provide remote interactive access to facilities and data are routinely available over wide-area-network connections to researchers. Audio and video communications, monitoring of control room information and synchronization of remote sites with experimental operations all enhance participation during experiments. Remote distributed computing capabilities allow utilization of off-site computers that now help support the demands of control room analyses and plasma modeling. A collaborative software development project is currently using object technologies with CORBA-based communications to build a network executable transport code that further demonstrates the ability to utilize geographically dispersed resources. Development to extend these concepts with security and naming services and possible applications to instrumentation systems has been initiated. An Information Technology Initiative is deploying communication systems, ISDN (telephone) and IP (network) audio/video (A/V) and web browser-based, to build the infrastructure needed to support remote physics meetings, seminars and interactive discussions.

In a magnetically diverted tokamak, the scrape-off layer (SOL) and divertor plasma separates the first wall from the core plasma, intercepting impurities generated at the wall before they reach the core plasma. The divertor plasma can also serve to spread the heat and particle flux over a large area of divertor structure wall using impurity radiation and neutral charge exchange, thus reducing peak heat and particle fluxes at the divertor strike plate. Such a reduction will be required in the next generation of tokamaks, for without it the divertor engineering requirements are very demanding. To successfully demonstrate a radiative divertor, a highly radiative condition with significant volume recombination must be achieved in the divertor, while maintaining a low impurity content in the core plasma. Divertor plasma properties are determined by a complex interaction of classical parallel transport, anomalous perpendicular transport, impurity transport and radiation, and plasma - wall interaction. In this paper we will describe a set of experiments on DIII-D designed to provide detailed two-dimensional documentation of the divertor and SOL plasma. Measurements have been made in operating modes where the plasma is attached to the divertor strike plate and in highly radiating cases where the plasma is detached from the divertor strike plate. We will also discuss the results of experiments designed to influence the distribution of impurities in the plasma using enhanced SOL plasma flow. Extensive modelling efforts will be described which are successfully reproducing attached plasma conditions and are helping to elucidate the important plasma and atomic physics involved in the detachment process.

Electron density, temperature and parallel pressure measurements at several locations along field lines connecting the midplane scrapeoff layer (SOL) with the outer divertor are presented for both attached and partially-detached divertor cases: Ip = 1.4 MA, q95 = 4.2 and Pinput ∼ 6.7 MW under ELMing H-mode conditions. At the onset of the Partially Detached Divertor (PDD), a high density, low temperature plasma forms in the divertor SOL (divertor MARFE). The electron pressure drops by a factor of 2 between the midplane separatrix and the X-point and then an additional ∼ 3–5 times between the X-point and the outboard separatrix strike point. These results are in contrast to the attached (non-PDD) case, where electron pressure in the SOL is reduced by, at most, a factor of two between the midplane and the divertor target. Divertor MARFEs generally have only marginal adverse impact on important H-mode characteristics, such as confinement time. In fact, PDD discharges at low input power (i.e., approximately twice the L-H-mode threshold power) maintain good H-mode characteristics until a high density, low temperature plasma abruptly forms inside the separatrix near the X-point (X-point MARFE). Concurrent with the appearance of this X-point MARFE is a degradation in both energy confinement and the plasma fueling rate and an increase in the carbon impurity concentration inside the core plasma. The formation of the X-point MARFE is consistent with a thermal instability resulting from the temperature dependence of the carbon radiative cooling rate in the range ∼ 7–30 eV.

We have studied the evolution of the edge plasma in VH-mode discharges in DIII-D as the discharge evolves from the ELM-free H-mode phase through the VH-mode phase to the final ELMing H-mode phase, by following the changes in the radial profiles of the density and temperature, in the core plasma near the separatrix and in the scrape-off-layer (SOL) plasma outside the separatrix. The electron density and temperature profiles in the SOL do not show any significant difference between the ELM-free H-mode and VH-mode phases. In the ELMing phase, the, density profile broadens during an ELM, forming a high density (n{sub e} > 1 x 10{sup 19}/m{sup 3}) plateau that extends out into the SOL to the limit of the measurement. This density plateau persists between the ELMs, although the density in the SOL does relax somewhat between the ELMs, with a characteristic time that can be larger than ten milliseconds, much longer than the sonic particle flow time to the divertor plates. The time average density scale length measured at the separatrix increases by about a factor of two after the ELMs begin. This density scale length increases with the ELM background, as measured by the photo-diodes nearest to, but outside the outer strike-point, suggesting that recycling from the divertor targets is supplying particles that maintain the density plateau. The electron temperature profile in the SOL also broadens during an ELM, but relaxes on a much faster time scale and does not persist between ELMs.

A comprehensive set of experiments has been carried out on the DIII-D tokamak to measure the 2-D (R,Z) structure of the divertor plasma in a systematic way using new diagnostics. Measurements cover the divertor radially from inside the X-point to the outer target plate and vertically from the target plate to above the X-point. Identical, repeatable shots were made, each having radial sweeps of the X-point and divertor strike points, to allow complete plasma and radiation profile measurements. Data have been obtained in ohmic, L-mode, ELMing H-mode, and reversed B{sub T} operation ({gradient}B drift away from the X-point). In addition, complete measurements were made of radiative divertor plasmas with a Partially Detached Divertor (PDD) induced by D{sub 2} injection and with a Radiating Mantle induced by Impurity injection (RMI) using neon and nitrogen. The data set includes first observations of the radial and poloidal profiles of the X-point, inner and outer leg plasmas in PDD and RMI radiative divertor operation. Preliminary data analysis shows that intrinsic impurities play a critical role in determining the SOL and divertor conditions.

We account for ≥85% of the injected power in DIII-D during single-null diverted ELMing H-mode discharges. Core plasma radiation accounts for ≤18% with the rest flowing through the scrape-off layer to the inboard and outboard divertors. The total power roughly splits in an in: out ratio of 1:1.3. Of the power entering the outboard divertor 75% leaves as direct heat flux to the divertor target plates with the rest in the form of radiation. In the inboard divertor, however, 75% of the power leaves as radiation with the rest as divertor target plate heat flux and charge exchange losses. The in: out ratio remains constant over a range of injected power and plasma current and even as divertor conditions change due to gas puffing.

In this paper, we examine a denser and broader scrape-off layer (SOL) plasma, one class of which was first seen in VH mode, in the DIII-D tokamak. An enhanced SOL appears in many types of double-null (DN) discharges and is not a property of VH mode only. The L and H mode DN enhanced SOL density and temperature profiles exhibit a 5–6 cm broad profile outside the separatrix. The VH mode enhanced SOL is associated with a higher separatrix density and temperature and is only 1.5–2 cm wide. Both types of SOL enhancement are only observed in high triangularity discharges. The origin of the enhanced SOL is, however, not yet understood.

We show that charge-exchange and other ion-neutral collisions can reduce the power transmission factor of the plasma sheath, thereby lowering the ion impact energy and target plate sputtering. The power transmission factor relates the heat flux reaching the divertor target to the plasma density and temperature just in front of the surface: δ=Qsurf/Jewk Te. Experimental data from the DIII-D tokamak suggests that δ could be as low as 2–3 near the region of peak divertor particle flux, instead of the 7–8 expected from usual sheath theory. Several effects combine to allow ion-neutral interactions to be important in the divertor plasma sheath. The shallow angle of incidence of the magnetic field (1–3° in DIII-D) leads to the spatial extension of the sheath from approximately ρi∼1 mm normal to the plate [R. Chodura, Phys. Fluids 25 (1982) 1628] to several centimeters along the field lines. Ionization reduces the sheath potential, and collisions reduce the ion impact energy.

We have developed an edge physics data base containing data for the plasma in the divertor region and the scrape-off layer (SOL) for the DIII-D tokamak. The data base provides many of the parameters necessary to model the power flow to the divertor and other plasma processes in the plasma edge. It will also facilitate the analysis of DIII-D data for comparison with other divertor tokamaks. In addition to the core plasma parameters, edge-specific data are included in this data base. Initial results using the data base show good agreement between the pressure profiles measured by the Langmuir probes and those determined from the Thomson data for the inner strike point, but not for the outer strike point region. We also find that the ratio of separatrix density to average core density, as well as the in/out asymmetry in the SOL power at the divertor in DIII-D do not agree with values currently assumed in modeling the international thermonuclear experimental reactor (ITER).

The CERN linear collider (CLIC) [W. Schnell, Proc. Workshop on Physics of Linear Colliders, Capri, 1988, eds. l. Palumbo, S. Tazzari and V.G. Vaccaro (1989) p. 345] is a two-beam-accelerator (TBA) scheme in which the driving beam consists of an intense 3 to 5 GeV electron beam bunched at 30 GHz. One possible way to produce this drive beam is to start with a low-energy (or order 10 MeV), high-current (about 5 kA) beam from an induction linac. In passing through a wiggler, this beam is bunched at 30 GHz into micropulses, each with about 10 12 electrons. To construct the format required for the CLIC drive beam, the bunched beam is subsequently chopped at 350 MHz. It is then accelerated to 3 to 5 GeV in an rf linac driven by conventional, low-frequency klystrons. Rf power is extracted at the bunch frequency of 30 GHz and fed into high-gradient structures to accelerate electron or positron beams to TeV energies. The drive beam is repeatedly reaccelerated in 350 MHz superconducting cavities. This study examines the design trade-offs of the proposed “front end” of the CLIC TBA, the linear induction accelerator and FEL. We examine the relevant figure of merit, the efficiency of bunching, as a function of beam energy, current, and emittance, and we consider the effects of wiggler errors, energy spread and slew, and beam misalignment.

This paper describes designs for 280-GHz and 560-GHz microwave sources based on free electron lasers (FELs). These 10-MW units are based on technology developed over the last 5 years. A first demonstration of high-average-power microwave production with an FEL system is expected in the Microwave Tokamak Experiment (MTX) facility. This paper gives details on the design and construction of that 250-GHz, 2-MW system and discusses specific applications for the Compact Ignition Tokamak (CIT).

We describe the proposed developement of a 17.1 GHz microwave source based on an induction-linac-driven free-electron laser that is expected to produce peak power levels at the several-GW level. This system is being designed to serve as the power source for a TeV linear collider or for other high-gradient accelerators. We will discuss the power-source requirements for TeV colliders and present the initial design of an FEL that will produce the desired power. In addition, we shall discuss efficiency enhancement schemes such as a relativistic klystron “afterburner”, the requirements for beam reacceleration, and concepts for extraction of the microwave power while preserving key properties of the beam such as emittance and phase.

The Microwave Tokamak Experiment (MTX) is under construction at Lawrence Livermore National Laboratory to investigate the feasibility of intense, pulsed microwave radiation for plasma heating on future ignition tokamaks. A high-average-power free-electron laser (FEL) will use the Experimental Test Accelerator (ETA-II), a linear induction accelerator, in combination with an advanced high-field wiggler, to produce 1-2 MW of power at 1-2-mm wavelengths for periods of up to 0.5 s. The design of the FEL, termed the Intense Microwave Prototype (IMP), is described, along with the status and major issues associated with the experiment. >

We describe an induction-linac based free electron laser amplifier design for producing multi-megawatt levels of microwave power for electron cyclotron resonance heating of tokamak fusion devices such as the Compact Ignition Tokamak or the International Thermonuclear Experimental Reactor. The wiggler design strategy incorporates a tapering algorithm suitable for FEL systems with moderate space charge effects and minimizes spontaneous noise growth at frequencies below the fundamental, while enhancing the growth of the signal at the fundamental. In addition, engineering design considerations of the waveguide wall loading and electron beam fill factor in the waveguide set limits on the waveguide dimensions, the wiggler magnet gap spacing, the wiggler period, and the minimum magnetic field strength in the tapered region of the wiggler. This FEL is designed to produce an average power of about 10 MW at frequencies in the range from 280 to 560 GHz. The achievement of this average power at a reasonable cost requires a high duty factor, which affects some component design. In addition, the desire to obtain a high extraction efficiency pushes the beam energy up and requires magnetic field strengths in the wiggler that are near or possibly larger than the Halbach limit. We used a methodology for our system study that had been developed earlier. We considered several FEL configurations and selected one that minimized total cost. We determined that increasing the beam energy requires that the wiggler use vanadium-permendur as the pole material. We discuss the basic design of the selected configuration and give the expected performance.

We have used the free electron laser particle simulation code FRED to examine the design of an FEL for amplifying radiation in the 1–2 mm wavelength range for use in electron heating and current drive in a tokamak device such as ALCATOR-C. We have used as a desired design goal a peak output power of 8 GW, with a minimum input power in the 1–100 W range. We have examined the effects of electron beam current, energy and brightness, laser frequency and input power as well as wiggler wavelength and overall wiggler length on performance of the FEL. We find that to obtain the desired power output, we require a 3 kA electron beam with a brightness of about 1 × 105 A/cm2. For a 1 mm wavelength system, the beam energy must be at least 10 MeV, the laser input power must be at least 100 W, and the wiggler wavelength is 8 cm, with an overall length of at least 6 m. For a 2 mm wavelength system, the output power goals can be met with a 7 MeV electron beam, with 10 W of input laser power, and a wiggler with an 8 cm wavelength and total length of at least 6 m. The beam and laser requirements to meet the design criteria are less stringent for the 2 mm wavelength system. The magnet requirements to meet the design power level goal is more difficult to meet with a wiggler wavelength of 8 cm compared to a 9.8 cm wiggler wavelength. With the 10 MeV beam energy of the 1 mm system and the 7 MeV beam energy of the 2 mm system, the peak magnetic field changes from about 0.4 to 0.6 T when the wiggler wavelength changes from 9.8 to 8 cm.

rf stoppering of a streaming plasma was observed in experiments in which the applied electric fields were too weak for stoppering by the ponderomotive effect on ions. The experimental parameters met two requirements for the electron ponderomotive effect: (a) The electron thermal speed v/sub 0/ or approx. =(2..omega../e)(mk(T/sub e/+T/sub i/))/sup 1/2/. The stoppering observed was the result of the ponderomotive effect on electrons.

Neutral molecular densities were measured just outside the plasma edge at 14 axial locations in the Tandem Mirror Experiment Upgrade (TMX-U). Bayard-Alpert gauges can be operated routinely as calibrated pressure gauges in magnetic fields between 0.3 and 0.7 T. The pressures at the outer turning point and at the midplane of the plug are moderately low, 1 × 10−6 torr, but during plasma operation the inner turning point pressure is as high as 2 × 10−5 torr. This high pressure is consistent with a wall reflux coefficient of approximately 14, assuming that local anomalous radial loss is the cause. Fokker-Planck modelling of the plasma densities with molecular and energetic atomic fuelling shows that this high inner turning point pressure does not account for the measured plasma densities. However, an as yet undetermined additional radial transport of sloshing ions with a 1 ms rate does produce agreement. Both measurements and modelling show that the charge exchange rate at the exposed sloshing ion turning points is high (0.5 ms), but the plasma is self-shielding for most of the remaining axial distance, and the bounce averaged, charge exchange radial loss time constant is 5 ms.

The standard synchronous tapering method used to design the wiggler magnetic field for free electron laser (FEL) amplifiers operating in the Compton regime will not work for amplifier systems where space-charge effects are important. The space-charge effects lower the overall gain in the amplifier system and, even more importantly, shift the peak in the gain curve to magnetic field values that are significantly less than the synchronous magnetic field value. As a result, the overall predicted gain using the synchronous tapering method is too low. Moreover, the synchronous magnetic field corresponds to the peak in the gain curve for a frequency below the fundamental frequency. Consequently, shot noise at frequencies below the fundamental frequency can grow to levels that may prevent amplification of the fundamental. We have developed a new tapering strategy that improves the predicted amplifier gain and circumvents the shot-noise growth for systems with moderate space-charge effects. For this new strategy, we hold the wiggler magnetic field constant at a value below the synchronous value but near the peak of the gain curve for the fundamental frequency, for some optimized length at the front end of the wiggler. Beyond this constant wiggler section, the field is tapered using the standard synchronous tapering algorithm. This new tapering scheme results in significant improvement in predicted amplifier gains and limits the growth of shot noise to insignificant levels. We demonstrate the effectiveness of this new tapering algorithm using the tapered wiggler design for the proposed microwave heating experiment (MTX) at the Lawrence Livermore National Laboratory (LLNL).

The suppression of the drift cyclotron loss cone (DCLC) instability by the localized electrostatic trapping of cold ions is considered. Calculations indicate that the trapped cold ions can suppress the DCLC mode with a cold-ion throughput and energy drain which are smaller than in present stream-stabilized systems.

We describe the base line design for the wiggler to be used in conjunction with the Experimental Test Accelerator‐II (ETA‐II) to produce high‐power microwave radiation in the Intense Microwave Prototype (IMP) free‐electron laser experiment. This design produces negligible noise growth for spontaneous noise sources as large as 20 dB above shot noise, and thus corrects the defects present in earlier iterations of the design, which resulted in diminished gain at the signal frequency because of space‐charge effects as well as enhanced spontaneous noise growth at frequencies lower than the signal frequency. In addition, more realistic engineering design constraints on the waveguide size and the wiggler gap spacing have been set by the waveguide wall loading and the electron beam fill factor and have been incorporated in the final base line design. The design assumes that the ETA‐II accelerator will supply an electron beam with an energy of 10 MeV, a current of 3 kA, and a brightness of 1.0×108 A/(m rad)2. With...

A feedback‐controlled particle suspension system has been developed for laser target plasma production in the LITE experiment. Suspension voltages derived from position and velocity (position derivative) feedback signals serve to capture an injected particle in high vacuum, damp the particle motion, and stably position the particle at the system null. In typical operations, LiH particles of ∼100‐μm diameter, projected by a spring plunger into the suspension region, are captured and positioned at the system center in ⩽0.25 sec with a jitter of ⩽20 μm, less than 1/5 the particle diameter. With this system the precise particle positioning required for spherically symmetric plasma generation is obtained even with large particle sizes, and the feedback particle suspension system is incorporated as an integral part of the LITE experimental facility for studies of neutral injection buildup on a dense, mirror‐confined, laser‐produced target plasma.

A two-dimensional criterion for distinguishing absolute instabilities with arbitrary propagation in an infinite magnetized plasma is developed and applied to mirror-confined plasmas. It is shown that this criterion follows directly from the one-dimensional criterion by careful consideration of the singularities in the inverse Fourier-Laplace transform integrals.A number of instabilities, generally referred to as loss-cone instabilities, are numerically investigated using the above criterion and the results compared to those available from one-dimensional calculations where growth is restricted to the direction parallel to the magnetic field. It is found that for the loss-cone instability the one-dimensional calculation gives good estimates of the transition density and real parallel wave number as a function of the electron temperature, but yields an overestimation of the maximum axial convective growth rate. When the effects of ion temperature anisotropy are examined it is shown that the two-dimensional calculation is required because of the great and sensitive dependence of the transition density on the value of the perpendicular wave number.Although the stabilizing effect of increased electron temperature on the loss cone, drift cone, and drift cyclotron instabilities is confirmed, it is found that this effect is smaller in the case of the drift-cyclotron mode. Moreover, the presence of a cool ion component in a hot loss-cone plasma is shown to have only a small stabilizing effect at the first ion cyclotron harmonic.The Dory-Guest-Harris instability associated with the ring distribution is shown to be absolute for flute-like modes, while for oblique propagation it is shown that an absolute instability cannot exist.For all modes, the one-dimensional calculation gives somewhat pessimistic results regarding the dimensions of a stable plasma. By allowing for spatial decay normal to the magnetic field the two-dimensional investigation shows that the plasma need not be as stubby to be stable against the loss-cone modes.

We describe the models used to calculate the equilibrium operating point for the MARS mode of the axicell MFTF-B, and present the model equations necessary to calculate the plasma and potential parameters in the central-cell, axicell, and anchor regions. In addition, we calculate the neutral beam, rf power, and gas inputs required to sustain the plasma and maintain the ion and electron-particle energy balance. After a brief description of the MFTF code, we present the results of a sample calculation using MFTF.

Experimental studies aimed at improving the reproducibility of the dense, laser produced plasmas were performed. Efforts in this area were concerned with the positioning of the target particle more accurately in the laser focal region and with preventing the premature displacement of the particle from the focal region. To address this latter problem, the single beam laser system of the previous experiments was modified to permit the simultaneous irradiation of the lithium hydride pellet by two opposed laser beams. In addition, the diagnostic capabilities of the present experiment were augmented so that more detailed measurements of the plasma lifetime, density, and energy can be made. A 4 mm microwave interferometer was installed to follow the plasma decay from higher densities. The measured density decay histories were analyzed to give estimates of the plasma lifetime and energy. Independent measures of the time dependence of the plasma radius were obtained from both microwave interferometry and optical streak photography. To measure the energy distribution of the ions which escape through the mirror loss cones, an electrostatic ion energy analyzer was developed. The theoretical effort in support of the experimental program dealt with the solution of the rate equations and Fokker-Planck equations which describemore » the confined plasmas in the Laser Initiated Target Experiment (LITE). The rate equation analysis of the plasma heating and evolution to steady state with neutral injection using the LITE parameters indicates that a 0.5 Ampere current of 10-keV neutrals with intensity 5 mA/cm/sup 2/ at the target will balance the Coulomb decay and charge exchange losses with the neutral background and will susiain a plasma of density)10/sup 12/ cm/sup -3/. In addition, self-consistent Fokker -- Planck calculations (now including the effects of charge exchange with the background neutrals) were performed to give the time evolution of the particle energies, density, and plasma potential. The design of the neutral beam line for the charge exchange heating and neutral injection experiments was completed. To accommodate the energetic neutral flux while maintaining the necessary high vacuum conditions, a self-consistent calculation, including recycling of the neutral flux from the walls, has shown the need for an enlarged experiment chamber to provide more effective pumping. The criteria on which the designs of the experiment chamber and the neutral beam line are based, are discussed. (auth)« less

In this report we document the operating scenario models and calculations as they exist and comment on those aspects of the models where performance is sensitive to the assumptions that are made. We also focus on areas where improvements need to be made in the mathematical descriptions of phenomena, work which is in progress. To illustrate the process of calculating performance, and to be very specific in our documentation, part 2 of this report contains the complete equations and sequence of calculations used to determine parameters for the MARS mode of operation in MFTF-B. Values for all variables for a particular set of input parameters are also given there. The point design so described is typical, but should be viewed as a snapshot in time of our ongoing estimations and predictions of performance.