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Higher electron temperature (Te>350eV) and reduced electron thermal diffusivity (χe

Density in a spheromak is lowered by action of a m∕n=2∕4 island, after which the electrons heat rapidly to give profiles of nTbefore=nTafter, indicating that the spheromak heats up to a pressure limit. The spheromak core remains at Te∼150eV for ∼1∕2ms during a period exhibiting low amplitude incoherent fluctuations consistent with a pressure-driven mode, giving the volume-averaged electron beta ⟨βe⟩∼3% and peak βe of ∼6%. Calculations show that measured pressure profiles are close to the Mercier limit.

The Sustained Spheromak Physics Experiment (SSPX) uses a magnetized coaxial gun to form and sustain spheromaks by helicity injection. Internal probes give the magnetic profile within the gun. Analysis of these data show that a number of commonly applied assumptions are not completely correct, and some previously unrecognized processes may be at work. Specifically, the fraction of the available vacuum flux spanning the gun that is stretched out of the gun is variable and not usually 100%. The n=1 mode that is present during sustained discharges has its largest value of {delta}B/B within the gun, so that instantaneously B within the gun is not axisymmetric. By applying a rigid-rotor model to account for the mode, the instantaneous field and current structure within the gun are determined. The current density is also highly non-axisymmetric and the local value of {lambda} {triple_bond} {mu}{sub 0}j{sub {parallel}}/B is not constant, although the global value {lambda}{sub g} {triple_bond} {mu}{sub 0}I{sub g}/{psi}{sub g} closely matches that expected by axisymmetric models. The current distribution near the gun muzzle suggests cross-field current exists, and this is explained as a line-tying reaction to plasma rotation.

During buildup of a spheromak by helicity injection, magnetic reconnection converts toroidal flux into poloidal flux. This physics is explored in the resistive magnetohydrodynamic code, NIMROD [C.R. Sovinec, A.H. Glasser, T.A. Gianakon, D.C. Barnes, R.A. Nebel, S.E. Kruger, D.D. Schnack, S.J. Plimpton, A. Tarditi, and M.S. Chu, J. Comp. Phys., 195, 355-386 (2004)], which reveals negative current sheets with {lambda} = {mu}{sub 0}j {center_dot} B/B{sup 2}reversed relative to the applied current. The simulated event duration is consistent with magnetic diffusion on the sheet thickness and is accompanied by cathode voltage spikes and poloidal field increases similar to those seen in the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion 39, 863 (1999)]. All magnetic fieldlines are open during reconnection and their trajectories are very sensitive to their starting points, resulting in chaos. The current sheets are most intense inside the separatrix near the X-point of the mean-field spheromak, suggesting that the reconnection occurs near fieldlines which are closed in the azimuthal average.

Simulation results are presented that illustrate the formation and decay of a spheromak plasma driven by a coaxial electrostatic plasma gun, and model the plasma energy confinement. The physics of magnetic reconnection during formation is also illuminated. The simulations are performed with the three-dimensional, time-dependent, resistive magnetohydrodynamic NIMROD code [C. R. Sovinec, A. H. Glasser, T. A. Gianakon, D. C. Barnes, R. A. Nebel, S. E. Kruger, D. D. Schnack, S. J. Plimpton, A. Tarditi, and M. S. Chu, J. Comput. Phys. 195, 355 (2004)]. The simulation results are compared to data from the Sustained Spheromak Physics Experiment (SSPX) [E. B. Hooper, L. D. Pearlstein, and R. H. Bulmer, Nucl. Fusion 39, 863 (1999)]. The simulation results are tracking SSPX with increasing fidelity (e.g., improved agreement with measured magnetic fields, fluctuation amplitudes, and electron temperature) as the simulation has been improved in its representations of the experimental geometry, the magnetic bias coils,...

In the Sustained Spheromak Physics Experiment, SSPX [E. B. Hooper, D. Pearlstein, and D. D. Ryutov, Nucl. Fusion 39, 863 (1999)], progress has been made in understanding the mechanisms that generate fields by helicity injection. SSPX injects helicity (linked magnetic flux) from 1 m diameter magnetized coaxial electrodes into a flux-conserving confinement region. Control of magnetic fluctuations (delta B/B similar to 1% on the midplane edge) yields T-e profiles peaked at > 200 eV. Trends indicate a limiting beta (beta(e)similar to 4%-6%), and so we have been motivated to increase T-e by operating with stronger magnetic field. Two new operating modes are observed to increase the magnetic field: (A) Operation with constant current and spontaneous gun voltage fluctuations. In this case, the gun is operated continuously at the threshold for ejection of plasma from the gun: stored magnetic energy of the spheromak increases gradually with delta B/B similar to 2% and large voltage fluctuations (delta V similar to 1 kV), giving a 50% increase in current amplification, I-tor/I-gun. (B) Operation with controlled current pulses. In this case, spheromak magnetic energy increases in a stepwise fashion by pulsing the gun, giving the highest magnetic fields observed for SSPX (similar to 0.7 T along the geometric axis). By increasing the time between pulses, a quasisteady sustainment is produced (with periodic good confinement), comparing well with resistive magnetohydrodynamic simulations. In each case, the processes that transport the helicity into the spheromak are inductive and exhibit a scaling of field with current that exceeds those previously obtained. We use our newly found scaling to suggest how to achieve higher temperatures with a series of pulses.

The first Thomson scattering measurements of ne and Te in the divertor region of a tokamak are reported. These data are used as input to boundary physics codes such as UEDGE and DEGAS and to benchmark the predictive capabilities of these codes. These measurements have also contributed to the characterization of tokamak disruptions. A Nd:YAG laser (20 Hz, 1 J, 15 ns, 1064 nm) is directed vertically through the lower divertor region of the DIII–D tokamak. A custom, aspherical collection lens (f/6.8) images the laser beam from 1 to 21 cm above the target plates into eight spatial channels with 1.5 cm vertical and 0.3 cm radial resolution. Two-dimensional mapping of the divertor region is achieved by sweeping the divertor X-point location radically through the fixed laser beam location. Fiber optics carry the light to polychromators whose interference filters have been optimized for low-Te measurements. Silicon avalanche photodiodes measure both the scattered and plasma background light. Temperatures and dens...

A 48‐channel platinum‐foil bolometer system on DIII‐D was installed to achieve better spatial and temporal resolution of the radiated power in diverted discharges. Two 24‐channel arrays provide complete plasma coverage with optimized views of the divertor. The divertor radiation profile was measured for a series of radiative divertor and power balance experiments. A significant change in the magnitude and distribution of divertor radiation with heavy gas puffing was observed. Unfolding the radiation profile with only two views requires one to treat the core and divertor radiation separately. The core radiation is fitted to a function of magnetic flux and is then subtracted from the divertor viewing chords. The divertor profile is then fit to a 2D spline as a function of magnetic flux and distance from divertor floor.

The composite‐based low voltage field emitter cathode offers distinct advantages for high‐frequency applications. It is based on a directionally solidified ZrO2⋅Y2O3–W eutectic composite containing aligned, single‐crystal tungsten fibers, ≊500 nm in diameter, with spatial densities of 2–50×106/cm2. The composite can be etched to form fibers of virtually any length with tip radii of 5–10 nm. A gridded structure can be formed around the fibers using a line‐of‐sight vapor deposition process. Because of the thickness of the composite chips used as the substrate (0.5 mm) and the high resistivity of the oxide matrix, the insulator film can be made extremely thin, thereby reducing the extractor aperture radius. Emission tests have verified this configuration results in measurably higher field enhancement values for the arrays.

Successful application of field emitter array cathodes at microwave frequencies is dependent upon significant progress in emitter development. Two factors are of primary concern, namely, the transconductance and the array capacitance. To provide guidance in development of structures with minimum capacitance, a scaled, physical model of an emitter array has been developed and used to study qualitatively the effect of several geometrical and materials parameters on device capacitance. In this article the effects of an insulating versus a conducting substrate, and variation of the insulator film thickness, extractor aperture diameter, and extractor film thickness are reported. It is shown that the array capacitance is primarily determined by the details of the extractor aperture–emitter tip geometry, and is not influenced by the extractor to substrate spacing unless the insulator film thickness is substantially less than one‐half the pin‐to‐pin spacing in the array. Although the results are not intended to b...

Field emitter array (FEA) devices were constructed using thin-film fabrication techniques based on in situ eutectic composites containing either TaC or NbC fibers in a NiCr alloy matrix. The emission characteristics of the TaC devices were measured and compared to those of W–ZrO2 FEA's. Both devices produced the linear Fowler-Nordheim plots common to all field emission devices; however, the array current density of the TaC FEA's was more than two orders of magnitude lower than that of the W–ZrO2 FEA's. This was primarily due to the lower fiber density and lower field enhancement of the carbide fiber devices. If it were possible to produce cathode geometries comparable to those of the W–ZrO2 FEA's, a theoretical analysis indicates that emission performance of the TaC devices would be superior because of the lower work function of TaC. However, post-emission observations indicated that the carbide fiber emitters failed more readily than the tungsten emitters under comparable operating conditions.

Ambipolar radial transport (equal ion and electron flux) is not directly measured in tandem mirror experiments because the particle flow does not produce a net electrical current. The first absolute measurements of the ionization source in the Tandem Mirror Experiment Upgrade (TMX-U) plasma have been obtained. These have permitted the determination of the magnitude of ambipolar radial transport from the particle balance. Furthermore, comparisons of the source measurements with a Monte Carlo neutral transport code have shown reasonable agreement. Measurements of the particle balance under several operating conditions are presented. For some of these cases, the ambipolar radial transport is smaller than the other measured losses.

Thermal-barrier experiments in the Tandem Mirror Experiment Upgrade (TMX-U) are reported, along with progress made at the Lawrence Livermore National Laboratory in plasma confinement and central-cell heating. Thermal barriers in TMX-U improved axial confinement by two orders of magnitude over a limited range of densities, compared with confinement in single-cell mirrors at the same ion temperature. It is shown that central-cell radial nonambipolar confinement scales as neoclassical theory and can be eliminated by floating the end walls. Radial ambipolar losses can also be measured and reduced. The electron energy balance is improved in tandem mirrors to near classical, resulting in T/sub e/ up to 0.28 keV. Electron cyclotron heating (ECH) efficiencies up to 42%, with low levels of electron microinstability, were achieved when hot electrons in the thermal barrier were heated to average betas as large as 15%. The hot-electron distribution was measured from X-rays and is modeled by a Fokker-Planck code that includes heating from cavity radio-frequency (RF) fields. >

Time resolved power deposition profiles on the D III-D divertor target plates have been measured for Ohmic and neutral beam injection heated plasmas using fast response infrared thermography (τ ≤ 150 μs). Giant Edge Localized Modes have been observed which punctuate quiescent periods of good H-mode confinement and deposit more than 5% of the stored energy of the core plasma on the divertor armour tiles on millisecond timescales. The heat pulse associated with these events arrives approximately 0.5 ms earlier on the outer leg of the divertor relative to the inner leg. The measured power deposition profiles are displaced relative to the separatrix intercepts on the target plates, and the peak heat fluxes are a function of core plasma density.

Optical pumping of long-lived quantum states is used to identify groups of ions in phase space and follow their transport. The principles and variants of a novel diagnostic method, as well as new physical aspects and applications, including space-dependent line narrowing, are demonstrated.

In this paper we report the development of the single frequency dye laser as a plasma diagnostic for measuring ion velocity distributions with a resolution limited only by the natural linewidth of the laser excited resonance transition. For the 6S1/2–6P1/2 transition in Ba+ this corresponds to a velocity uncertainty of 103 cm/s. Velocity selection is performed by the laser as it scans the Doppler broadened plasma absorption line, in contrast with the usual method of scanning an emission line with a high‐resolution spectrometer. Both sensitivity and resolution are improved by 2 orders of magnitude, allowing nonperturbing measurements to be made at densities below 107 cm−3. In addition to a description of the technique, sample measurements of time‐resolved wave induced modifications of ion velocity distributions are shown.

Reduced recycling, reduced edge neutral pressure, improved density control, and improved discharge reproducibility have been achieved in the DIII-D tokamak by in situ helium condition of the graphite tiles. An improvement in energy confinement has been observed in hydrogen discharges with hydrogen beam injection after helium preconditioning. After the graphite wall coverage in DIII-D was increased to 40%, helium glow wall conditioning, routinely applied before each tokamak discharge, has been necessary to reduce recycling and obtain H-mode. The utilization of helium glow wall conditioning was an important factor in the achievement of an ohmic H-mode, i.e. no auxiliary heating, with significant improvement in ohmic energy confinement.

A laser diagnostic yielding detailed space-tme resolution of the ion velocity distribution function reveals that ions ejected by electrostatic instabilities can form a frequency-coherent beam circulating diamagnetically at large radii around the unstable region. This results in anomalous transport with fluxes and over distance scales much larger than conventional processes.

In thermal-barrier experiments in the tandem mirror experiment upgrade axial confinement times of 50 to 100 ms have been achieved. During enhanced confinement we measured the thermal-barrier potential profile using a neutral-particle-beam probe. The experimental data agree qualitatively and quantitatively with the theory of thermal-barrier formation in a tandem mirror.

The authors' data indicate that the L-mode to H-mode transition in the DIII-D tokamak is associated with the sudden reduction in anomalous, fluctuation-connected transport across the outer midplane of the plasma. In addition to the reduction in edge density and magnetic fluctuations observed at the transition, the edge radial electric field becomes more negative after the transition. They have determined the scaling of the H-mode power threshold with various plasma parameters; the roughly linear increase with plasma density and toroidal field are particularly significant. Control of the ELM frequency and duration by adjusting neutral beam input power has allowed us to produce H-mode plasmas with constant impurity levels and durations up to 5 s. Energy confinement time in ohmic H-mode plasmas and in deuterium H-mode plasmas with deuterium beam injection can exceed saturated ohmic confinement times by at least a factor of two. Energy confinement times above 0.3 s have been achieved in these beam-heated plasmas with plasma currents in the range of 2.0 to 2.5 MA. Local transport studies have shown that electron and ion thermal diffusivities and angular momentum diffusivity are comparable in magnitude and all decrease with increasing plasma current.

Tokamak discharges using the expanded boundary divertor in the DIII-D device exhibit H-mode confinement. With neutral-beam power up to 6 MW, energy confinement remains comparable to the Ohmic value at a plasma current of 1 MA. Confinement is also independent of plasma density and toroidal field. Confinement increases with plasma current, but the exact functional dependence is, as yet, uncertain. These results show that the H mode can be achieved in a reactor-compatible open divertor configuration.

The first observation of $H$-mode confinement with electron heating as the sole auxiliary heating method has been made in divertor discharges in the DIII-D tokamak. These discharges exhibit the usual characteristics of the $H$ mode, including improved confinement of particles and energy, when electron cyclotron heating is added at a power level above 0.7 MW. The $H$-mode transition is accompanied by the development of an electron temperature pedestal of 0.25 keV and a dramatic steepening of the density gradient near the separatrix.

Field emitter array cathodes were fabricated from unidirectionally solidified composites of tungsten fibers in an insulating yttria-stabilized-zirconia (YSZ) matrix. A close-spaced molybdenum gate film (extractor) was formed utilizing c-beam evaporation of alumina as an insulator, which was overlayed by the molybdenum extractor. The high resistivity of the composite matrix coupled with the alumina insulator resulted in low leakage current and permitted dc operation of the device. Emission testing demonstrated current densities of 1–5 A/cm2 with leakage in the μA range for applied potentials of 125–200 V. Variation of emitter tip geometries from hemispheres to right circular cylinders to pointed cones produced increases in emission consistent with reduced tip radii.

We have developed an optical diagnostic system suitable for characterizing the performance of energetic neutral beams. An absolutely calibrated CCD video camera is used to view the neutral beam as it passes through a relatively high‐pressure (10−5 Torr) region outside the neutralizer: collisional excitation of the fast deuterium atoms produces Hα emission (λ=6561 A) that is proportional to the local atomic current density, nearly independent of the species mix of accelerated ions over the energy range 5–20 keV. Digital processing of the video signal provides profile and aiming information for beam optimization.

The Tandem‐Mirror Experiment‐Upgrade (TMX‐U) has a large and complex system of Bayard–Alpert, magnetron, and Penning gauges, in addition to mass spectrometers (RGA), all of which measure neutral pressures in the many internal regions of TMX‐U. These pressure measurements are used as part of the confinement physics data base as well as for management of the TMX‐U vacuum system. Dynamic pressures are modeled by a coupled‐volumes simulation code, which includes wall reflux, getter pumping, and plasma pumping.

Several diagnostics measure the particle sources and losses in the Tandem Mirror Experiment‐Upgrade (TMX‐U) plasma. An absolutely calibrated high‐speed (0.5 ms per frame) filtered (6561 A) video camera measures the total ionization source as a function of radius. An axial view of the plasma automatically integrates the axial variations within the depth of field of the system. Another camera, viewing the plasma radially, measures the axial source variations near the deuterium fueling source. Axial ion losses are measured by an array of Faraday cups that are equipped with grids for repelling electrons and are mounted at each end of the experiment. Unequal ion and electron (nonambipolar) radial losses are inferred from net current measurements on an array of grounded plates at each end. Any differences between the measured particle losses and sources may be attributed to ambipolar radial losses and/or azimuthal asymmetries in the particle‐loss profiles. Methods of system calibration, along with details of co...

Infrared thermography in the 8–12 μ wavelength range was used to measure surface heating on the low energy neutral source (LENS) plates in the Tandem Mirror Experiment Upgrade (TMX‐U). A surface temperature rise of 3.5 °C was observed at the plasma edge during discharges lasting 70 ms with edge densities of approximately 1012 cm−3. However, surface heating appeared to be nonuniform azimuthally in that the bottom plates showed a greater temperature rise. A few seconds after the termination of the discharge an axial view of the TMX‐U revealed an additional form of surface heating. This is consistent with observations from the x‐ray diagnostic which showed that a significant concentration of energetic electrons remained after the bulk thermal plasma had disappeared; these electrons disappeared only after the magnets were turned off. Details of the experiment and our experiences with this diagnostic tool will be presented.

A rapid, controllable process for CVD of superconducting Yba2Cu30x has been developed. The new process relies on slowly feeding and pneumatically transporting powdered solid reagents directly into the CVD furnace; vaporizers are not used. Deposition rates two orders of magnitude greater than those achievable by reagent sublimation have been obtained using a powder feed mixture of Y, Ba, and Cu tmhd complexes (tmhd is tetramethylheptanedionate). Potentially, the process is applicable to thermally unstable, less volatile, lower cost reagents such as the acetylacetonates.

Neutral-beam current-drive experiments in the DIII-D tokamak with a single null poloidal divertor are described. A plasma current of 0.34 MA has been sustained by neutral beams alone, and the energy confinement is of $H$-mode quality. Poloidal $\ensuremath{\beta}$ values reach 3.5 without disruption or coherent magnetic activity suggesting that these plasmas may be entering the second stability regime.

NASA is investigating high power, high specific impulse propulsion technologies that could enable ambitious flights such as multi-body rendezvous missions, outer planet orbiters and interstellar precursor missions. The requirements for these missions are much more demanding than those for state-of-the-art solar-powered ion propulsion applications. The purpose of the NEXIS program is to develop advanced ion thruster technologies that satisfy the requirements for high power, high specific impulse operation, high efficiency and long thruster life. The nominal design point for the NEXIS thruster is 20 kWe at a specific impulse of 7500 s with an efficiency over 78% and a xenon throughput capability of greater than 2000 kg. These performance and throughput goals will be achieved by applying a combination of advanced technologies including a large discharge chamber, erosion resistant carbon-carbon grids, an advanced reservoir hollow cathode and techniques for increasing propellant efficiency such as grid masking and accelerator grid aperture diameter tailoring. This paper provides an overview of the challenges associated with these requirements and how they are being addressed in the NEXIS program.

The radiation of divertor heat flux on DIII-D is shown to greatly exceed the limits imposed by assumptions of energy transport dominated by electron thermal conduction parallel to the magnetic field. Approximately 90% of the power flowing into the divertor is dissipated through low Z radiation and plasma recombination. The dissipation is made possible by an extended region of low electron temperature in the divertor. A one-dimensional analysis of the parallel heat flux finds that the electron temperature profile is incompatible with conduction dominated parallel transport. Plasma flow at up to the ion acoustic speed, produced by upstream ionization, can account for the parallel heat flux. Modeling with the two-dimensional fluid code UEDGE has reproduced many of the observed experimental features.

By operating a magnetized coaxial plasma gun continuously with just sufficient current to enable plasma ejection, large gun-voltage spikes (approximately 1 kV) are produced, giving the highest sustained voltage approximately 500 V and highest sustained helicity injection rate observed in the Sustained Spheromak Physics Experiment. The spheromak magnetic field increases monotonically with time, exhibiting the lowest fluctuation levels observed during formation of any spheromak (B/B>/=2%). The results suggest an important mechanism for field generation by helicity injection, namely, the merging of helicity-carrying filaments.

Summary form only given, as follows. We will report recent results from the Sustained Spheromak Physics Experiment (SSPX) in which we measure peak electron temperatures greater than 300eV in driven spheromak discharges. The SSPX spheromak plasma (0.31m major radius, 0.13m minor radius) is driven for up to 4msec by a DC coaxial source (a Marshall gun), which is powered by a 2MJ capacitor bank. The plasma is contained inside a tungsten-coated copper flux conserver which maintains stability against global tilting modes.

This paper presents a comparison of the total radiated power profile and impurity line emission distributions in the SOL and divertor of DIII-D. This is done for ELMing H-mode plasmas with heavy deuterium injection (partially detached divertor operation, PDD) and those without deuterium puffing. Results are described from a series of dedicated experiments performed on DIII-D to systematically measure the 2D ( R, Z ) structure of the divertor plasma. The discharges were designed to optimize measurements with new divertor diagnostics including a divertor Thomson scattering system. Discharge sequences were designed to produce optimized data sets against which SOL and divertor theories and simulation codes could be benchmarked. During PDD operation the regions of significant radiated power shift from the inner divertor leg and SOL to the outer leg and X-point regions. D α emission shifts from the inner strikepoint to the outer strikepoint. Carbon emissions (visible CII and CIII) shift from the inner SOL near the X-point to a distributed region from the X-point to partially down the outer leg during moderate D 2 puffing. In heavy puffing discharges the carbon emission coalesces on the outer separatrix near the X-point and for very heavy puffing it appears inside the last closed flux surface above the X-point. Calibrated spectroscopic measurements indicate that hydrogenic and carbon radiation can account for all of the radiated power. L α and CIV radiation are comparable and when combined account for as much as 90% of the total radiated power along chords viewing the significant radiating regions of the outer leg.

The KSTAR team is carrying out the design and research and development for a steady-state-capable advanced superconducting tokamak to establish the basis for an attractive fusion reactor as a future energy source. The physics requirements are driven by the plasma control and exhaust capabilities needed to extend the performance and pulse length of tokamak plasmas. The tokamak has major radius 1.8 m, minor radius 0.5 m, toroidal field 3.5 T and plasma current 2 MA, a strongly shaped plasma cross-section shaping (elongation 2.0 and triangularity 0.8), and a double-null poloidal divertor. The initial pulse length is 20 s, long enough to study physics on confinement timescales, but short enough to permit economical plasma-facing component technology. The pulse length can be increased to 300 s through upgrades. The machine will be operable in either hydrogen or deuterium, but neutron yields will be constrained to avoid the cost and inconvenience of remote maintenance and low-activation materials. The magnet system provides an inductively driven 20 s pulse with full current, beta, and shaping. With non-inductive current drive, steady-state plasmas can be sustained over a wide range of profile shapes and plasma pressures, Passive structures are provided to stabilize the vertical instability and high-beta modes and internal coils are provided for fast position control. The divertor structures are designed for particle removal, recycling control, impurity control, and flexibility for advanced divertor operation. The plasma heating system is designed for heating, current-drive, profile control, and flexibility. It will deliver power via neutral beams (8 MW), ion-cyclotron waves (6 MW), and lower-hybrid waves (1.5 MW), each of which can be expanded through upgrades. A comprehensive set of diagnostics is planned for plasma control, performance evaluation, and physics understanding.

Summary form only given. In the Sustained Spheromak Physics Experiment (SSPX), a large toroidal n=1 magnetic mode is observed within the coaxial gun (/spl delta/B/B/spl ap/15-20%). The fraction of gun flux used for helicity injection is accurately determined using internal probes and an asymmetric rigid rotor model. Current and field distributions in the gun show plasma there is neither relaxed nor complexity force-free A j/sub /spl perp// produces a torque opposite then n=1 rotation direction. This is interpreted as a result of line tying acting to counter any motion associated with the n=1 mode.

A major portion of the DIII-D program includes studies of the L-H transition, of the VH-mode, of particle transport and control and of the power-handling capability of a divertor. Significant progress has been made in all of these areas and the purpose of this paper is to summarize the major results obtained during the last two years. An increased understanding of the origin of improved confinement in H-mode and in VH-mode discharges has been obtained, good impurity control has been achieved in several operating scenarios, studies of helium transport provide encouraging results from the point of view of reactor design, an actively pumped divertor chamber has controlled the density in H-mode discharges and a radiative divertor is a promising technique for controlling the heat flux from the main plasma.

A thermionic source of singly charged alkali ions consisting of an aluminosilicate‐molybdenum emitter assembly rigidly mounted behind a Pierce‐type electrostatic focusing structure is described. This ion source is compatible with an ultra‐high vacuum environment and has generated intense, high purity ion beams for a continuous operating period of one month.