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We have observed reduction of the runaway electron synchrotron radiation, hard X-ray (HXR) intensity, and HXR energy after applying 110 GHz 2nd harmonic electron cyclotron resonant heating (ECRH) during runaway electron (RE) discharges at low density with startup runaway electrons. However, we did not see a significant reduction of X-rays from 170 GHz 2nd harmonic ECRH at a higher field. A recently installed IR TV camera was used to observe the forward cone of synchrotron radiation from high energy REs in KSTAR. We have observed changes to the synchrotron images and reduction of the HXR by application of resonant magnetic perturbations (RMP) from in-vessel control coils (IVCC) installed inside KSTAR in the n = 1 configuration.

The neutral beam injection (NBI) system was designed to provide plasma heating and current drive for high performance and long pulse operation of the Korean Superconducting Tokamak Advanced Research (KSTAR) device using two co-current beam injection systems. Each neutral beam injection system was designed to inject three beams using three ion sources and each ion source has been designed to deliver more than 2.0 MW of deuterium neutral beam power for the 100-keV beam energy. Consequently, the final goal of the KSTAR NBI system aims to inject more than 12 MW of deuterium beam power with the two NBI for the long pulse operation of the KSTAR. As an initial step toward the long pulse (∼300 s) KSTAR NBI system development, the first neutral beam injection system equipped with one ion source was constructed for the KSTAR 2010 campaign and successfully commissioned. During the KSTAR 2010 campaign, a MW-deuterium neutral beam was successfully injected to the KSTAR plasma with maximum beam energy of 90 keV and the L-H transition was observed with neutral beam heating. In recent 2011 campaign, the beam power of 1.5 MW is injected with the beam energy of 95 keV. With the beam injection, the ion and electron temperatures increased significantly, and increase of the toroidal rotation speed of the plasma was observed as well. This paper describes the design, construction, commissioning results of the first NBI system leading the successful heating experiments carried in the KSTAR 2010 and 2011 campaign and the trial of 300-s long pulse beam extraction.

A compact diagnostic system for monitoring Hard X-Ray (HXR) in the KSTAR tokamak has been developed in this work. Its development aims for an extension of the counting rate limit, and a good time r...

Hard X-ray (HXR) emission from KSTAR superconducting Tokamak has been measured during discharges which includes ECH assisted start up. Since the HXR signal indicates energetic electrons we can assu...

Wave enhanced runaway generation is expected to play an important role in the conversion of plasma current into runaway current during major disruptions. The fast electrons created by electron cyclotron heating (ECH) were used to study this issue in KSTAR. It is found that the fast electrons driven by ECH can enhance runaway production in the flat top phase with high loop voltage. The runaway current in disruptions was not enhanced by the ECH produced fast electron population due to the strong magnetic fluctuations which inhibited the generation of runaway electrons. It is found that a complete loss of existing REs during thermal quench has occurred in KSTAR limiter configuration discharges.

It is possible to detect the presence of small field errors in a tokamak with an electron beam. This was demonstrated earlier on T-15 and TEXTOR. This paper discusses the concept, past experience on these tokamaks, calculations for the Korea Superconducting Tokamak Advanced Research (KSTAR) device, an electron beam source, measurement devices for these measurements, and some results. It is shown that small toroidally averaged field errors can be detected by this method. A low voltage electron beam (e-beam) gun and fluorescent screen were mounted in a vertical port and inserted into the vacuum vessel at the end of the KSTAR 2nd campaign plasma experiments. A camera with a narrow field of view was mounted in midplane port in a tube tangent to the field lines at R ∼ 1.3 m and photographed the beam striking the screen. The poloidal field (PF) currents were held constant during the camera exposure period. Many shots with various PF coils energized were made and the deflections of the e-beam were measured. The measurements were made with a camera integration time of 300 ms because of the low light intensity. The results show that there are large field errors that diminish as the PF currents are raised. There appears to be no significant up–down asymmetry for static fields. Measurements with a 7 PF coil scenario with a calculated field null located at e-beam radial position show much larger fields than calculated. KSTAR was constructed with Incoloy 908 conduit using cable-in-conduit conductors (CICC) in 10 of the 14 PF coils and all 16 of the toroidal field (TF) coils. Incoloy 908 has a relative magnetic permeability, μ, of about 10. The field errors appear to be largely due to Incoloy 908.

There is strong evidence that the ICRH applied to the central cell (CC) of Hanbit does not result in complete ionization. Two techniques are being tried in an attempt to improve the degree of ionization. The first of them is ECRH preionization by application of microwave power from one 2-kW CPI klystron at 14 GHz as well as two 1.8-kW VA-806 klystrons at 7.67 GHz and 7.87 GHz. The second technique is production of a reflex discharge using a 76 mm diameter LaB{sub 6} cathode heated to incandescence and biased negatively in the cusp end cell of Hanbit. The ECRH preionization has shown beneficial effects on the plasma. There are no results from the reflex discharge.

The HANBIT device is a simple mirror-type device of which the length, radius, and magnetic field are about 5 m, 0.18 m, and 0.1-0.3 T, respectively, in the central cell. In HANBIT, two antenna systems are used for the plasma production, heating, and MHD stabilization; one is the slot antenna located near the center region with the maximum power of 500 kW and the typical frequency of 3.5 MHz, and the other DHT antenna located near the mirror throat with the maximum power of 100 kW and the frequency of 3.75 MHz. Recent experimental studies in HANBIT indicate that the slot antenna system can produce stable, high-density plasmas in apparently two different regimes; one is the fast wave regime with the ratio w/Wci∼2 and the other is the slow wave regime with w

Low density slide away discharges with anomalous Doppler resonance (ADR) effects have been observed in the KSTAR tokamak. When the line averaged electron density was lower than 0.6 × 10 19 m − 3 , the discharges went into the slide-away regime with relaxations in the electron cyclotron emission due to the ADR effects which transferred the runaway electron energy from parallel to perpendicular motion. The suppression of the ADR effects has been achieved by electron cyclotron resonance heating which enhanced the perpendicular energy of electrons and led to an isotropization of the electron distribution function.

HANBIT is a magnetic mirror confinement device. Recently, after finishing the first campaign for the basic system development, it started the second campaign for high-temperature plasma confinement physics study in a mirror configuration. Here, we introduce briefly the HANBIT device and report initial physics experiment results on RF-plasma heating and confinement in the simple mirror configuration. It appears that the discharge characteristics of HANBIT are quite different from those in other mirror devices, and an explanation is presented to clarify the difference.

The HANBIT device is a non axi-symmetric mirror being operated as a national users’ facility. Plasmas are routinely produced by ICRF at 3.5 MHz with a slot antenna with gas puffing and the line-int...

The first phase of the HANBIT project that started in 1996, is ending this year. The goal of the first phase is to establish a decent experimental facility based on the HANBIT mirror device and to ...

New theoretical and experimental tools are applied to the analysis of ICRF antenna-edge plasma interactions in the TFTR tokamak. A new numerical method for computing the three dimensional (3-D) rf sheath voltage distribution is used, and the quantitative predictions of rf sheath theory are compared with measurements of the edge density profile obtained by microwave reflectometry and with titanium impurity concentration data. It is shown that the local density depletion at the antenna is consistent with density pump-out by strong E × B convection into the Faraday screen (FS). Modelling of the FS impurity influx shows that the calculated titanium impurity concentration based on this direct influx agrees with the measured concentration for π phasing. It is also shown that screening of impurity neutrals by ionization in the SOL is a large effect and increases with rf power. At high power over many shots, a fraction of the metal impurities migrates around the machine and is deposited on the limiters, providing a secondary source of titanium. The data show that the central titanium concentration is strongly dependent on antenna phasing. Possible explanations for this phasing dependence are discussed.

The Hanbit device' is a magnetic mirror machine which has a central cell, one anchor cell and one plug cell. The Hanbit device has been involved in a series of experiments on stabilization of the MHD flute type mode including stability experiments with a divertor. We have undertaken investigations to see if the Kinetic Stabilizer (KS) of R. F. Post 2-5 can stabilize the MHD instability. According to the theory, by locating a stabilizing plasma pressure on the field lines at a region with a strong second derivative and large radius in the expanding field region outside the mirrors, the main plasma in the mirror central cell in regions with unfavorable field line curvature can be stabilized. The Hanbit KS uses microwave produced plasmas on field lines in the cusp tank region. Two coils on the cusp tank are configured to produce expanding field lines with a large positive radius of curvature. A 5-kW 2.45 GHz magnetron is used to produce the stabilizing electron cyclotron resonant heated (ECRH) plasma pressure in this region. Details of the experimental arrangement and stabilizing plasma parameters were previously reported. 6 For normally terminating plasmas, a reduction in the instability duration has been observed and the range of density where the instability occurs has decreased. However, for higher density plasmas which disrupt due to an m=-1 instability, a prevalent m=+1 instability is removed while the duration of the m=-1 instability is increased.

If the ion Bernstein wave (IBW) heating power in an H mode discharge of the PBX-M experiment exceeds a threshold power of about 200 kW, a core transport barrier is created in the central region of the plasma. At lower neutral beam injection (NBI) powers, the core barrier is accompanied by an edge L mode. The high edge localized mode (ELM) repetition frequency (1 kHz) prevents the creation of a strong barrier, so the edge first has to make an H-to-L transition before a strong core transport barrier can be created. At higher NBI powers, the ELM repetition frequency is lowered to less than 200 Hz, which allows the immediate creation of a strong core barrier. Edge localized mode loss, which propagates radially first on a fast (non-diffusive) and then on a slow (diffusive) time-scale all the way to the plasma core, is strongly reduced in the core barrier region. Correlated with the reduced ELM loss, the fluctuations in the core barrier region are also strongly reduced, both during the ELM and during the quiet periods between the ELMs. There is strong evidence that the IBW induced poloidal flow shear is responsible for the stabilization of core turbulence and the creation of the core transport barrier. The large perpendicular E ? B flow shear component of the measured toroidal velocity in co-injection neutral beam heated discharges seems to be largely cancelled by the ion diamagnetic drift shear produced by large ion pressure gradients in the core barrier region. The value of IBW induced poloidal flow has not been experimentally determined, but its numerical value is found to be a factor of 4 larger than either the toroidal velocity or the ion diamagnetic drift shear components, leaving only IBW induced flow shear as the most probable cause for the turbulence stabilization. The core turbulence suppression and the creation of the core transport barrier is also consistent with expectations from a comparison between the E ? B flow shear rate and a rough estimate of the linear ion temperature gradient (ITG) growth rate. The presence of the core barrier region also strongly modifies the other MHD events: crashes on the q = 1.5, 2 surfaces and the disruption.

Two new types of high frequency (HF) MHD mode activity are observed when a core transport barrier is formed during ion Bernstein wave (IBW) heating in PBX-M. These modes, localized in the core transport barrier, seem to be mainly responsible for the soft saturation in the strength of the core transport barrier. The two types of HF mode activity are determined by the presence or absence of a strong 1/1 mode. The n = m modes are observed with the 1/1 mode, while the n = m - 1 modes are seen when the 1/1 mode is absent. The m numbers of both of these modes vary between 4 and 13. They all seem to be coupled and rotate as a rigid body. The n = m - 1 modes seemed to be narrowly localized on their corresponding rational surface, i.e. q = m/(m - 1). The modes are ballooning-like, and their amplitudes increase with increasing β. Calculations with the CASTOR code show that the n = m - 1 modes are most probably pressure driven tearing modes, but the possibility of kinetic ballooning modes (KBMs) cannot be fully excluded. The n = m modes appear on or around the q = 1 surface, and are localized at the X point of the 1/1 mode island. The frequency offset of the n = m modes, which is too low for a TAE, is shown to be consistent with β-induced toroidal Alfven eigenmodes (BAEs).

The Princeton Beta Experiment-Modification (PBX-M) was run both with elliptical and with bean-shaped plasmas during the 1992 and 1993 operating periods. Two deuterium-fed neutral beams were used for auxiliary heating, and during 1992 the average power was . This will be referred to as the lower neutral-beam power (LNBP) period. As many as four deuterium-fed neutral beams were used during 1993, and the average power was . This will be referred to as the medium neutral-beam power (MNBP) period. The neutron source strength, Sn, showed a scaling with injected power , for both the LMBP and MNBP periods. A much wider range of shaping parameters was studied during the MNBP as compared with the LNBP period. A weak positive dependence on bean shaping was observed for the LNBP, and a stronger positive dependence on shaping was observed for MNBP, viz . High values of Sn were obtained in bean-shaped plasmas for the highest values of at for the LNBP. For the MNBP the highest values of Sn and stored energy were obtained at , and the highest values of were obtained at . The achievement of high Sn is aided by high neutral-beam power, high toroidal field, strong shaping, high electron temperature, and broad profiles. The achievement of high is aided by low toroidal field, high density, less shaping, broad profiles, and access to the H-mode, viz . The achievement of high is aided by strong shaping, high density, broad profiles, and access to the H-mode, viz . Some comparisons with the previous higher neutral-beam (HNBP) period in 1989 are also made.

Recently GOL-3 has been reconfigured to a multimirror trap with improved confinement and high ion temperature. A dense plasma is created with a life time in the millisecond range. BTI neutron bubbl...

Continuing the experiments reported previously, 1 , 2 additional microwave power has been applied to the plug region of Hanbit in order to increase the stored energy and beta of the hot-electron plasma created there. Two new 1.5-kW VA-806 klystrons at 7.67 GHz and 7.87 GHz have been used in conjunction with the existing 2-kW CPI klystron at 14 GHz. The plasma is created in order to provide a high-beta ring to stabilize the Hanbit central cell plasma against ballooning instabilities. An array of Hall probes mounted on the outside of the Hanbit plug cavity was installed to measure the axial profile of the Bz fields. The total stored energy was measured by diamagnetic loops and the radial location of the plasma was determined by a Si-PIN diode detector measuring the energetic electron end loss. All three measurements were to be used to determine the radial and axial location of the plasma, the plasma volume, the stored energy, and hence the plasma beta. However, the Bz signal was too small to measure and the diamagnetic signal was smaller than previously found. The ring was found to be very wide and not adequate to stabilize the central cell plasma.

The three dimensional (3-D) nature of antennas for fusion applications in the ion cyclotron range of frequencies (ICRF) requires accurate modelling to design and analyse new antennas. In this article, analysis and design tools for radiofrequency (RF) antennas are successfully benchmarked with experiment, and the 3-D physics of the launched waves is explored. The systematic analysis combines measured density profiles from a reflectometer system, transmission line circuit modelling, detailed 3-D magnetostatics modelling and a new 3-D electromagnetic antenna model including plasma. This analysis gives very good agreement with measured loading data from the Tokamak Fusion Test Reactor (TFTR) Bay-M antenna, thus demonstrating the validity of the analysis for the design of new RF antennas. The 3-D modelling is contrasted with 2-D models, and significant deficiencies are found in the latter. The 2-D models are in error by as much as a factor of 2 in real and reactive loading, even after they are corrected for the most obvious 3-D effects. Three dimensional effects play the most significant role at low parallel wavenumbers, where the launched power spectrum can be quite different from the predictions of 2-D models. Three dimensional effects should not be ignored for many RF designs, especially those intended for fast wave current drive.

The confinement characteristics and behaviour of energetic ions injected during tangential neutral beam injection (NBI) experiments on the Advanced Toroidal Facility (ATF) are examined using several experimental and computational methods. Measurements by a two dimensional scanning neutral particle analyser of the fast ion, slowing down spectra of the injected ions have been used to examine the characteristics of the fast ion distribution in low density plasmas (ne ≤ 1.0 × 1019 m-3). The energetic ions are found to behave in a manner consistent with the classical slowing down process embodied in the fast ion Fokker-Planck equation. Neutron measurements have been used to extend these results to higher density plasmas (ne ≈ 7.5 × 1019 m-3). Because of experimental uncertainties, it is difficult to determine whether the measured neutron rates are consistent with classical predictions; however, there are indications in some ATF magnetic configurations that the measured neutron source rates are much lower than expected, indicating an enhanced orbit loss of fast ions in these configurations. In addition, computational studies suggest that the effectiveness of tangential NBI in certain operating regimes in ATF may be limited owing to the presence of loss regions at the trapping boundary

A 4667-second long discharge has been obtained in the ATF torsatron. The total amount of the energy input was 330 MJ. The set-up procedures, the control of plasma parameters, and the effect on the wall conditioning for the long pulse discharge are reported in this paper. Fast density feedback control is not useful for a long-pulse operation; control of the neutral pressure on a slower time scale is necessary. Long-pulse discharges are quite effective for wall conditioning. Valuable experiences have been accumulated in the future experiments planned for LHD. 10 refs., 4 figs.

Thirty one real-time boronizations were applied to PBX-M using the plasma erosion of solid target probes. More than 17 g of boron were deposited in PBX-M using this technique. The probes were positioned at the edge plasma to optimize vaporization and minimize spallation. Auger depth profile analysis of poloidal and toroidal deposition sample coupon arrays indicate that boron was transported by the plasma around the torus and deep into the divertors. During discharges with continuous real-time boronization, low- Z and high- Z impurities decreased rapidly as plasma surfaces were covered during the first 20–30 discharges. After boronization, a short-term improvement in plasma conditions persisted prior to significant boron erosion from plasma surfaces, and a longer term, but less significant improvement persisted as boron farther from the edge continued gettering. Real-time solid target boronization has been found to be very effective for accelerating conditioning to new regimes and maintaining high performance plasma conditions.

Density fluctuations in low-collisionality, low-beta ({beta} nearly equals 0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron have been studied using a 2-mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher {kappa}{sub PRP}{rho}{sub s} (nearly 1) than is typically seen in tokamaks. The in-out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii {rho} from 0.5 to 0.85) increases from n/n >= 1% when the density profile flat to n/n >= 3% when the density profile is steepened. These observations are in qualitative agreement with theoretical expectations for helical dissipative trapped electron modes (DTEMs), which are drift-wave instabilities associated with particle trapping in the helical stellaratormore » field. 5 refs., 5 figs.« less

Application of Ion Bernstein Wave Heating (IBWH) into the Princeton Beta Experiment‐Modification (PBX‐M) [Phys. Fluids B 2, 1271 (1990)] tokamak stabilizes sawtooth oscillations and generates peaked density profiles. A transport barrier, spatially correlated with the IBWH power deposition profile, is observed in the core of IBWH‐assisted neutral beam injection (NBI) discharges. A precursor to the fully developed barrier is seen in the soft x‐ray data during edge localized mode (ELM) activity. Sustained IBWH operation is conducive to a regime where the barrier supports large ∇ne, ∇Te, ∇νφ, and ∇Ti, delimiting the confinement zone. This regime is reminiscent of the H(high) mode, but with a confinement zone moved inward. The core region has better than H‐mode confinement while the peripheral region is L(low)‐mode‐like. The peaked profile enhances NBI core deposition and increases nuclear reactivity. An increase in central Ti results from χi reduction (compared to the H mode) and better beam penetration. Boot...

Density fluctuations in low‐collisionality, low‐beta (β∼0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] have been studied using a 2 mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher k⊥ρs (∼1) than is typically seen in tokamaks. The in–out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii ρ from 0.5 to 0...

A new type of reflectometer not previously used on tokamaks has been installed on TFTR to measure edge electron density profiles. This reflectometer gives unambiguous edge profiles despite large edge density fluctuations and the presence of strong auxiliary heating. The effects of the large density fluctuations in the edge gradient region are overcome by reducing the multiplicity of fringes and eliminating phase excursions produced by scattering from density fluctuations at the reflecting layer. This is accomplished by using differential-reflectometry: the difference phase between two probing signals reflecting from cut-off layers separated by a distance much less than the correlation length of the density fluctuations is used. This system probes the TFTR plasma using the extraordinary mode (X-mode) with two signals swept from 90-118 GHz while maintaining a fixed difference frequency of 125 MHz between these signals. It has been used to obtain density profiles in the range of 1*1011 to 3*1013 cm-3 in high-field (4.5-4.9 T) full size (R0=2.62 m, a=0.96 m) TFTR plasmas. The reflectometer launcher is located in an ion cyclotron range of frequencies (ICRF) antenna and views the plasma through a small penetration in the centre of the Faraday shield. Initial measurements demonstrated that this technique is an effective way to measure the electron density profile in the edge gradient region.

The Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] has completed experiments focusing on microwave scattering measurements of density fluctuations and transport studies utilizing the modulation of dimensionless parameters. Microwave scattering measurements of electron density fluctuations in the core of low‐collisionality electron cyclotron heated (ECH) plasmas show features that might be evidence of trapped electron instabilities. Starting from gyro‐Bohm scaling, the additional dependence of confinement on the dimensionless parameters ν* and β (collisionality and beta) has been investigated by modulating each of these parameters separately, revealing the additional favorable dependence, τE∝τgBν*−0.18β+0.3.

Effects of deliberately introduced magnetic field perturbations on plasma conditions have been studied in the Advanced Toroidal Facility (ATF) torsatron. Magnetic dipole coils, located above the plasma at two positions 180 degrees apart, created a perturbation Bz0/B0 ≤ 1.4%, which produced a large l=1/2 island. Two modes of plasma operation were employed: (1) constant gas feed allowing the electron density to vary and (2) constant density with the gas feedback controlled. Without density control, the density and the stored energy were approximately halved. With density control, the stored energy decreased only slightly and the potential increased in the vicinity of the island. It was found that both with and without density control, the pressure profile narrowed and the particle confinement was lowered substantially when the perturbations were applied. Thus, field perturbations, provide an external control on the pressure profile and the particle confinement time

Positive limiter biasing on the currentless Advanced Toroidal Facility (ATF) torsatron produces a significant increase in the particle confinement with no improvement in the energy confinement. Experiments have been carried out in 1-T plasmas with {approximately}400 kill of electron cyclotron heating ECM. Two rail limiters located at the last closed flux surface (LCFS), one at the top and one at the bottom of the device, are biased at positive and negative potentials with respect to the vessel. When the limiters are positively biased at up to 300 V, the density increases sharply to the ECH cutoff value. At the same time, the H{sub {alpha}} radiation drops, indicating that the particle confinement improves. When the density is kept constant, the H{sub {alpha}} radiation is further reduced and there is almost no change in the plasma stored energy. Under these conditions, the density profile becomes peaked and the electric field becomes outward-pointing outside the LCFS and more negative inside the LCFS. In contrast, negative biasing yields some reduction of the density and stored energy at constant gas feed, and the plasma potential profile remains the same. Biasing has almost no effect on the intrinsic impurity levels in the plasma.

A two-frequency correlation reflectometer has been operated on the Advanced Toroidal Facility (ATF) to measure plasma electron density fluctuations. This reflectometer uses quadrature phase detection to permit true phase measurement of the reflected microwave signal (probing beam). By measuring the phase fluctuations in the reflected probing beam, the amplitude of the density fluctuations can be estimated. Simultaneous two-frequency operation makes it possible to measure the coherence between fluctuations at two radially separated cut-off layers, from which the radial correlation lengths and wavenumbers can be estimated. This reflectometer has been used to study the density fluctuations in the edge gradient region of low density ATF plasmas produced by electron cyclotron heating. These studies have revealed globally coherent turbulence with a radial correlation length of up to approximately 5 cm, a radial wavenumber kr ≈ 0 cm-1 and a poloidal wavenumber kθ ≈ 1 cm-1. The rms amplitude of the fluctuations reaches a maximum of ≈ 5% at the plasma edge (ρ = 1, where ρ is the flux surface normalized radius) and decreases with decreasing radius to a level of 1%. Simultaneous measurements of the fluctuations with the reflectometer, the heavy ion beam probe and the fast reciprocating Langmuir probe provide consistent results. A comparison of the measurements with simplistic mixing length estimates, ne/ne = 1/kθLn or ne/ne = 1/krLn, shows that these estimates are too high by factors of two to more than 100, while a comparison with a more detailed estimate for the pressure gradient driven resistive interchange turbulence yields reasonable agreement with the experimentally measured fluctuation characteristics

Fast wave heating experiments in the ion cyclotron range of frequencies (ICRF) were performed on target plasmas produced by 350 kW of electron cyclotron heating at 53 GHz and also by neutral beam injection in the Advanced Toroidal Facility (ATF). Various heating regimes were investigated in the frequency range between 9.2 MHz and 28.8 MHz with magnetic fields of 0.95 T and 1.9 T on axis. The nominal pulse lengths of up to 200 kW RF power were in the range between 100 and 400 ms. Data from spectroscopy, loading measurements, and edge RF and Langmuir probes were used to characterize the RF induced effects on the ATF plasma. In the hydrogen minority regime at low plasma density, large suprathermal ion tails were observed with a neutral particle analyser. At high density (ne ≥ 5.0 × 1013 cm-3) substantial increases in antenna loading were observed, but ICRF power was insufficient to produce definitive heating results. A two-dimensional RF heating code, ORION, and a Fokker-Planck code, RFTRANS, were used to simulate these experiments. A simulation of future high power, higher density experiments in ATF indicates improved bulk heating results due to the improved loading and more efficient thermalization of the minority tail

Thermal plasma collapses have been observed in the Advanced Toroidal Facility (ATF) and in some cases these collapses limit the plasma performance. A 15-channel bolometer array has been used to measure the radiated power profile. It has been found that thermal collapses, defined by a drop in the plasma stored energy, occur when the local radiated power exceeds the local power input to the electrons. The local radiated power at the collapse first increases at the edge and then begins to propagate inward, leading to a peaking of the radiated power profile. This finding indicates that additional central heating would be effective in preventing the thermal collapse

Recent experiments in the Advanced Toroidal Facility (ATF) [Fusion Technol. 10, 179 (1986)] have been directed toward investigations of the basic physics mechanisms that control confinement in this device. Measurements of the density fluctuations throughout the plasma volume have provided indications for the existence of theoretically predicted dissipative trapped electron and resistive interchange instabilities. These identifications are supported by results of dynamic configuration scans of the magnetic fields during which the magnetic well volume, shear, and fraction of confined trapped particles are changed continuously. The influence of magnetic islands on the global confinement has been studied by deliberately applying error fields which strongly perturb the nested flux‐surface geometry, and the effects of electric fields have been investigated by means of biased limiter experiments.

Runaway electron formation and confinement occur readily in pulsed torsatrons and heliotrons because of the high loop voltages during initiation and termination of the helical and vertical fields (‘‘field ramping’’) and the inherently good containment of the electrons on the flux surfaces in the vacuum fields. This has been confirmed for the Advanced Toroidal Facility (AFT) [Fusion Technol. 10, 179 (1986)] and other stellarators by orbit calculations. Since runaway electrons can cause an unacceptable level of hard x rays near the machine, a runaway electron suppression system was incorporated in ATF. The main component of the system is a movable paddle, which is normally left in the center of the plasma chamber during the field ramps. This device, in conjunction with programmed vertical field ramping, which reduces the volume of good flux surfaces, has proved to be very effective in reducing the runaway electron population. Measurements of hard x rays from ATF have shown that the runaway electrons are pro...

Measurements of poloidal magnetic field fluctuations outside currentless, finite-beta (β ≤ 0.5%, β0 ≤ 3%) plasmas with peaked pressure profiles in the Advanced Toroidal Facility (ATF) torsatron reveal bands of small (θ/B ~ 10−5), coherent fluctuations in the frequency range 5-60 kHz. The geometrical structure of these fluctuations shows n = 1 toroidal mode symmetry, with inferred poloidal mode numbers of m = 2 and 3. The coherent Bθ amplitudes increase with B for B 0.3%, and then saturate and begin to decrease with B for B 0.3%.

The toroidal current observed during electron cyclotron heating in the Advanced Toroidal Facility torsatron is identified as bootstrap current. The observed currents, ranging between +4 and {minus}2 kA, agree well with predictions of neoclassical theory in magnitude and parametric dependence, as determined by systematic scans of quadrupole (shaping) and dipole (magnetic axis shift) moments of the poloidal magnetic field. It has been shown that the bootstrap current in a stellarator can be externally controlled, and zero-current operation can be achieved.

The characteristics of plasmas in the Advanced Toroidal Facility (ATF) have proven to be strongly dependent on the type of wall conditioning employed. A succession of techniques, beginning with glow discharge cleaning and baking, and evolving to gettering with chromium and titanium, have led to progressive improvement of the plasma parameters. Gettering with titanium has reduced the low-Z impurity content by a factor of 3, lowered the radiated power by a factor of 2.5–3.5, and improved the control over the electron density. The maximum values achieved for stored energy, line averaged density and confinement times are 28 kJ, 1.2 × 1014cm−3 and 25 ms, respectively. These parameters are comparable to the best results achieved in the ISX-B tokamak which had the same average minor radius and one half the major radius of ATF. Quasi-steady operation for 200 ms of neutral beam injection (NBI) has been obtained in high density, titanium gettered plasmas without the collapses that were typical earlier periods of operation. Neon injection experiments have helped to delineate the limits on the global levels of radiation that can be maintained and have supported the conclusion that mechanisms other than radiative losses are important for initiating the collapses still observed in low density NBI plasmas.

An overview of recent AFT (Advanced Toroidal Facility) experimental results and program plans is presented, with emphasis on the role of magnetic configuration controls in transport studies. The ATF operating space is bounded by a density limit that effectively sets a limit on the energy confinement time tau /sub E/. Although this limit is not solely due to impurities, it has recently been raised by improved cleanliness following titanium gettering. This has led to collapse-free neutral beam injection discharges with global tau /sub E/ approximately=16 ms. Preliminary experiments show that stored energy and bootstrap current are sensitive to details of the magnetic configuration. >

Plasmas in the Advanced Toroidal Facility (ATF), an l = 2 torsatron with 12 field periods, are produced by 200–400 kW of electron cyclotron heating (ECH) and up to 1.5 MW of neutral-beam injection (NBI). The characteristics of the plasmas are sensitive to the type of wall conditioning employed. A progression of techniques, beginning with glow discharge cleaning and baking and evolving to gettering with chromium and titanium, has significantly reduced the low-Z impurity content, lowered the fraction of radiated power, and permitted improved control over the electron density. As a result, plasma-parameters and machine performance have been enhanced significantly. The maximum values achieved for stored energy, line-averaged density, and confinement times are 28 kJ, 1.2 × 1020m−3, and 25 ms. These parameters are comparable to those obtained in the ISX-B tokamak. In addition, with titanium gettering, quasisteady operation for 200 ms of neutral beam injection has been obtained without the collapses that characterized earlier periods of operation.

A workshop on 1–3 August 1989 reviewed the techniques, uncertainties, and experiences of neutron calibration on PLT, TFTR, JET, Tore Supra, JT‐60, JIPPT‐IIU, Alcator C‐Mod, ATF, FT, ASDEX, Textor, and DIII‐D. In the summary session, the workshop participants discussed possible consensus neutron calibration techniques appropriate to D‐D plasmas in tokamaks. The application of such techniques would facilitate a more accurate comparison of neutron yields from different devices, and also allow new calibration techniques to relate their precision to a reference value. General agreement was reached on the suitability of two techniques: (1) a 252Cf source calibration of epithermal neutron detectors, and (2) threshold neutron activation of Ni foils placed vertically above or below the plasma. This paper will present details on detector positioning, neutron transport calculations, and interlab normalization needed to accomplish the standardized calibration using a Cf neutron source.

Experiments involving plasma improvement, confinement scaling, bootstrap currents, and edge fluctuations have been carried out in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Average densities ne≤9×1019 m−3 have been obtained, with global energy confinement times τ*E≤20 msec. Confinement times generally follow the stellarator/torsatron empirical scaling law, τSL =0.17×P−0.58n0.69eB0.84a2R0.75 (with τSL in seconds, power P in megawatts, density ne in 1020 m−3, and plasma radius a and major radius R in meters). Gas injection during neutral beam injection (NBI) causes increases in ne, so that τ*E does not decrease during NBI. Edge plasma fluctuations are found to exhibit a mode change near the peak of the energy confinement time. Plasma currents observed during electron cyclotron heating have been identified as bootstrap currents.

Access to the magnetohydrodynamic (MHD) second stability regime has been achieved in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Operation with a field error that reduced the plasma radius and edge rotational transform resulted in peaked pressure profiles and increased Shafranov shift that lowered the theoretical transition to ideal MHD second stability to β0≊1.3%; the experimental β values (β0≤3%) are well above this transition. The measured magnetic fluctuations decrease with increasing β, and the pressure profile broadens, consistent with the theoretical expectations for self‐stabilization of resistive interchange modes. Initial results from experiments with the field error removed show that the pressure profile is now broader. These later discharges are characterized by a transition to improved (×2–3) confinement and a marked change in the edge density fluctuation spectrum, but the causal relationship of these changes is not yet clear.

The Advanced Toroidal Facility is a large torsatron device with a major radius R0 = 2.1 m, an average plasma minor radius a ≈ 0.3 m, and a magnetic field B0 ≤ 2T. The sheared magnetic configuration...

Edge electron density profile measurements, including the scrape‐off layer, have been made during ion cyclotron range of frequency (ICRF) heating with the two‐frequency differential‐phase reflectometer installed on an ICRF antenna on the Tokamak fusion test reactor (TFTR). This system probes the plasma using the extraordinary mode with two signals swept from 90 to 118 GHz, while maintaining a fixed‐difference frequency of 125 MHz. The extraordinary mode is used to obtain density profiles in the range of 1×1011–3×1013 cm−3 in high‐field (4.5–4.9 T) full‐size (R0=2.62 m, a=0.96 m) TFTR plasmas. The reflectometer launcher is located in an ICRF antenna and views the plasma through a small penetration in the center of the Faraday shield. A 26‐m‐long overmoded waveguide run connects the launcher to the reflectometer microwave electronics. Profile measurements made with this reflectometer system will be presented along with a discussion of the characteristics of this differential phase reflectometer and data ana...

Summary form only given, as follows. Initial attempts have been made at Ohmic (Joule) heating, stabilization, and preionization in Hanbit using an electron-emitting hot cathode in conjunction with a reflex discharge. The hot cathode was constructed using a set of LaB/sub 6/ discs heated internally with current through carbon rods. The cathode was mounted in the cusp section of Hanbit on magnetic field lines connected to the central cell (CC). The reflex discharge was initiated between the cathode and a CC biased limiter. This provided Joule heating of the electrons in the plasma Normally, ion cyclotron resonant heating (ICRH) produces the plasma. In the CC with a slot antenna supplied with power from a 500-kW transmitter at 3.5 MHz. In addition, the reflex discharge could be initiated prior to the ICRH discharge in order to provide a target plasma for the ICRH power. The ICRF power was thus not needed to breakdown the plasma and could be used solely for ion heating. Further experiments have been performed with the heated cathode but, without a reflex discharge to attempt line-tying stabilization of the CC plasma with all electron emitting end wall. The preliminary results for these three experiments will be presented. In addition, the electron-emitting cathode could be used to remove gas on the interior surfaces of the CC by electron impact desorption (EID) when the magnetic fields are arranged to cause the electrons to strike the CC walls arid internal structures. Preliminary results from this experiment will also be presented.

Summary form only given, as follows. Additional microwave power has been installed at Hanbit in an attempt to increase the beta of the hot-electron ring plasma created in the plug section. Two VA-806 series klystrons with a frequency of /spl sim/8 kW have been installed to operate in conjunction with an existing 2-kW 14-GHz klystron already, used in previous experiments. A hot-electron ring is created in the plug in order to provide a high-beta ring to stabilize the Hanbit central cell plasma against ballooning instabilities. Previous experiments had shown that a hot-electron ring plasma had been created with the 14-GHz klystron. The hot-electron ring was in a favorable location to stabilize the CC plasmas but the plasma beta was too low and could not provide stability. The present experiments were designed to increase the beta with additional microwave power. In addition, diagnostics were installed to determine the length of the plasma New diagnostic techniques and measurements will be presented.

In stellarators, external coil currents rather than internal currents are used to produce the confinement fields. This allows for greater flexibility in the design and operation of a stellarator. This flexibility can be exploited in a series of experiments on one device, such as the ATF. MHD stability boundaries will be tested by varying the vertical field. Access to the second stability regime has been observed in initial experiments on ATF. In this regime, the relationship between stabilization of pressure-driven modes and transport improvement can be studied. The density profile peaking factor can be varied using pellet injection in order to understand the role of eta /sub j/ modes. As an example, the ATF-TEXT collaboration on edge fluctuations will compare the levels of resistive-gradient-driven fluctuations observed in the two devices. In addition, global confinement and local transport comparisons have been made to tokamaks and to other stellarators of roughly similar size and capabilities but with widely varying configuration properties. >

The PBX-M passive plate stabilization system provides significant stabilization of long-wavelength external kink modes, the slowing of vertical instability growth rates, and the amelioration of disruption characteristics. The passive plate stabilization system has allowed the use of LHCD and IBW to induce current density and pressure profile modifications, and m=1 divertor biasing for modifying edge plasma transport. Improvements in the passive plate system insulators and support structures have provided reliable operation. Impurity influxes with the close-fitting passive plates are low. Solid target boronization is applied routinely to reduce conditioning time and maintain clean conditions.