Table of contents

The L-H transition in the ASDEX tokamak was investigated. The level of fluctuations both in the scrape-off layer and in a narrow region inside the separatrix drops on a 100 mu s timescale. Density turbulence coming from the edge and magnetic turbulence, as measured with magnetic probes, show a similar behavior. Before and during the transition no frequency shift in the fluctuation spectra could be detected which would indicate the rapid appearance of a radial E-field in the region of the H-mode barrier. Within milliseconds after the L-H transition new fluctuations propagating in the electron diamagnetic direction are destabilized which however exhibit no marked influence on transport across the barrier. In the bulk of the plasma fluctuation spectra show a frequency shift which is dominated by toroidal plasma rotation due to neutral beam injection. The initial transport barrier is deduced to lie inside the separatrix with a width of <or=3 cm.

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 aim is to summarize the major results. 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.

An account is given of the high performance plasmas established by development of the H-mode regime in JET in the experimental campaigns up to 1992. High performance in this case is measured in terms of the confinement enhancement achieved over the L-mode scaling as measured using the plasma diamagnetism. Three JET H-mode regimes have achieved enhancement factors (H_G^DIA) over Goldston L-mode scaling of 2.5<H_G^DIA<4.0. These are the Pellet Enhanced Performance (PEP) H-mode, the high bootstrap fraction (high beta _POL) H-mode and the Hot Ion (HI) H-mode. The phenomenology of these three regimes is reviewed and contrasts and common threads are elucidated. All the regimes are transient (on a resistive diffusion timescale) and the mechanisms terminating the period of enhanced confinement are discussed. Common features between the high beta _POL and HI regimes are identified which suggest that they are effectively two aspects of a unified Very High (VH) confinement regime. The experimental conditions for accessing this regime are reviewed.

The recent experimental results on H-mode in JT-60U and JFT-2M are reported. The enhanced confinement mode with the edge transport barrier ( H-mode) in JT-60U is characterized by the high edge ion temperature. The confinement improvement is well correlated with the edge ion temperature. The H-mode at low magnetic field ( approximately 2 T) in JT-60U has similar features to the H-mode in JFT-2M: rapid reduction of D_alpha emission. However, in the H-mode at high magnetic field ( approximately 4 T) and at low density, no clear transition is observed in JT-60U, although the confinement is still improved with the edge transport barrier. Collisionality and the width of the transport barrier are also investigated in both tokamaks, and are compared with Shaing's theory (1989). Improvement of the high-pp mode discharge with the H-transition was demonstrated in JT-60U. Owing to this improvement, the record value of the fusion triple product (1.1*10²¹ m^-3 s keV) has been achieved. Quasi-steady H-mode operation with H-factor of approximately 1.8 has been also demonstrated in JT-60U by controlling the repetition rate of edge localized modes (ELMs). In JFT-2M, ELM activity has been successfully induced by a new and active method using helical magnetic field perturbation, and as a result, a steady H-mode has been achieved.

The PBX-M tokamak has an array of unique features that allow it to study regimes of enhanced confinement and stability. In particular, because of its divertor operation, PBX-M has proved to be a valuable contributor to identifying and understanding various features of H-mode physics. To be reported here are recent results of H-mode studies of m=1 bias induced H-modes, L- to H-transitions, including suppression of turbulent transport, ELMs and other related energy loss mechanisms, and Ion Bernstein Wave modification of H-mode discharges.

In W7-AS the H-mode has been observed for the first time in a currentless stellarator plasma. H-modes are achieved with 0.4 MW Electron Cyclotron Resonance Heating with 140 GHz at 2.5 T and high density, with 70 GHz at 1.25 T and lower density and with neutral beam injection. The H-phases display all characteristics known from tokamak H-modes including the development of an edge transport barrier, an increase of the poloidal impurity flow velocity at the edge, the reduction of edge turbulence and ELMs. The power threshold for the H-mode seems to be lower than that in tokamaks and is in agreement with an n_eB_T scaling. Major differences to the divertor H-mode is the small increase in energy content of maximally 30%, the lack of a strong isotope effect both in threshold and in H-mode characteristics and a peculiarly narrow operational range in iota.

It is shown that the bifurcation dynamics in the H-mode theory based on the ion orbit loss and nonlinear viscosity is not sensitive to the radial pressure and temperature gradients after bifurcation. This allows the poloidal flow velocity to be opposite to that of the poloidal E*B velocity in the H-mode phase if the pressure gradient is steep. Here, E(B) is the electric (magnetic) field. The theory is also extended to stellarator configurations to demonstrate that the tokamak H-mode theory can be tested in a controlled manner in those devices. A set of poloidal and toroidal momentum evolution equations is derived in which the radial current density outside the last closed flux surface is expressed in terms of the radial integral of the poloidal current density into the divertor plates. This facilitates the calculation of the radial electric field profile across the separatrix.

Rapid changes of the main ion energy distribution at transitions from L-to-H, H-to-L and during ELMs are studied by time of flight neutral measurements in the JFT-2M tokamak. At the L to H transition, 200-400 mu s prior to the start of H_alpha drop, an increase of the high energy neutral flux above an energy of 200 eV is observed. An energy of higher than 200 eV for hydrogen corresponds to the collisionless condition v*_i<l just inside the separatrix. The change of the main ion energy distribution after sawtooth crash during an L-mode phase is also studied. The change of the ion energy distribution just after sawtooth crash is the same as that at L/H-transition. The floating potential measured in the SOL also shows the rapid jump to more positive just after the sawtooth crash (at the same time as the change of an ion energy distribution). This shows the increase of ion outflux in the SOL and might correspond to the change in the ion energy distribution. This may be the reason why most H-modes are triggered by a sawtooth. These observations support theories predicting an important role for ion loss for the L/H-transition.

Various features of H-mode observed in the JT-60U tokamak have been intensively investigated with respect to the effective edge ion collisionality v_i*_eff, which takes the impurity contribution into consideration. Cumulated data exhibits v_i*_eff at the transition does vary, depending upon target density, magnetic field, equilibrium profile and the heating beam power. Scatters in the v_i*_eff have been categorized into three regions: low field, hot-ion mode and high- beta p mode regime. In all the regimes reduced turbulent density fluctuation was observed across the transition, which is a result of the decrease of coherence in the edge density fluctuation and increase in the radial shearing of the poloidal flow velocity. Reflectometric technique and fast-sample magnetic probes were used for the turbulence diagnostics in this work. It was also documented that turbulent density fluctuation inversely scales with shear in the poloidal flow velocity in the low-field H-mode. Particularly in high- beta p H-mode discharge, the poloidal rotation is in the ion diamagnetic direction, and enhanced magnetic turbulence was observed, which may invoke the theoretical considerations regarding the role of magnetic turbulence as a cause of the H-mode transition.

A self-consistent model of the L to H transition is derived from coupled nonlinear envelope equations for the fluctuation level, radial electric field shear E_r', and pressure gradient Del P_i. These equations exhibit a bifurcation between dual L-mode and H-mode fixed points. They show that the L to H transition has similar characteristics to a phase transition of the second kind for which the order parameter is E_r'. The transition occurs when the turbulence drive is large enough to overcome the damping of the E*B flow. This leads to a power threshold condition for the transition P_crit approximately=( mu nT)_edgeaRL_s.

The H-mode obtained in ASDEX Upgrade by three heating methods (Ohmic, NBI, ICRF) is analysed. The power threshold is shown to be relatively low compared with other tokamaks and to be independent of the heating method. The operational window for the different types of H-phases, (dithering, ELMing with type-III and I ELMs, ELM-free) is given and discussed. The features of the H-mode discharges, in particular quasi-stationary ELMing phases with type I ELMs, are described and discussed. Finally, it is shown that the power flux through the plasma edge is a key parameter for H-mode operation and control.

The H-mode in CCT can be obtained in a wide range of densities due to advanced titanium hydride based recycling control. In this paper we study the radial particle barrier associated by the rotational E layer. The m=1 and the n=0 toroidal mode is used for the analysis. Mach probe (a two sided Langmuir probe) and poloidal rake probe data along with reflectometer data have been used to define the flow properties of the layer. The experimental results (including recent lower hybrid heating experiments) indicate that a nearly absolute particle barrier is achieved. The low frequency fluctuations found in ohmic discharges are eliminated in H-mode discharges due to "rotational averaging" in the interior and "scraping/shaving" of the fluid boundary at the separatrix.

The COMPASS-D tokamak has recently obtained H-mode plasmas in a single-null X-point configuration with no additional heating. Clear H-mode signatures have been observed including reduction in D_alpha light and the unmistakable appearance of ELMs and ELM-free periods. Recycling and gas puff can play crucial roles in achieving and terminating the H-modes. The emergence of distinct ELMs, from the apparently-random fluctuations, of the background D_alpha light, is usually one of progression, taking some 10s of ms rather than a sudden discontinuity. The relatively slow evolution allows for detail examination of important phenomena, such as improvements to confinement and fluid rotation. The H-mode quality improves with density, unless impurity accumulation becomes important. Improvements to the energy and particle confinement are observed and are accompanied by positive increases in the radial electric field at approximately=4 cm inside the last closed flux surface (LCFS). The ohmic powers in discharges with clearly defined H-modes have not been less, so far, than the power obtained from power threshold scalings from larger machines.

Characteristics of the H-mode transition observed in CHS are described. The transition is achieved with modification of the rotational transform profile by inducing a small ohmic heating current. The transition is often initiated by a sawtooth crash caused by a coherent mode with the poloidal mode number m=2 and toroidal one n=1.

By modification of current and heat deposition profiles, the integrated core plasma performance was improved in ELMy H-mode. In a quasi steady state, beta _p approximately 2.5-3, beta _N approximately 2.5-3.1 and H-factor approximately 1.8-2.1 were sustained simultaneously under the full current drive condition (bootstrap current 55%, beam driven current 45% at I_p=0.5MA). The achievable beta _p value and the energy confinement performance increases systematically with safety factor at the edge q_eff and a high q_eff operation (q_eff>6) is required to obtain favorable integrated performance. To obtain these improved high beta ELMy plasmas, it is essential to control MHD activities; suppression of beta _p-collapse and control of ELM activity.

The dynamic behaviour of the L-H transition in ASDEX Upgrade is described. We focus on two dynamic phenomena: Dithering Cycles and ELMs. The Dithering Cycles are characterized experimentally; then, a theoretical model is presented which explains the cycle as a limit cycle oscillation at the transition. With regard to ELMs, the modulation of heat and particle fluxes into the divertor due to ELMs is studied, quantitative results from thermography of the target plates are given.

Edge Localized Modes have been studied in PBX-M discharges with high data sampling rate magnetic diagnostics with good spatial coverage. High frequency (>or=250 kHz) activity appears prior to the increase in the D_alpha emission. While this high frequency activity is global, with high coherence for all coils both on the inside and outside midplane, there is no identifiable mode structure. A very intense outward ballooning feature appears simultaneously with the increase in the D_alpha emission, and lasts for about 5 msec. The toroidal mode structure of this feature is clearly identifiable, consisting of a series of toroidal harmonics from n=3 to 8, at frequencies given by f_n=f₀+n Delta f, where Delta f=20-30 kHz and f₀=30-50 kHz. This feature may be caused by the kinetic ballooning mode.

Edge density fluctuations have been observed with microwave scattering in limiter H-mode plasmas on TFTR. The primary feature of these fluctuations is a strong average frequency shift in the electron diamagnetic drift direction corresponding to poloidal velocities of 1-3*10⁶ cm/s and an inward radial electric field of 60 kV/m. The rotation begins to increase at the H alpha drop and rises to a steady state in 5 to 50 ms. well before ELMs begin. ELMs modulate the rotation feature, eliminating it for the duration of the ELM. After the ELM, the rotation feature resumes its pre-ELM value in about 0.1 ms. Short duration (0.01 ms) precursor bursts propagating in the ion diamagnetic direction are observed to precede the ELM event by approximately 0.5 ms.

We present the global analysis of a recent survey of the H-mode power threshold in DIII-D using D degrees to D⁺ NBI after boronization of the vacuum vessel. Single parameter scans of B_T, I_p, density, and plasma shape have been carried out on the DIII-D tokamak for neutral beam heated single-null and double-null diverted plasmas. In single-null discharges, the power threshold is found to increase approximately linearly with B_T and n_e but remains independent of I_p. In double-null discharges, the power threshold is found to be approximately independent of both B_T and n_e. Various shape parameters such as plasma-wall gaps had only a weak effect on the power threshold. Imbalancing the double null configuration resulted in a large increase in the threshold power.

Supershot to limiter H-mode transitions have been studied on TFTR. In several cases an edge poloidal rotation was observed prior to the initial D_alpha drop at the transition. This spin-up is consistent with formation of a radial electric field, -E_r. The Del _pI contribution to E_r is approximately 30 kV/m and the density fluctuation level is reduced by a factor of 1 to 1.5 over a 5 cm region at the plasma edge across the transition.

In order to study the role of the radial electric field in the tokamak transport, a one-dimensional transport code which describes the toroidal and poloidal plasma rotation as well as the radial electric field has been developed. The neoclassical transport, turbulent transport and ion orbit loss are included. Using a simple turbulent transport model with a constant diffusion coefficient and a fixed temperature profile, the density profile in a steady state and the transient behavior during the co and counter neutral beam injection are studied. More consistent analysis has been initiated with a turbulent transport model based on the current diffusive high-n ballooning mode. The enhancement of the radial electric field due to ion orbit losses and the reduction of the transport due to the poloidal rotation shear are demonstrated. The preliminary calculation indicates a sensitive temperature dependence of the density profile.

New features of the energy confinement were observed in the Ohmic regime with reduced transport - Ohmic H-mode. Parametric dependencies as well as absolute values of the energy confinement time are in agreement with scaling proposed for description of the ELM-free H mode in bigger devices with powerful auxiliary heating ("DIII/JET H-mode" scaling). In particular, strong dependencies of tau _E on plasma current and input power and weak dependencies on density were found. Energy confinement time is enhanced by a factor of 15 compared to predictions of the usual ohmic scalings ("Neo-Alcator", "Merezhkin-Mukhovatov"). In the small tokamak TUMAN-3 30ms energy confinement time was achieved.

A collaborative program has been initiated on DIII-D to study helium (He) transport and exhaust in H-mode plasmas. To simulate the presence of He ash in DIII-D, a 50 ms He puff is injected into a DIII-D plasma, resulting in a He concentration of approximately 15%. The time dependence of the He density profiles in the plasma core is measured with charge-exchange recombination spectroscopy and the He spatial distribution on the divertor floor is studied with an impurity monitor array. The dependence of core transport diffusivities as a function of edge-localized modes (ELM) frequency has been studied and the first demonstration made of He exhaust from an ELMing H-mode plasma in a diverted tokamak. A measured value of tau *_He/ tau _E of 14 was obtained, which is within the acceptable range for a fusion reactor.

Heat flux and particle flux exhaust during ELMy H-mode were investigated using infrared TV camera, Langmuir probes and H_alpha detectors. The peak heat flux density reaches 400MW/m² and most of the power is deposited within a few milliseconds with PNB approximately 10MW. While the scrape-off layer (SOL) current usually flows from the outer divertor to the inner divertor during ELM, occasional reversed current at the outer SOL layer accompanies broader heat flux profile.

Poloidal and toroidal rotation of the fuel ions (He²⁺) and the impurity ions (C⁶⁺ and B⁵⁺) in H-mode helium plasmas have been investigated in the DIII-D tokamak by means of charge exchange recombination spectroscopy, resulting in the discovery that the fuel ion poloidal rotation is in the ion diamagnetic drift direction while the impurity ion rotation is in the electron diamagnetic drift direction. The radial electric field obtained from radial force balance analysis of the measured pressure gradients and rotation velocities is shown to be the same regardless of which ion species is used and therefore is a more fundamental parameter than the rotation flows in studying H-mode phenomena. It is shown that the three contributions to the radial electric field (diamagnetic, poloidal rotation, and toroidal rotation terms) are comparable and consequently the poloidal flow does not solely represent the E*B flow. In the high-shear edge region, the density scale length is comparable to the ion poloidal gyroradius, and thus neoclassical theory is not valid there. In view of this new discovery that the fuel and impurity ions rotate in opposite sense, L-H transition theories based on the poloidal rotation may require improvement.

In this paper, standard neoclassical theory has been extended into the shear layer. Main ion and impurity ion rotation velocity and bootstrap current within shear layer in H-mode are discussed. Inside the H-mode shear layer, standard neoclassical theory is not valid since the ion poloidal gyroradius becomes comparable to pressure gradient and electric field gradient scale length. We present modified expressions of impurity flows and bootstrap current in a shear layer neglecting ion temperature gradient. Comparisons with DIII-D measurements are also discussed.

Spontaneous formation of internal transport barrier was observed in JT-60U high- beta _p discharges, where q=3 surface is the most likely explanation for its radial location. Further improved confinement was brought about by the subsequent transition to the edge transport barrier.

Fluctuations of JFT-2M scrape-off plasma are measured by triple probes in detail. Electrostatic fluctuation can account for the cross field particle flux. When the radial electric field is made by the divertor biasing, reductions of the fluctuations and fluctuation induced flux are observed at the shear layer.

Measurements of interior microturbulence in the DIII-D tokamak have indicated that during double null discharges with low radiated power and strong neutral beam injection power, interior microturbulence is observed to reduce prior to the onset of the so-called VH-mode, when the peak energy confinement time is greater than two times that predicted by the JET/DIII-D H-mode scaling law. The core broadband turbulence level begins to modulate at frequencies of hundreds of Hertz, and over 100-200 ms following the L to H transition, the modulation period increases while the average fluctuation level reduces. The bursts are associated with momentum transfer events because the toroidal rotation increases significantly after their suppression. The associated E_r*B convective drift increases significantly as well. During this period, confinement increases continuously and is often observed to peak several hundred ms after the modulation ends. The question arises as to whether shear flow stabilization might be reducing core turbulence in VH-mode and reducing the fluctuation driven energy transport. In recent experiments, it has been shown that independently reducing E_r in the core limits the fluctuation reduction and prevents access to the VH-mode regime.

Published neoclassical results are misleading as concerns the plasma edge for they do not adequately take the peculiar local conditions into account, in particular the fact that the density and temperature variation length-scales are here quite small: L_N,T <<a, R, qR. In the high collisionality regime (qR nu _J/cJ>>1), gyro-stress tensor and ion inertia play an essential role if the ratio q²R²/( Omega _i tau _i)aL_N,T is of the order of, or larger than unity. Coupled novel neoclassical equations then obtain, not only for the evolution of the density and temperatures, but also for the radial electric field and the evolution of the parallel ion momentum: gyro-stresses and inertia indeed upset the otherwise de facto ambipolarity of particle transport and a radial electric field necessarily builds up. The increased nonlinear character of these revisited neoclassical equations widens the realm of possible plasma behaviors.

The theory of transport across a low mode number magnetic island belt driven by the ionization instability, elongated along the last entire q rational surface, and extending beyond the separatrix leads to a number of predictions which are in agreement with observations in DIII-D, TEXTOR, ASDEX and CCT. It also suggests that transition to a regime characterized by an improved confinement should occur at a sufficiently high heating power level.

In PBX-M, periods of enhanced transport are often observed in high beta _polH-mode discharges. They can reduce the energy confinement time by a factor of two. Their duration is typically between 2 to 20 ms. They are followed by periods of normal transport of a similar duration. These periods of enhanced transport consist of a series of turbulent m=even MHD events, starting several cm inside the plasma edge. They propagate into the confinement region in 600 mu s, causing a progressive erosion of the profile. Each m=even event can also cause an energy loss of 1% or less, mainly from the confinement region. These m=even events are reminiscent of grassy ELMs.

Particle transport in ECRH plasmas has been investigated for both cases of edge or central heating at the second harmonic cyclotron frequency. Global particle confinement is improved for both cases, and a transition phenomenon occurs for the case of edge heating. The effects of the trapped electron loss on the particle confinement improvement are discussed.

A regime of improved H-mode energy confinement, VH-mode, is obtained in the DIII-D tokamak with adequate vessel conditioning. The improved confinement in VH-mode is consistent with the extension of the region of high E*B velocity shear turbulence suppression zone further in from the plasma boundary. The energy confinement enhancement in VH-mode can be limited by ELMs, localized momentum transfer events, or operation at high heating power or low q. Energy confinement enhancement improves with increasing triangularity of the plasma cross section and is independent of elongation. The termination of VH-mode is associated with an edge localized kink mode which is destabilized by both the large edge pressure gradient and edge current density.

Plasmas with the energy confinement time enhanced by a factor 1.4 to 2.1 over the ITER93H-P H-mode scaling expression have been obtained in 3 different H-mode regimes (excluding pellet injection) in JET. The phase of enhanced confinement has strong similarities with the VH-mode observed in DIII-D in all 3 cases. The JET data clearly show VH-mode confinement increasing with plasma current and degrading with loss power.

We summarize results from DIII-D in regard to issues for reactor application of H-mode. Recently, DIII-D has begun to operate a cryopump (D₂ pumping speed =31,000 l/s at a pressure of 2 mTorr). Initial results are very favorable for density control in H-mode. The plasma density could be reduced by 50%. Energy confinement is unchanged so the temperature rises in proportion to the density drop. Ability to access these less collisional plasmas in H-mode is favorable to current drive application. With the all-graphite wall, impurity accumulation has been eliminated. The exceedingly good confinement of VH-mode offers the possibility of retaining good confinement while radiating copious power from the plasma edge using injected noble gas impurities.

Two H-mode regimes have been identified in JET in which edge localized modes (ELMs) maintain steady-state conditions. In the first regime, strong gas puffing was used in combined (ICRF plus NBI) heating experiments at powers of up to 20MW. Rapid ELM activity occurred and at moderate powers ( approximately 8MW) steady-state H-modes with durations of up to 18s and energy confinement times of up to 95% of the JET/D-IIID scaling were established. At high toroidal beta ( beta _N>or=1.5) H-mode plasmas were also found to exhibit regular ELM behaviour which resulted in steady-state H-modes with confinement enhancement of approximately 90% of the JET/D-IIID scaling. This paper examines the plasma properties of these regimes and assesses their implications for steady-state H-mode operation in ignited plasmas.

Anomalous transport coefficients, which express the H-mode and L-mode in a unified manner, are obtained by use of a theory of self-sustained turbulence. The reduction of transport coefficients and the fluctuations in the H-mode are shown quantitatively. The L-H mode transition condition and the structure of the transport barrier are discussed. The existence of various transport barriers (particle, momentum and heat) is also pointed out. The dynamics of the barrier formation and the relation to ELM are discussed.

The change in thermal transport across the L to H transition is studied in detail for those JET high performance H-modes which have a very fast transition. It is found that in these pulses the transport changes very rapidly (<4 msecs) over a very large radial region 0.5< rho <1, and a very large transport barrier is formed. The reasons for the formation of this barrier are discussed.

The physics which determines poloidal rotation, and its role in the development of profiles during H- and VH-modes, is discussed. A simple phenomenological transport model, which incorporates the E*B flow shear suppression of turbulence, is shown to predict profile evolution similar to that observed experimentally during H-mode and VH-mode.

The poloidal rotation velocity profiles both in L- and H-mode measured in JAERI Fusion Torus 2 Modified (JFT-2M) (Phys. Rev. Lett. 65, 1364 (1990)) are compared with H-mode models based on ion orbit loss. The profiles of poloidal rotation velocity measured in L- and H-mode are consistent with the calculation which consists of ion orbit loss model. The observed dependence of the thickness of the layer of high shear E_r on poloidal gyro-radius is explained by the radial transport of poloidal rotation velocity.

Measurements of edge profiles, turbulence, and turbulent-driven transport were made inside the last-closed flux surface (LCFS) and in the scrape-off layer (SOL) of PBX-M L-mode and H-mode plasmas using a fast reciprocating Langmuir probe diagnostic. Direct measurements of the potential profile confirm the generation of a strong inward radial electric field (E_r approximately -100 V/cm) just inside the LCFS in H-mode. Density and potential fluctuation levels are reduced at the L-H transition, resulting in significantly lower turbulent transport. The edge density gradient then steepens in response to the reduced cross-field transport. After a stable H-mode edge equilibrium is formed, the fluctuation amplitudes recover back to nearly L-mode values. The poloidal wavenumber spectrum is narrowed in the transport barrier, and the density and potential fluctuations are decorrelated. The reduction in turbulent transport occurs across the LCFS and SOL regions and is not localized to the region of strong radial electric field.

Since the last H-mode workshop in 1991, there has been significant progress in a number of areas. In addition to H-modes in tokamaks, H-mode has been achieved in a current-free stellarator, a heliotron/torsatron with some net toroidal current and in a linear, tandem mirror. Because H-mode has been seen in a variety of magnetic confinement devices and has been produced by a variety of methods, a universal explanation is needed for the H-mode confinement improvement. The hypothesis of turbulence stabilization by sheared E*B flow has this universality. New diagnostics have confirmed the structure of E_r at the plasma edge in tokamaks and have lead to direct determination of the reduction in turbulence-driven transport in the H-mode. Improved measurements of the edge rotation and pressure gradients in tokamaks have lead to better understanding of the physics of the E_r formation and to tighter tests of L to H transition theories. Significant improvements in core confinement have also been seen in several tokamaks; the data here indicate that sheared E*B flow may be playing a role in the core confinement improvement. Power balance studies of the change in core thermal diffusivity after the L to H transition have raised fundamental questions about the relationship between heat flux and the temperature gradient. Finally, helium transport studies have shown that helium transport is quite similar to deuterium transport indicating that helium ash removal in a reactor operating in ELMing H-mode should be feasible.

Theoretical research into H-mode physics is reviewed. The topics cover elementary processes such as bifurcation physics, collisional transport theory and anomalous transport theory. Associated with these investigations, a global picture of H-mode phenomena, such as the spatial structure and temporal evolution has been constructed from theory. The author tries to clarify the contributions of theoretical presentations in this H-mode workshop, illustrating the relation to the overall progress on the H-mode theory.

Next-step devices and advanced tokamaks and stellarators are discussed. Improved wall conditioning allows the power threshold to decrease and H-mode operation under ohmic conditions occurs. Density-, helium exhaust-, and ELM control has been demonstrated for H-mode plasmas albeit in a rather rudimentary form. Schemes of enhanced edge radiation can be reconciled with the H-mode specific edge conditions. Modelling of the SOL with ELMs is being studied. There are several routes for further enhancement in confinement. As the stability quality can also be improved, prospects of advanced tokamak operation with a large bootstrap current fraction in the H-mode seems favourable. As the advanced stellarator concept addresses primarily collisional transport and stability aspects, this further improves its reactor prospects with the recently discovered H-mode in stellarators.