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We have constructed a Bayesian neural network model that predicts the change, due to neutron irradiation, of the Charpy ductile-brittle transition temperature (ADBTT) of low-activation martensitic steels given a set of multi-dimensional published data with doses

The development of fusion power requires a facility for assessing the behaviour of materials subjected to damage from 14 MeV neutrons with displacement damage levels of up to about 150 displacements per atom (dpa). The proposed International Fusion Materials Irradiation Facility (IFMIF) will enable experiments to be conducted that closely fit these requirements. When designing experiments it is at first sight natural to suggest a uniformly sampled test matrix over the domain of interest. However, as the irradiation volume in IFMIF will be limited, it is appropriate to consider how the set of experiments may be optimised. In the present work we suggest a set of experiments designed using predictive Bayesian neural network models created using published data for irradiated reduced-activation martensitic steels. It has been possible to identify gaps in knowledge on the basis of model-perceived uncertainties, and to assess trends in order to determine more optimal experimental sampling.

Small platelike clusters of self-interstitial atoms (SIAs) in irradiated metals are extremely mobile. This mobility can be greatly reduced by foreign atoms. Where the plates are large enough to form edge dislocation loops, their immobilization is analysed as a solid solution hardening. The misfitting substitutional solute atoms can significantly reduce the mobility of small SIA loops when in the central cores of their edge dislocation lines. An activation energy is required to unpin a loop from such atoms and this – unlike in conventional solid solution hardening – remains finite even with no applied stress driving the dislocation. In dilute solutions break-away occurs by the thermally activated escape from single atom obstacles on the loops. Application to a proposed fusion power plant alloy (EUROFER 97) shows that the W alloy atoms provide the most severe immobilization, although Mn atoms produced by transmutation run a close second. The contribution of Cr is evaluated.

A recent analysis of the migration of voids and bubbles, produced in neutron irradiated fusion materials, is outlined. The migration, brought about by thermal hopping of atoms on the surface of a void, is normally a random Brownian motion but, in a temperature gradient, can be slightly biassed up the gradient. Two effects of such migrations are the transport of voids and trapped transmutation helium atoms to grain boundaries, where embrittlement may result; and the coalescence of migrating voids, which reduces the number of non-dislocation sites available for the capture of knock-on point defects and thereby enables the dislocation bias process to maintain void swelling. A selection of candidate fusion power plant armour and structural metals have been analysed. The metals most resistant to void migration and its effects are tungsten and molybdenum. Steel and beryllium are least so and vanadium is intermediate.

Experimental investigation of neutron-induced irradiation damage in structural materials is fundamental to the development of magnetic confinement fusion. Proposals for the testing of candidate materials are described, indicating that a period of at least 10 years will elapse before a suitable high neutron fluence fusion test facility becomes available. In this circumstance, the possibility that neutron spallation sources could be exploited to shorten the time-scale of fusion materials development is attractive. Although fusion displacement and transmutation reaction rates can be replicated in spallation sources, there are significant differences arising from the harder neutron spectra and the presence of energetic protons. These differences, including higher energy PKA, electron heating effects, transmutation rates and pulsing are described and their consequences discussed, together with the concomitant development of theoretical models, needed to understand the effects. It is concluded that spallation source experiments could make a significant contribution to the database required for the validation of theoretical models, and hence reduce the time scale of fusion materials development.

Neutron irradiation in a fusion power plant will cause helium bubbles and voids to form in the armour and blanket structural materials. If sufficiently large densities of such defects accumulate on the grain boundaries of the materials, the strength and the lifetimes of the metals will be reduced by helium embrittlement and grain boundary failure. This Letter discusses void migration in metals, both by random Brownian motion and by biassed flow in temperature gradients. In the assumed five-year blanket replacement time of a fusion power plant, approximate calculations show that the metals most resilient to failure are tungsten and molybdenum, and marginally vanadium. Helium embrittlement and grain boundary failure is expected to be more severe in steel and beryllium.

The main limitation to the performance of JET optimized shear (OS) discharges is due to MHD instabilities, mostly in the form of a disruptive limit. The structure of the MHD mode observed as a precursor to the disruption, as measured from SXR and ECE diagnostics, shows a global ideal MHD mode. The measured mode structure is in good agreement with the calculated mode structure of the pressure driven kink mode. The disruptions occur at relatively low normalized beta (1 < βN < 2), in good agreement with calculated ideal MHD stability limits for the n = 1 pressure driven kink mode. These low limits are mainly due to the extreme peaking factor of the pressure profiles. Other MHD instabilities observed in JET OS discharges include, usually benign, chirping modes. These modes, which occur in bursts during which the frequency changes, have the same mode structure as the disruption precursor but are driven unstable by fast particles.

High fusion performance has been achieved in optimized shear discharges in JET with the early appearance of internal transport barriers (ITBs) in the high power phase in both DD and DT plasmas. An ITB has been produced in a large variety of conditions, including LHCD only, ICRH only, NBI only and NBI + ICRH with LHCD preheat. An ITB may coexist with the edge barrier. The high pressure gradient produced by ICRH combined with high power NBI heating starting in a low density plasma with q(0) ≈ 1.5-2 is the main prerequisite for an efficient ITB in high performance discharges. Strong coupling between q profiles, MHD activity, energy confinement and fusion yield has been observed. The transition from L to H mode can be triggered by MHD events. The transition is accompanied by significant momentary changes in the density fluctuation spectrum in the peripheral plasma. The ITB may persist for some time in both ELM-free and ELMy H mode.

The unique roles played by ICRH in the preparation, formation and sustainment of internal transport barriers (ITBs) in high fusion performance JET optimized shear experiments using the Mark II poloidal divertor are discussed. Together with LHCD, low power ICRH is applied during the early ramp-up phase of the plasma current, `freezing in' a hollow or flat current density profile with q(0)>1. In combination with up to ~20 MW of NBI, the ICRH power is stepped up to ~6 MW during the main low confinement (L mode) heating phase. An ITB forms promptly after the power step, revealed by a region of reduced central energy transport and peaked profiles, with the ion thermal diffusivity falling to values close to the standard neoclassical level near the centre of both DD and DT plasmas. At the critical time of ITB formation, the plasma contains an energetic ICRF supported hydrogen minority ion population, contributing ~50% to the total plasma pressure and heating mainly electrons. As both the NBI population and the thermal ion pressure develop, a substantial part of the ICRF power is damped resonantly on core ions (ω = 2 ωcD = 3ωcT), contributing to the ion heating. In NBI step-down experiments, high performance has been sustained by maintaining central ICRH; analysis shows the efficiency of central ICRH ion heating to be comparable to that of NBI. The highest DD fusion neutron rates (RNT = 5.6 × 1016 s-1) yet achieved in JET plasmas have been produced by combining a low magnetic shear core with a high confinement (H mode) edge. In DT, a fusion triple product niTiτE = (1.2 ± 0.2) × 1021 m-3 keV s was achieved with 7.2 MW of fusion power obtained in the L mode and with up to 8.2 MW of fusion power in the H mode phase.

Transport analysis of high performance JET plasmas with optimized magnetic shear (OMS) has revealed many interesting features which cannot be explained by the existing JET empirical transport model (Erba, M., et al., Plasma Phys. Control. Fusion 39 (1997) 261). TRANSP analysis shows that transport coefficients in OMS plasmas are often reduced in the plasma core (Cottrell, G.A., et al., in Controlled Fusion and Plasma Physics (Proc. 24th Eur. Conf. Berchtesgaden, 1997), Vol. 21A, Part I, European Physical Society, Geneva (1997) 81) to the level of ion neoclassical transport. TRANSP analysis and predictive modelling with JETTO show that this region of improved confinement appears near the plasma centre and then expands outwards in a way which does not follow either the evolution of the region with the negative magnetic shear or the propagation of the region with a large shear in plasma rotation. The best agreement with experiment has been achieved by using a transport model which combines the effect of a long wavelength decoupling due to small magnetic shear with its suppression by strong rotational shear. Predictive modelling of some of the characteristic JET OMS plasmas gives quite good agreement between such a model and the experimental data.

Steady state high performance with improved core confinement and sustainable plasma edge conditions has been approached on JET in a double barrier (DB) mode. The DB mode combines an internal transport barrier of the optimized shear regime with an edge transport barrier of an ELMy H?mode regime. Improved confinement with an H?factor HITER-89 ? 2 has been maintained for four energy confinement times. Ion and electron temperature profiles remain peaked in the DB mode, while the density profile is broad and similar in shape to the conventional ELMy H?mode profile. The energy confinement improves across the whole plasma cross-section, and the ion heat conductivity falls to the neoclassical level in the core. Particle transport studies show that impurity accumulation can be avoided in the DB mode. In DT discharges the DB mode has attained a fusion gain of Q ? 0.4, producing 6.8?MW of fusion power, compared with Q ? 0.2 in the conventional sawtoothing ELMy H?mode. The ELMs are more benign, with an amplitude an order of magnitude smaller in the DB mode. The DB mode has a high potential to improve performance in reactor relevant conditions.

Reactor relevant ICRH scenarios have been assessed during DT experiments on the JET tokamak using H mode divertor discharges with ITER-like shapes and safety factors. Deuterium minority heating in tritium plasmas was demonstrated for the first time. For 9% deuterium, an ICRH power of 6 MW gave 1.66 MW of fusion power from reactions between suprathermal deuterons and thermal tritons. The Q value of the steady state discharge reached 0.22 for the length of the RF flat-top (2.7 s), corresponding to three plasma energy replacement times. The Doppler broadened neutron spectrum showed a deuteron energy of 125 keV, which was optimum for fusion and close to the critical energy. Thus, strong bulk ion heating was obtained at the same time as high fusion efficiency. Deuterium fractions around 20% produced the strongest ion heating together with a strong reduction of the suprathermal deuteron tail. The ELMs had low amplitude and high frequency and each ELM transported less plasma energy content than the 1% required by ITER. The energy confinement time, on the ITERH97-P scale, was 0.90, which is sufficient for ignition in ITER. 3He minority heating, in approximately 50:50 D:T plasmas with up to 10% 3He, also demonstrated strong bulk ion heating. Central ion temperatures up to 13 keV were achieved, together with central electron temperatures up to 12 keV. The normalized H mode confinement time was 0.95. Second harmonic tritium heating produced energetic tritons above the critical energy. This scheme heats the electrons in JET, unlike in ITER where the lower power density will allow mainly ion heating. The inverted scenario of tritium minority ICRH in a deuterium plasma was demonstrated as a successful heating method producing both suprathermal neutrons and bulk ion heating. Theoretical calculations of the DT reactivity mostly give excellent agreement with the measured reaction rates.

Ion cyclotron resonance heating (ICRH) of JET high-performance (optimized shear and hot-ion H-mode) deuterium discharges, characterized by strong neutral beam injection (NBI), high ion and electron temperatures and high deuterium-deuterium fusion reactivity, is analysed using numerical simulations. The main aim is to investigate under which conditions good bulk ion heating can be obtained with ICRH tuned to the fundamental hydrogen resonance, coinciding with the second-harmonic deuterium resonance, near the plasma centre. The analysis has been facilitated by adding NBI source terms in the ion cyclotron range of frequencies (ICRF) code PION, which calculates the time evolution of the ICRF power deposition and the resonating ions distribution functions in a self-consistent way. The results show that damping ICRF power on deuterons results in a more central bulk ion heating profile than minority hydrogen ions, which is an advantage. The highest bulk ion heating fractions of up to 50% of the total ICRF power were obtained by using multiple frequencies, the effects of which have been simulated in detail for the first time. Furthermore, operating with high plasma densities has been found to be beneficial for maximizing the bulk ion heating.

Formation of core regions in Tore Supra and JET tokamaks with reduced transport coefficients is reported. Characteristics of the enhanced confinement regions and the physics process involved in their formation and maintenance should be considered separately when the electron or ion components are predominantly heated. In Tore Supra and JET, central electron temperature transitions are observed by injecting lower hybrid waves at modest power levels during the current ramp-up phase of the discharges. Transport analyses stress the importance of the low magnetic shear in the core to explain the anomalous electron transport reduction. With high-power dominant ion heating schemes in JET (neutral beam injection and ion cyclotron resonance heating), internal transport barriers have been obtained in plasmas fuelled with a mixture of deuterium-tritium (D-T) ions leading to a successful production of fusion power (8.2 MW) in this regime. Similar additional power levels to those applied in pure deuterium (D-D) plasmas are required to establish internal transport barriers in D-T plasmas. In D-D and D-T plasmas, ion thermal diffusivities are reduced close to their neoclassical levels in the plasma core and electron thermal diffusivities decrease by one order of magnitude at midplasma radius. The combined role of magnetic shear and velocity shear can explain the formation and evolution of plasma core regions with low energy transport coefficients.

Plasmas with the highest deuterium fusion neutron rates yet achieved in JET have been produced by combining a hollow or flat central current profile with a high-confinement (H-mode) edge. In these discharges, lower hybrid current drive (LHCD) and ion cyclotron resonance heating (ICRH) preheating, applied early in the current ramp-up phase, `freezes in' a hollow or flat current density profile. When the combined neutral beam injection (NBI) and ICRH heating power is increased, a region of reduced transport and highly peaked profiles forms during the L-mode phase and persists into the later H-mode phase when the fusion reactivity reaches a maximum. Transport analysis shows the formation of a central region of good confinement (the internal transport barrier or ITB) which expands with radial velocity . The clearest signatures of this region are large gradients in the ion temperature and toroidal rotation profiles. Ion thermal diffusivities in the central region are of the order of the neoclassical value. The position and rate of expansion of the ITB radius correlates well with the calculated rational q = 2 surface. The confinement improvement can also be seen in electron density, and, to a lesser extent, in electron temperature. The ITB can persist in combination with the edge transport barrier characteristics of the H-mode.

The observation of internal transport barriers (ITBs) in which ion thermal diffusivity is reduced to a neoclassical level has been made for the first time in tokamak plasmas fueled with deuterium and tritium ions using a broad current density profile. The heating and current profiles required to obtain an ITB are similar in D-T and D-D plasmas. Central ion temperatures of 40 keV and plasma pressure gradients of ${10}^{6}\mathrm{Pa}/\mathrm{m}$ were observed in a D-T plasma, leading to a fusion triple product ${n}_{i}{T}_{i}{\ensuremath{\tau}}_{E}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.10}^{21}{\mathrm{m}}^{\ensuremath{-}3}\mathrm{keV}\mathrm{s}$ and 8.2 MW of fusion power. There is potential for further optimization as a step towards the development of efficient tokamak fusion reactors.

Heating during the early part of the current rise phase gives a low or negative magnetic shear (= (dq/dr)) in the centre of JET plasmas. Under these conditions the confinement improves with high additional heating power heating during the current ramp-up phase of the discharge. The reduction in the transport manifests itself as a peaking of the profiles with a large gradient region near = 0.55. The best discharges have no transport barrier at the edge of the plasma (L-mode). This allows central power deposition by the neutral beams in JET. A control of the plasma pressure, using feedback of the additional heating power in real-time, minimizes the impact of magnetohydrodynamic instabilities. As a result, these discharges achieve the highest D-D neutron rates in JET; , with , and .

Ion cyclotron resonance heating (ICRH) experiments have been carried out in JET D-T plasmas using scenarios applicable to reactors. Deuterium minority heating in tritium plasmas is used for the first time and produces 1.66 MW of D-T fusion power for an ICRH power of 6 MW. The Q value is 0.22, which is a record for steady state discharges. Fundamental He-3 minority ICRH, in both 50:50 D-T and tritium dominated plasmas, generates strong bulk ion heating and ion temperatures up to 13 keV. Second harmonic tritium ICRH is seen to heat mainly the electrons as expected for JET conditions. All three schemes produce H-mode plasmas.

Experiments using high-power heating during the current ramp-up phase of the discharge have obtained the highest D-D neutron rates in JET; neutrons , with , keV and keV. The best discharges ( MA and tesla) have peaked pressure profiles with a transport barrier located at = 0.55. The pressure peaking is limited by MHD modes and requires active input power control to achieve the best performance. Deuterium neutral beam injection into a tritium-rich target plasma has established internal transport barriers at power levels close to the lowest threshold for pure deuterium plasmas.

All major JET systems have been fully commissioned for D-T and the DTE1 series of experiments has started with the D-T fuel mixture and operating conditions foreseen for ITER. In the area of ITER physics, significant results have been produced in both D-D and D-T. In D-D, the LH threshold power database has been extended, the bounds on edge-electron temperature and density in ELMy H-modes have been defined and the advantages of Types I and III ELMy discharges have been compared. In D-T plasmas, the isotope effect on H-mode threshold power and transport has been determined so that a more accurate assessment can be made of the ignition margin and heating requirements for ITER. Trace tritium experiments have provided first particle transport measurements and an assessment of the ITER reference ion-cyclotron resonance-frequency heating scenarios has been started, In the area of fusion performance, record D-D neutron yields have been obtained by controlling the plasma and current profiles in hot ion ELM-free H-modes and optimized shear modes. In D-T, internal transport barriers have been readily established in optimized shear discharges and Alfven eigenmodes have been observed.

Ion cyclotron emission (ICE) has been observed during neutral beam‐heated supershots in the Tokamak Fusion Test Reactor (TFTR) [Phys. Rev. Lett. 72, 3526 (1994)] deuterium–tritium campaign at fusion product cyclotron harmonics. The emission originates from the outer midplane edge plasma, where fusion products initially have an anisotropic velocity distribution, sharply peaked at a sub‐Alfvenic speed. It is shown that the magnetoacoustic cyclotron instability, resulting in the generation of obliquely propagating fast Alfven waves at fusion product cyclotron harmonics, can occur under such conditions. The time evolution of the growth rate closely follows that of the observed ICE amplitude. Instability is suppressed if the fusion products undergo a moderate degree of thermalization, or are isotropic. In contrast, the super‐Alfvenic fusion products present in the outer midplane of the Joint European Torus (JET) [Nucl. Fusion 33, 1365 (1993)] can drive the instability if they are isotropic or have a broad speed distribution. This may help to account for the observation that fusion product‐driven ICE in JET persists for longer than fusion product‐driven ICE in TFTR supershots.

After five years in a fusion power plant, neutrons will have transmuted a plasma-facing tungsten component into an alloy of about 75 W, 13 Os and 12 Re (atomic percent), a composition close to the homogeneous σ field of the phase diagram. Since the σ phase is extremely brittle, it is important to know whether the transformation will occur. To avoid this problem, the use of tantalum or molybdenum, as alternatives, is proposed.

A comprehensive set of fast particle diagnostics is routinely used at JET. Some are in the process of being upgraded and others, completely new, are being prepared for use during the forthcoming tritium experiments. For fusion product studies, the strength and profile of the charged particle birth distribution is obtained by measuring the neutron emission with three pairs of absolutely calibrated fission chambers and a two-camera profile monitor. Information on the DD neutron energy spectrum is deduced from a time-of-flight neutron spectrometer. Neutrons of 14 MeV energy from triton burnup are measured using silicon diodes and a high energy branch of the profile monitor. Absolute calibration is obtained with an activation system. A prototype lost alpha particle detector (Faraday cup) has been tested in the laboratory and has been installed inside JET so that its noise immunity can be tested. Two 14 MeV neutron spectrometers are under commissioning and a third is under construction. A high energy neutral particle analyser is routinely used to diagnose fast RF driven particles and will also be available for alpha particle studies. The intensity of RF driven fast particles is also deduced by the spectroscopy of gamma rays emitted in reactions with impurity ions in the plasma. The gamma ray measuring branch of the neutron profile monitor adds spatial resolution to the measurement. A new antenna for measuring ion cyclotron emission (ICE) and a gyrotron for alpha particle scattering experiments are in the process of being commissioned. Active charge exchange spectroscopy to investigate the low energy range of the alpha particle population is being considered. Where appropriate, information gathered with these systems is presented to illustrate their performance

Ion cyclotron emission (ICE) has been observed from neutral beam heated TFTR, and JET tritium experiments at sequential cyclotron harmonics of both fusion products and beam ions. The emission originates from the outer midplane plasma, where fusion products and beam ions are likely to have a drifting ring-type velocity-space distribution that is anisotropic and sharply peaked. Fusion product driven ICE in both TFTR and JET can be attributed to the magnetoacoustic cyclotron instability, which involves the excitation of obliquely propagating waves on the fast Alfven/ion Bernstein branch at cyclotron harmonics of the fusion products. Differences between ICE observations in JET and TFTR appear to reflect the sensitivity of the instability growth rate to the ratio vbirth/cA where vbirth is the fusion product birth speed and cA is the local Alfven speed for fusion products in the outer midplane edge of TFTR supershots, vbirth < cA for alpha particles in the outer midplane edge of JET, the opposite inequality applies. If sub-Alfvenic fusion products are isotropic or have undergone even a moderate degree of thermalization, the magnetoacoustic instability cannot occur. In contrast, the super-Alfvenic alpha particles that are present in the outer midplane of JET can drive the magnetoacoustic cyclotron instability even if they are isotropic or have a relatively broad distribution of speeds. These conclusions may account for the observation that fusion product driven ICE in JET persists for longer than fusion product driven ICE in TFTR. Moreover, the time evolution of the maximum growth rate, obtained using the Sigmar model for the alpha particle distribution and TFTR data for the fusion product source rate, closely follows the observed time evolution of the ICE amplitude in TFTR supershot discharges. Other observed features of fusion product driven ICE that match the linear instability include the scaling with fusion product density, doublet splitting of spectral peaks, the relative strength of certain harmonics and source localization. A separate mechanism is proposed for the excitation of beam driven ICE in TFTR: electrostatic ion cyclotron harmonic waves, supported by strongly sub-Alfvenic beam ions, can be destabilized by a low concentration of such ions with a very narrow spread of velocities in the parallel direction. Sufficiently narrow distributions are likely to exist in the edge plasma, close to the point of beam injection.

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 β 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

The theory of the magnetoacoustic cyclotron instability, which has been proposed as a mechanism for suprathermal ion cyclotron harmonic emission observed in large tokamaks, is generalized to include finite parallel wave number k∥. This extension introduces significant new physics: the obliquely propagating fast Alfven wave can undergo cyclotron resonant interactions with thermal and fusion ions, which affects the instability driving and damping mechanisms. The velocity–space distribution of the fusion ions is modeled by a drifting ring, which approximates the distribution calculated for the emitting region in tritium experiments on the Joint European Torus (JET) [Cottrell et al., Nucl. Fusion 33, 1365 (1993)]. Linear instability can occur simultaneously at the fusion ion cyclotron frequency and all its harmonics when the fusion ion concentration is extremely low, because the finite k∥ gives rise to a Doppler shift, which decouples cyclotron damping due to thermal ions from wave growth associated with fusi...

A common mechanism for the observed excitation of waves at sequential multiple ion cyclotron harmonics in the JET tokamak and in various space plasmas is proposed. It is suggested that, in each case, waves on the fast Alfven-ion Bernstein branch are excited by the magnetoacoustic cyclotron instability, which is driven by an energetic ion distribution with a local maximum at v⊥ ≠ 0, where v⊥ is the velocity component perpendicular to the magnetic field.

In the course of the Preliminary Tritium Experiment in JET, where combined deuterium and tritium neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) was measured in the frequency range v ≤ 180 MHz. The ICE spectra contain superthermal, narrow, equally spaced emission lines, which correspond to successive cyclotron harmonics of deuterons or alpha particles at the outer midplane, close to tile last closed flux surface at major radius R approximately 4.0 m. Above about 100 MHz the lines merge into a relatively intense continuum. The ICE signal fluctuates rapidly in time, and is extinguished whenever a large amplitude edge localized mode (ELM) occurs. In pure deuterium and mixed DT discharges ICE spectra are similar in form, but on changing from pure D to mixed D+T neutral beam injection at constant power, the intensity of the ICE rises in proportion to the increased neutron flux: this indicates that fusion alpha particles-and not beam ions-provide the free energy to generate ICE. The JET ICE database, which now extends over a range of six decades in signal intensity, shows that the time averaged ICE power increases almost linearly with total neutron flux. The rise and fall of the neutron flux during a single discharge is closely followed by that of the ICE signal, which is delayed by a time of the order of the fusion product slowing down time. This feature is well modelled by a TRANSP code simulation of the density of deeply trapped fusion products reaching the plasma edge. Calculations reveal a class of fusion products, born in the core, which make orbital excursions of sufficient size to reach the outer midplane edge. There, the velocity distribution has a ring structure, which is found to be linearly unstable to relaxation to obliquely propagating waves on the fast Alfven-ion Bernstein branch at all ion cyclotron harmonics. The paper shows how ICE provides a unique diagnostic for fusion alpha particles

Two-ion ion cyclotron resonance (ICR) heating in a tokamak can produce superthermal minority ions having orbital radii comparable with the plasma radius. The drift motion causes the trapped fast ions to precess around the torus, thus forming a fast ion current. A semi-analytic model and a Monte Carlo radiofrequency (RF) orbit code have been used to calculate the direction, magnitude and profile of this current. For typical high power central ICR heating of JET, a fast ion current density of j|| ~ 0.7 MA/m2 is found which is peaked at a radius ~ 30 cm from the centre where it enhances the local current density. The effect of the fast ion current is to flatten the q-profile within the region of the inversion radius of MHD sawteeth, where the sawtooth stability is expected theoretically to be sensitive to the radial profile of the current density. The fast ion current could explain the observed sawtooth stabilization during ICR heating. Data for RF heating of JET have been examined for evidence of a correlation between the natural duration of sawtooth-free periods and the expected magnitude of the off-axis fast ion current density. The data show a threshold current density below which sawteeth are not stabilized. Above this threshold, the duration of sawtooth stable periods increases monotonically with the magnitude of the off-axis RF fast ion current density

Preliminary experiments were carried out in the Joint European Torus (JET) using 120 keV helium (He) neutral beam injection (NBI). Injected power levels up to 5 MW with 3He and 7 MW with 4He, lasting up to 3 s were reached. The 3 s helium NBI produced efficient ion heating and similar global and local energy confinements to those obtained with deuterium (D) NBI, in both limiter and X-point plasma geometries, in L-mode and H-mode plasma regimes. The elimination of beam-plasma and beam-beam fusion reactions by replacing D NBI with He NBI extended the range for measuring ion temperatures with the JET neutron profile monitor and neutron spectrometers.

The D-D fusion reactivity from JET plasmas has been optimized by using combined neutral beam injection and radiofrequency heating. Reaction rates of up to 2.9 x 10(16) reactions per second (equivalent to neutron emission rates of 1.45 x 10(16) neutrons per second) have been achieved. The best results were obtained in the double-null magnetic configuration or with plasmas limited by the inner wall. By far the largest fraction of the neutron production originates from beam induced reactions, although, in some cases, about 25% of the production can be attributed to the acceleration of deuterium ions by the radiofrequency. The instantaneous neutron yield was limited by a strong influx of carbon impurity ions due to excessive, but local, wall heating.

Localized and intense ion cyclotron resonance heating (ICRH) of H and 3He minority ions in plasmas produced in the Joint European Torus (JET) tokamak create fast ion distributions having average energies in the multi-MeV range, similar to 3.5 MeV fusion alpha particles expected in a reactor. For typical values of the JET poloidal magnetic field, the radii of the particle orbits can significantly exceed the size of the RF deposition. Moreover, in the presence of a steep radial gradient in the fast ion slowing-down time, the large radial excursions of the heated ions take them into the outer plasma region where the frictional drag is larger than in the hot plasma core. In modelling the fast ion distribution with ICRH, therefore, it is important to include a correction for the finite orbital size since this gives rise to a significant reduction in the calculated energy content of the minority component. A large orbit width model for ICRH is described which has been used to predict the global fast ion energies of both H and 3He minority ions during ICRH in the JET tokamak. The model is capable of explaining certain discrepancies which have previously been observed on JET between the measured fast ion energy contents and those which have been calculated using a zero orbit width model. The largest corrections to the global energy content of the fast ions are found to be ≈56% for low current H minority discharges in which the ion-electron slowing-down time is long (≈ 1.6 s at the magnetic axis). Within experimental scatter, we can place an upper limit of Dfast < 0.18 m2s−1 on the fast ion diffusion coefficient for any additional non-classical energy loss processes.

The energy confinement properties of low density, high ion temperature L- and H-mode plasmas are investigated. For L-mode plasmas it is shown that, although the global confinement is independent of density, the energy confinement in the central region is significantly better at low densities than at higher densities. The improved confinement appears to be associated with the steepness of the density gradient. For the H-mode phase, although the confinement at the edge is dramatically improved, which is once again associated with the steep density gradient in the edge region, the central confinement properties are essentially the same as for the standard L-mode. The results are compared in a qualitative manner with the predictions of the ion temperature gradient instability theory and appear to be in disagreement with some aspects of this theory.

RF heating and current drive with ion cyclotron waves and Lower Hybrid waves have been crucial for the development of the Optimized Shear scenario on JET to high performance. Peaked electron temperature profiles and improved energy confinement could be obtained with electron heating both from LHCD and ICRH during plasma current ramp up. ICRH and NBI comparisons allow to separate heating and fueling and suggest a dominant role of core heating in the formation of an Internal Transport Barrier (ITB). ICRH and NBI powers are there equivalent. Pressure profile control by varying the composition of centrally peaked ICRF and broader NBI deposition improves MHD stability. Current profile modifications in a wide range have been obtained with LHCD and in combination with NBI during current ramp up. During the high performance phase, however, LH coupling degrades strongly due to the steep edge density gradient resulting in a drop of the density in front of the LH antenna to the cut-off density. High fusion performance achieving simultaneously high beta values and bootstrap currents is predicted in scenario modeling using pressure and current profile control with ICRF and LHCD.

In a rotating plasma, with co-neutral beam injection (NBI), the Doppler shift of the NBI particles, as viewed in the frame of the plasma, can result in a significant reduction in the beam-driven (Ohkawa) current when the rotation is strong (i.e. with rotational Mach numbers, M ⩾ 0.5). The correction applies to the toroidal fast ion current calculated for a non-rotating plasma and is independent of the normal Z eff and electron trapping terms. A simple analytical model is presented to estimate the magnitude of the effect for plasmas with arbitrary toroidal rotation and the conditions where this is important have been identified. This model has been compared with the results from existing Monte Carlo neutral beam codes and found to reproduce their results. The important parameters in this problem are the ratio, , of the NBI injection particle velocity (in the laboratory frame) to the critical velocity of the plasma, and the ratio ρϕ = v ϕ/v crit which is related to the rotational Mach number. A phase plot in dimensionless (ρLab, ρϕ) space is presented which enables the fast ion current drive efficiencies to be compared for different tokamaks. For strongly rotating plasmas, the degradation in fast ion current efficiency is significant for ρLab ⩽ 1. However, when ρLab is larger than this, the degradation in fast ion current drive is less severe. Approaches to improve the fast ion current drive efficiency are briefly discussed.

Summary form only given, as follows. In the course of the preliminary tritium experiment (PTE) in JET, where combined D and T neutral beam injection generated a DT fusion power of 1.7 MW, ion cyclotron emission (ICE) in the frequency range up to 180 MHz was measured using an ICRF heating antenna as probe. The ICE spectrum showed a number of superthermal narrow, equally spaced emission lines which correspond to deuteron or alpha -particle ion cyclotron harmonics at the outer mid-plane edge (major radius R approximately 4 m). DD and DT power spectra are similar in form, and show split lines which, above approximately 100 MHz, merge into a continuum. With tritium injection, the ICE power increased by a factor comparable to the increase of neutron flux, indicating that 3.5 MeV fusion alpha -particles provide the free energy for generating ICE. The ICE power increases almost linearly with neutron flux over a range of six decades. The evolution of the ICE follows the rise of the neutron flux, but is delayed by a time comparable with the slowing-down time of the alpha -particles. The ICE intensity is anti-correlated with large amplitude edge-localized modes.

The performance of high current discharges can be improved by applying central ICRF heating before or shortly after the onset of sawtooth activity in the plasma current rise phase. Long sawtooth-free periods have been obtained which result in a transiently-enhanced discharge performance. High Te(0)=9-10.5 keV with peaked profiles Te(0)/(Te)=3-4 were obtained giving values of Ne(0)Te(0) up to 6*1020 (keV m-3). Improvements in Ti(0) and neutron production are observed. A best value of nD(0)Ti(0) tau E=1.65*1020 (m-3 keV s) was achieved. Local transport simulation shows that the electron and ion thermal diffusivities do not differ substantially in the two cases of current-rise (CR) and flat-top (FT) heating, the performance of the central plasma region being enhanced, in the case of current-rise, entirely by the elimination of the sawtooth instability. The maximum D-D reaction rate is enhanced by a factor of 2 compared to the flat-top value. An appreciable part of the reaction rate is attributed to 2nd harmonic deuterium (2 omega CD) heating. In all current-rise discharges radiation amounts to 25-50% of total power and Zeff remains roughly constant.

Observations of superthermal ion cyclotron emission from JET Ohmic-- and neutral-beam--heated discharges are presented. Previously unobserved narrow-band spectral features correspond in both cases to multiple harmonics and half-harmonics of the proton gyrofrequency at the outer edge of the plasma. In this region, fusion products with large radial excursions and injected fast ions produce anisotropic velocity distributions with positive perpendicular gradient. It is discussed how these may relax by maser action, giving rise to the observed localized radiation.

1. The identifiable C3 neurone of the snail (Helix aspersa) is a motoneurone for the tentacle retractor muscle (TRM) and is immunoreactive for the molluscan peptide FMRFamide. 2. Using bioassay, we show that this cell, as well as its target muscle, contains acetylcholine (ACh) and can synthesize ACh from its usual precursors. 3. Acetylcholinesterase was also detected in the TRM.

The site of action of corticosteroids in avoidance learning was investigated in 110 rats. Injection of cycloheximide, 30 min before one-trial training on a passive avoidance task suppressed corticosteroid secretion in response to footshock, and produced an avoidance deficit in a test 6 days later. However, an additional injection of hydrocortisone, either subcutaneously or intra-hippocampally within 5 min of training, restored the avoidance response in the test. Septal and hypothalamic injections of the hormone were ineffective in reversing the cycloheximide effect, whereas the effect of hormone injection into the amygdala was equivocal because of an increased level of activity. Corticosteroids secreted following an aversive experience appear to act upon the steroid-sensitive neurons in the hippocampus to influence the animal's later performance of passive avoidance response.

1. Embryonic mouse spinal cord neurones were maintained in a primary dissociated culture with a medium free from antibiotics. 2. Intracellular recordings were made and the effects of local application of the neuropeptide FMRFamide were tested on selected neurones. Two types of response were seen. 3. One response was depolarizing, with an accompanying decrease in conductance. This response was probably caused by a reduction in permeability to potassium ions. 4. The second type of response was accompanied by an increase in conductance. Such responses showed a wide variation in their reversal potentials between different neurones. A combination of permeability changes to sodium and chloride ions appeared to be responsible for these responses.

Four computer programs written for the BBC Model B microcomputer (coupled to a Unilab 8-bit interface) are discussed. These programs enable the system to be used as (1) a transient recorder, (2) a rapid signal averager, (3) a spike-train analyser and (4) an instrument for measuring the amplitude of single channel currents. Flow-charts illustrating the operation of each program are given along with a detailed discussion of how the programs may be used in the laboratory. The discussion is illustrated using recordings taken from experiments conducted on a range of neurobiological preparations.

A model is presented which permits selfconsistent calculation of the power deposition and velocity distribution of cyclotron resonance heated ions. The model is applied to minority heating of 3He in deuterium plasmas in the JET experiment. Good agreement is found between calculated and measured values of fusion yield and energy content of 3He ions. The results indicate that the power deposition is adequately described and that the fast ions slow down on electrons by Coulomb collisions. Measurements of the electron heating after the sawtooth crash show that about 50% of the energy content of the fast ions is retained in the centre after a sawtooth crash.

Projections of the (D)T fusion yield expected during fundamental ICRH heating of D in JET tritium plasmas are presented. The highest fusion multiplication factor, Q( identical to Pfus/PRF), is achieved for a relatively high plasma density (ne0>5*1019 m-3) and minority concentration ratio nD/nT approximately=20-40% with dipole antenna (k/sub /// approximately=7 m-1). The latter reduces mode conversion and maximizes the RF power coupled to the minority ions. The authors have used ray-tracing and global wave ICRH codes to calculate power deposition profiles; 80% is cyclotron damped by deuterium and 17% is coupled directly to electrons via TTMP and Landau damping. With launched RF power PRF=12 MW deposited approximately=0.3 m off-axis, the authors predict fusion powers Pfus up to approximately=8 MW for a range of JET plasmas with achieved plasma pressure ne0Te0=6*1020 keV m-3 and Zeff=2. Projecting to PRF=25 MW, Pfus increases in 17 MW with Zeff=2.

Measurements are presented of Balmer alpha line profiles from plasma ions which have been neutralized by charge exchange with injected neutral hydrogen beams in the DITE tokamak. Doppler broadening of the H? line is interpreted in terms of the ion temperature and line profiles are compared with neutral particle energy spectra in the regime where the plasma is transparent to the transport of neutrals from the core to the boundary. An asymmetry in the charge-exchange line profile is interpreted in terms of plasma rotation during neutral beam injection. The advantage of this method over conventional neutral analysis is seen in the context of the diagnosis of large, hot and dense tokamak plasmas.

An analysis of the attenuation behaviour of neutral beams injected for plasma heating is discussed, which yields a beam deposition profile for a known plasma density profile or which may provide a plasma density profile. The population of surviving beam trals at a penetration depth into a plasma is inferred from the excitation process of the fast neutrals. The excitation light is roughly independent of the electron temperature, and the line spectra can easily be distinguished by means of Dopplershift spectroscopy. Hydrogenic neutral beams typically consist of three energy species (E, E/2,E/3) with different survival probabilities. By comparing the excitation light from each species at several points along the beam path, it is possible to decode information on the plasma density through which the beam has passed. On the other hand, if the plasma density profile is known from other diagnostics, the fast ion deposition profile can be obtained. A preliminary result of single-chord measurements in Doublet III is presented.

Enregistrement intracellulaire des courants ioniques au niveau des neurones du ganglion cerebroide chez H.a. apres excitation par l'acetylcholine

In kindled rats, the administration of cysteamine (CSH, 200 mg/kg, i.p.) 4 h prior to a kindled seizure leads to long-term (up to 10 days) inhibition of kindled seizures. CSH (200 mg/kg, i.p.) also induces myoclonic seizures in kindled rats. We suggest that the long-term inhibition of kindled seizures might be the result of the myoclonus, not the somatostatin depletion as previously suggested. Prior administration of the short-acting benzodiazepine midazolam (5 mg/kg, i.p.) eliminated the CSH-induced myoclonus and prevented the long-term inhibition of kindled seizures. These results suggest that the CSH-induced the long-term inhibition of kindled seizures is the result of an interaction between the myoclonic seizure and a subsequent kindled seizure.