Real-time simultaneous control of several radially distributed magnetic and kinetic plasma parameters is being investigated on JET, in view of developing integrated control of advanced tokamak scenarios. This paper describes the new model-based profile controller which has been implemented during the 2006–2007 experimental campaigns. The controller aims to use the combination of heating and current drive (H&CD) systems—and optionally the poloidal field (PF) system—in an optimal way to regulate the evolution of plasma parameter profiles such as the safety factor, q(x), and gyro-normalized temperature gradient, . In the first part of the paper, a technique for the experimental identification of a minimal dynamic plasma model is described, taking into account the physical structure and couplings of the transport equations, but making no quantitative assumptions on the transport coefficients or on their dependences. To cope with the high dimensionality of the state space and the large ratio between the time scales involved, the model identification procedure and the controller design both make use of the theory of singularly perturbed systems by means of a two-time-scale approximation. The second part of the paper provides the theoretical basis for the controller design. The profile controller is articulated around two composite feedback loops operating on the magnetic and kinetic time scales, respectively, and supplemented by a feedforward compensation of density variations. For any chosen set of target profiles, the closest self-consistent state achievable with the available actuators is uniquely defined. It is reached, with no steady state offset, through a near-optimal proportional-integral control algorithm. Conventional optimal control is recovered in the limiting case where the ratio of the plasma confinement time to the resistive diffusion time tends to zero. Closed-loop simulations of the controller response have been performed in preparation for experiments, and typical results are shown. Finally, in the last section of the paper, the first experimental results using this dynamic-model approach to control the plasma current and the safety factor profile on JET, either with the three H&CD systems or also with the PF system as an additional actuator, are presented and discussed.

A one-dimensional analytical model of the sheath in a negative ion source, such as those proposed for heating and diagnostic beams on present and future fusion devices, has been developed. The model, which is collisionless, describes the transport of surface produced negative ions from a cathode, across the sheath to a plasma containing electrons, positive ions and negative ions. It accounts for the situation where the emitted flux of negative ions is greater than the space charge limit, where the electric field at the cathode is negative, and a virtual cathode is formed. It is shown that, in the presence of a virtual cathode, there is a maximum current density of negative ions that can be transported across the sheath into the plasma. Furthermore, for high rates of surface production the virtual cathode persists regardless of the negative bias applied to the cathode, so that the current density transported across the sheath is limited. This is a significant observation and implies that present negative ion sources may not be exploiting all of the surface production available. The model is used to calculate the transported negative ion flux in a number of examples. The limitations of the model and proposed future work are also discussed.

Several Collisional-Radiative (CR) models [1, 2, 3] have been developed to calculate the attenuation and the population of excited states of hydrogen or deuterium beams injected into tokamak plasmas. The datasets generated by these CR models are needed for the modelling of beam ion deposition and (excited) beam densities in current experiments, and the reliability of this data will be crucial to obtain helium ash densities on ITER combining charge exchange and beam emission spectroscopy. Good agreement between the different CR models for the Neutral Beam (NB) is found, if corrections to the fundamental cross sections are taken into account. First the H a and H b beam emission spectra from JET are compared with the expected intensities. Second, the line ratios within the Stark multiplet are compared with the predictions of a sublevel resolved model. The measured intensity of the full multiplet is ≈30% lower than expected on the basis of beam attenuation codes and the updated beam emission rates, but apart from the atomic data this could also be due to the characterization of the NB path and line of sight integration and the absolute calibration of the optics. The modelled n = 3 to n = 4 population agrees very well with the ratio of the measured H a to H b beam emission intensities. Good agreement is found as well between the neutral beam power fractions measured with beam emission in plasma and on the JET Neutral Beam Test Bed. The Stark line ratios and s/p intensity ratio deviate from a statistical distribution, in agreement with the CR model in parabolic states from Marchuk et al. [4]. MOTIVATIONPowerful neutral hydrogen or deuterium beams provide the dominant external heating and momentum input in most large scale tokamak experiments. For the interpretation of neutral beam (NB) heated discharges, detailed knowledge is required about the energy distribution of the neutrals (power fractions) and the attenuation of the beams in order to obtain radial proles of the fast ion deposition and hence of the heating, torque and beam driven current. For the quantitative interpretation of Charge eXchange (CX) spectra, the local NB fluxes and population of excited states in the beam are needed to convert CX emissivities into local impurity densities. All these calculations strongly rely on the accuracy of the atomic data for the NB that is provided by Collisional-Radiative (CR) models of the beam [1, 2, 3, 5, 6].When the Beam Emission Spectrum (BES) was recorded for the first time, it was immediately proposed to monitor the beam attenuation, and hence the accuracy of the effective beam stopping cross sections, by using the observed beam emission intensities [7, 8]. This replaces the accumulated error on the beam attenuation along the beam path [9], by a local error in the beam emission rate. Beam emission, when combined with Charge eXchange Recombination Spectroscopy (CXRS), also has the potential of reducing the need of an absolute calibration of the CXRS spectra and a calculation of the intersection integral between a lin...

Current hole plasmas in JET are those in which the current density within r/a < 0.3 is close to zero. Tritium ions injected quasi-tangentially into such plasmas can fulfil a stagnation condition whereby their vertical drift is cancelled by the poloidal component of their parallel velocity. These ions remain trapped at approximately 0.2 m from the plasma axis and can be detected by a distortion in the neutron emission profile. Numerical modelling of the steady-state distribution reproduces the experimental results while the decay of neutron emission after the cessation of injection is found to be sensitive to small changes in the q-profile.

A one dimensional model of the magnetic multipole volume plasma source has been developed for use in intense ion/neutral atom beam injectors. The model uses plasma transport coefficients for particle and energy flow to create a detailed description of the plasma parameters along an axis parallel to that of the extracted beam. Primarily constructed for applications to neutral beam injection systems on fusion devices, the model concentrates on the hydrogenic isotopes but can be extended to any gas by substitution of the relevant masses, cross sections and rate coefficients. The model considers the flow of fast ionizing electrons that create the ratios of the three hydrogenic isotope ion species, H + , H 2 + , H 3 + (and similarly for deuterium and tritium) as they flow towards the beam extraction electrode, together with the production of negative hydrogenic ions through volume processes. The use of detailed energy balance in the discharge allows the determination of the fraction of the gas density that is in an atomic state and also the gas temperature as well as the electron temperatures and plasma potential. Comparisons are made between the results of the model and experimental measurements in deuterium from a number of different filament driven sources used on beam heating facilities.

Trace tritium experiments (TTE) on JET were analysed using Monte Carlo modelling of the neutron emission resulting from the neutral beam injection (NBI) of short (∼300 ms) tritium (T) beam blips into reversed shear, hybrid ELMy H-mode and L-mode deuterium plasmas for a wide range of plasma parameters. The calculated neutron fluxes from deuterium-tritium (DT) reactions could only be made consistent with all plasmas by applying an artificial reduction of the T beam power in the modelling of between 20% and 40%. A similar discrepancy has previously been observed in both JET (Gorini et al 2004 Proc. 31st EPS Conf. on Plasma Physics (London, UK) vol 28G (ECA)) and TFTR (Ruskov et al 1999 Phys. Rev. Lett. 82 924), although no mechanism has yet been found that could explain such a difference in the measured T beam power. Applying this correction in the T beam power, good agreement between calculated and measured DT neutron emission profiles was obtained in low to moderate line averaged density (n e < 4 × 10 19 m −3 ) ELMy H-Mode plasmas assuming that the fast beam ions experience no, or relatively small, anomalous diffusion (D an 0.5 m 2 s −1 ). However, the modelled neutron profiles do not agree with measurements in higher density plasmas using the same assumption and the disagreement between the measured and calculated shape of the neutron profile increases with plasma density. In this paper it is demonstrated that large anomalous losses of fast ions have to be assumed in the simulations to improve agreement between experimental and simulated neutron profiles, characterized

Results are presented from the JET Trace Tritium Experimental (TTE) campaign using minority tritium (T) plasmas (n T /n D < 3%). Thermal tritium particle transport coefficients (D T , v T ) are found to exceed neo-classical values in all regimes, except in ELMy H-modes at high densities and in the region of internal transport barriers (ITBs) in reversed shear plasmas. In ELMy H-mode dimensionless parameter scans, at q 95 ∼ 2.8 and triangularity δ = 0.2, the T particle transport scales in a gyro-Bohm manner in the inner plasma (r/a < 0.4), whilst the outer plasma particle transport scaling is more Bohm-like. Dimensionless parameter scans show contrasting behaviour for the trace particle confinement (increases with collisionality, ν * and β) and bulk energy confinement (decreases with ν * and is independent of β). In an extended ELMy H-mode data set, with ρ * , ν * , β and q varied but with neo-classical tearing modes (NTMs) either absent or limited to weak, benign core modes (4/3 or above), the multiparameter fit to the normalized diffusion coefficient in the outer plasma (0.65 < r/a < 0.8) gives D T /B φ ∼ ρ * 2.46 ν * −0.23 β −1.01 q 2.03 . In hybrid scenarios (q min ∼ 1, low positive shear, no sawteeth), the T particle confinement is found to scale with increasing triangularity and plasma current. Comparing regimes (ELMy H-mode, ITB plasma and hybrid scenarios) in the outer plasma region, a correlation of high values of D T with high values of v T is seen. The normalized diffusion coefficients for the hybrid and ITB scenarios do not fit the scaling derived for ELMy H-modes. The normalized tritium diffusion scales with normalized poloidal Larmor radius (ρ * θ = qρ * ) in a manner close to gyro-Bohm (∼ρ * 3 θ ), a See annex of Pamela et al this conference, paper OV/1-2.

Demountable superconducting magnet coils would offer significant benefits to commercial nuclear fusion power plants. Whether large pressed joints or large soldered joints provide the solution for demountable fusion magnets, a critical component or building block for both will be the many, smaller-scale joints that enable the supercurrent to leave the superconducting layer, cross the superconducting tape and pass into the solder that lies between the tape and the conductor that eventually provides one of the demountable surfaces. This paper considers the electrical and thermal properties of this essential component part of demountable high temperature superconducting (HTS) joints by considering the fabrication and properties of jointed HTSs consisting of a thin layer of solder (In52Sn48 or Pb38Sn62) sandwiched between two rare-earth-Ba2Cu3O7 (REBCO) second generation HTS coated conductors (CCs). The HTS joints are analysed using numerical modelling, critical current and resistivity measurements on the joints from 300 to 4.2 K in applied magnetic fields up to 12 T, as well as scanning electron microscopy studies. Our results show that the copper/silver layers significantly reduce the heating in the joints to less than a few hundred mK. When the REBCO alone is superconducting, the joint resistivity (RJ) predominantly has two sources, the solder layer and an interfacial resistivity at the REBCO/silver interface (∼25 nΩ cm2) in the as-supplied CCs which together have a very weak magnetoresistance in fields up to 12 T. We achieved excellent reproducibility in the RJ of the In52Sn48 soldered joints of better than 10% at temperatures below Tc of the REBCO layer which can be compared to variations of more than two orders of magnitude in the literature. We also show that demountable joints in fusion energy magnets are viable and need only add a few percent to the total cryogenic cost for a fusion tokamak.