We present an ultrafast neural network (NN) model, QLKNN, which predicts core tokamak transport heat and particle fluxes. QLKNN is a surrogate model based on a database of 300 million flux calculations of the quasilinear gyrokinetic transport model QuaLiKiz. The database covers a wide range of realistic tokamak core parameters. Physical features such as the existence of a critical gradient for the onset of turbulent transport were integrated into the neural network training methodology. We have coupled QLKNN to the tokamak modelling framework JINTRAC and rapid control-oriented tokamak transport solver RAPTOR. The coupled frameworks are demonstrated and validated through application to three JET shots covering a representative spread of H-mode operating space, predicting turbulent transport of energy and particles in the plasma core. JINTRAC-QLKNN and RAPTOR-QLKNN are able to accurately reproduce JINTRAC-QuaLiKiz T i,e and n e profiles, but 3 to 5 orders of magnitude faster. Simulations which take hours are reduced down to only a few tens of seconds. The discrepancy in the final source-driven predicted profiles between QLKNN and QuaLiKiz is on the order 1%-15%. Also the dynamic behaviour was well captured by QLKNN, with differences of only 4%-10% compared to JINTRAC-QuaLiKiz observed at mid-radius, for a study of density buildup following the L-H transition. Deployment of neural network surrogate models in multi-physics integrated tokamak modelling is a promising route towards enabling accurate and fast tokamak scenario optimization, Uncertainty Quantification, and control applications.

The JET 2019-2020 scientific and technological programme exploited the results of years of concerted scientific and engineering work, including the ITER-like wall (ILW: Be wall and W divertor) installed in 2010, improved diagnostic capabilities now fully available, a major Neutral Beam Injection (NBI) upgrade providing record power in 2019-2020, and tested the technical & procedural preparation for safe operation with tritium. Research along three complementary axes yielded a wealth of new results. Firstly, the JET plasma programme delivered scenarios suitable for high fusion power and alpha particle physics in the coming D-T campaign (DTE2), with record sustained neutron rates, as well as plasmas for clarifying the impact of isotope mass on plasma core, edge and plasma-wall interactions, and for ITER pre-fusion power operation. The efficacy of the newly installed Shattered Pellet Injector for mitigating disruption forces and runaway electrons was demonstrated. Secondly, research on the consequences of long-term exposure to JET-ILW plasma was completed, with emphasis on wall damage and fuel retention, and with analyses of wall materials and dust particles that will help validate assumptions and codes for design & operation of ITER and DEMO. Thirdly, the nuclear technology programme aiming to deliver maximum technological return from operations in D, T and D-T benefited from the highest D-D neutron yield in years, securing results for validating radiation transport and activation codes, and nuclear data for ITER.

In this paper we present in-situ satellite data, theory and laboratory validation that show how small scale collisionless shocks and mini-magnetospheres can form on the electron inertial scale length. The resulting retardation and deflection of the solar wind ions could be responsible for the unusual "lunar swirl" patterns seen on the surface of the Moon.Miniature magnetospheres have been found to exist above the lunar surface [1] and are closely related to features known as "lunar swirls" [2]. Mini-magnetospheres exhibit features that are characteristic of normal planetary magnetospheres namely a collisionless shock. Here we show that it is the electric field associated with the small scale collisionless shock that is responsible for deflecting the incoming solar wind around the minimagnetosphere. These ions impacting the lunar surface resulting in changes to the appearance of the albedo of the lunar "soil" [2]. The form of these swirl patterns therefore, must be dictated by the shapes of the collisionless shock.Collisionless shocks are a classic phenomena in plasma physics, ubiquitous in many space and astrophysical scenarios [3]. Well known examples of collisionless shocks exist in the heliosphere, where the shock is formed by the solar wind interacting with a magnetised planet. What is a surprise is the size of the mini-magnetospheres, of the order of several 100 km; orders of magnitude smaller than the planetary versions. Results from various lunar survey missions have built up a good picture of these collisionless shocks.These collisionless shocks have a characteristic structure in which the ions are reflected from a rather narrow layer, of the order of the electron skin depth c/ω pe (where c is the speed of light and ω pe is the electron plasma frequency), by an electrostatic field that is a consequence of the magnetised electrons and unmagnetised ions. The narrow discontinuity in the shock structure produces a specular reflected ion component with a velocity equal to or greater than the incoming solar wind velocity. The reflected ions from a counter-propagating component to the solar wind flow that form the magnetic foot region, which extends about an ion Larmor orbit upstream from the shock. This occurs when the Mach number (the ratio of flow velocity to Alfvén velocity) is of the order 3 or less.We have carried out laboratory experiments using a plasma wind tunnel, to investigate mini-magnetospheres

Alpha particles with energies on the order of megaelectronvolts will be the main source of plasma heating in future magnetic confinement fusion reactors. Instead of heating fuel ions, most of the energy of alpha particles is transferred to electrons in the plasma. Furthermore, alpha particles can also excite Alfvénic instabilities, which were previously considered to be detrimental to the performance of the fusion device. Here we report improved thermal ion confinement in the presence of megaelectronvolts ions and strong fast ion-driven Alfvénic instabilities in recent experiments on the Joint European Torus. Detailed transport analysis of these experiments reveals turbulence suppression through a complex multi-scale mechanism that generates large-scale zonal flows. This holds promise for more economical operation of fusion reactors with dominant alpha particle heating and ultimately cheaper fusion electricity.

The spheromak device SPHEX has been modified by adding a current-carrying rod along the geometric axis, providing a preexisting toroidal field. We show that plasma can be successfully injected into such a field from a helicity source; the field assists plasma ejection from the gun and improves the coupling between gun and plasma, so that T^., Ti, and the toroidal current all increase with rod current. The mechanism of plasma sustainment appears to be the same as that of the spheromak. These results represent a step towards the achievement of steady-state tokamak operation.PACS numbers: 52.55.Hc SPHEX is a gun-injected spheromak device similar to the Compact Toroid Experiment (CTX) [1] at the Los Alamos National Laboratory; it is described in the preceding Letter [2], which presents results suggesting the outline of a relaxation mechanism involving a largescale coherent mode of oscillation which, we believe, drives the toroidal current in the plasma.In this paper we describe the modification of SPHEX in which the plasma is injected into a pre-existing toroidal field. This is generated by a current-carrying rod placed along the geometric axis of both the gun and the flux conserver (see Fig. 1 of Ref. [2]). This configuration was suggested by results from the Heidelberg HSE experiment [3]; a similar scheme has been proposed [4] to sustain a tokamak discharge by helicity injection. Our results (presented briefly in Ref.[5]) show for the first time that a coaxial helicity source can form and sustain a plasma in an externally generated toroidal field; although it is not clear that our configuration can properly be described as a tokamak, our results suggest that sustainment in a tokamak regime may be possible.In an ideal spheromak configuration the toroidal field vanishes at the wall but the safety factor q does not [6]; in fact, in a closed spheromak it varies by < 20% over the radius. We expected that because of the tight aspect ratio and strong toroidicity, the addition of toroidal field might lead to a significant shear even at modest rod currents. We have therefore studied numerically the effects of toroidal field on force-free equilibria described by VxB=/iB, both for constant p (the relaxed state [7]) and for p =='p^,+c\i//y/o, where if/ is the poloidal flux coordinate, y/G is the gun solenoid flux, and p^ is the value of p at the wall. This form has been used to describe spheromak equilibria in CTX [8]; with j/^ = 0 at the wall, a driven spheromak has <: < 0, the relaxed state r=0, and a decaying plasma c > 0 [8]. Solutions are obtained [9] by the SOR (successive over-relaxation) method applied to the corresponding Grad-Shafranov equation (linearized when p is not a constant). We have so far studied the driven or relaxed cases, c :< 0.We first consider spheromaklike solutions with IR =0. Figure 1 (a) shows a typical set of flux surfaces. Since this system has flux entering and leaving through the electrodes, it includes a separatrix dividing the field into regions of *'short open flux,'' *'long open flux,'' and...

Steady-state current drive by helicity injection has been demonstrated in the SPHEX spheromak. In order to understand the processes by which plasma toroidal current is driven on the closed flux, it is necessary first to diagnose the internal magnetic field and current structure. A method is described which fits force-free equilibria to magnetic field data from a linear insertable probe. It is shown that the overall field configuration and associated global parameters can be well fitted, though the detailed shape of the internal current profile is sensitive to small errors in the data and the choice of fitting function. The current drive process is closely associated with fluctuations having toroidal mode number n = 1. We investigate in detail shots with Ti gettering, when the n = 1 mode is episodic, showing that the current drive mechanism is switched on and off for periods during a single shot, corresponding to the presence or absence of the mode. This provides definitive evidence that the n = 1 mode is responsible for the current drive.

The Mega Amp Spherical Tokamak (MAST) was a low aspect ratio device (R/A = 0.85/0.65 ~ 1.3) with similar poloidal cross-section to other medium-size tokamaks. The physics programme concentrates on addressing key physics ___________________________________________________________________________

Here we describe a new experiment to test the shielding concept of a dipolelike magnetic field and plasma, surrounding a spacecraft forming a 'mini magnetosphere'. Initial laboratory experiments have been conducted to determine the effectiveness of a magnetized plasma barrier to be able to expel an impacting, low beta, supersonic flowing energetic plasma representing the solar wind. Optical and Langmuir probe data of the plasma density, the plasma flow velocity and the intensity of the dipole field clearly show the creation of a narrow transport barrier region and diamagnetic cavity virtually devoid of energetic plasma particles. This demonstrates the potential viability of being able to create a small 'hole' in a solar wind plasma, of the order of the ion Larmor orbit width, in which an inhabited spacecraft could reside in relative safety. The experimental results have been quantitatively compared with a 3D particle-incell 'hybrid' code simulation that uses kinetic ions and fluid electrons, showing good qualitative agreement and excellent quantitative agreement. Together the results demonstrate the pivotal role of particle kinetics in determining generic plasma transport barriers.