Demonstrating improved confinement of energetic ions is one of the key goals of the Wendelstein 7-X (W7-X) stellarator. In the past campaigns, measuring confined fast ions has proven to be challenging. Future deuterium campaigns would open up the option of using fusion-produced neutrons to indirectly observe confined fast ions. There are two neutron populations: 2.45 MeV neutrons from thermonuclear and beam-target fusion, and 14.1 MeV neutrons from DT reactions between tritium fusion products and bulk deuterium. The 14.1 MeV neutron signal can be measured using a scintillating fiber neutron detector, whereas the overall neutron rate is monitored by common radiation safety detectors, for instance fission chambers. The fusion rates are dependent on the slowing-down distribution of the deuterium and tritium ions, which in turn depend on the magnetic configuration via fast ion orbits. In this work, we investigate the effect of magnetic configuration on neutron production rates in W7-X. The neutral beam injection, beam and triton slowing-down distributions, and the fusion reactivity are simulated with the ASCOT suite of codes. The results indicate that the magnetic configuration has only a small effect on the production of 2.45 MeV neutrons from DD fusion and, particularly, on the 14.1 MeV neutron production rates. Despite triton losses of up to 50 %, the amount of 14.1 MeV neutrons produced might be sufficient for a time-resolved detection using a scintillating fiber detector, although only in high-performance discharges.

After completing the main construction phase of Wendelstein 7-X (W7-X) and successfully commissioning the device, first plasma operation started at the end of 2015. Integral commissioning of plasma start-up and operation using electron cyclotron resonance heating (ECRH) and an extensive set of plasma diagnostics have been completed, allowing initial physics studies during the first operational campaign. Both in helium and hydrogen, plasma breakdown was easily achieved. Gaining experience with plasma vessel conditioning, discharge lengths could be extended gradually. Eventually, discharges lasted up to 6 s, reaching an injected energy of 4 MJ, which is twice the limit originally agreed for the limiter configuration employed during the first operational campaign. At power levels of 4 MW central electron densities reached 3  ×  1019 m−3, central electron temperatures reached values of 7 keV and ion temperatures reached just above 2 keV. Important physics studies during this first operational phase include a first assessment of power balance and energy confinement, ECRH power deposition experiments, 2nd harmonic O-mode ECRH using multi-pass absorption, and current drive experiments using electron cyclotron current drive. As in many plasma discharges the electron temperature exceeds the ion temperature significantly, these plasmas are governed by core electron root confinement showing a strong positive electric field in the plasma centre.

The behaviour of the fast particle population during 18 keV hydrogen and 26 keV deuterium neutral beam injection in deuterium plasmas is investigated. Experiments reveal large fast ion losses. The experimental results are confirmed using different types of modelling: simulation using the NUBEAM module, solution of the Boltzmann kinetic equation and solution of the 3D fast ion tracking algorithm. The dynamics of the energetic particle redistribution and losses during sawtooth oscillation and toroidal Alfvén eigenmodes are studied. A method to decrease fast ion losses under the current conditions (0.4 T, 0.2 MA) is shown. The influence of the plasma parameters on the energetic ion confinement rate is investigated. Modelling for the Globus-M2 conditions (1 T, 0.5 MA) is performed.

The existing Globus-M machine [1] is a low aspect ratio compact tokamak (R = 0.36 m, a = 0.24 m) with high specific ohmic and auxiliary heating power. First plasma was achieved in Globus-M in 1999. The machine has demonstrated practically all of the project objectives ever since. Target design parameters (aspect ratio-1.5, 2 − X-point configuration, vertical elongation-2.2, traiangularity-0.45, average density-1.0•10 20 m −3 , plasma current-0.3 MA, toroidal beta-12%, auxiliary heating power-1 MW) [2] were achieved and some of them overcame [3,4]. Also Globus-M

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The targeted plasma parameters of the compact spherical tokamak (ST) Globus-M have basically been achieved. The reasons that prevent further extension of the operating space are discussed. The operational limits of Globus-M together with an understanding of the limiting reasons form the basis for defining the design requirements for the next-step, Globus-M2. The recent experimental and theoretical results achieved with Globus-M are discussed, the operational problems and the research programme are summarized and finally, the targeted Globus-M2 parameters are presented. The magnetic field and plasma current in Globus-M2 will be increased to 1 T and 0.5 MA, respectively. The plasma dimensions will remain unchanged. With auxiliary heating at a high average plasma density, the temperatures will be in the keV range and the collisionality parameter with ν * 1 will define the operational conditions. Noninductive current drive will be a major element of the programme. The engineering design issues of Globus-M2 tokamak are discussed and the technical tokamak parameters are confirmed by thermal load and stress analysis simulations. The experimental results obtained on Globus-M2 and the limits of its performance should clarify the feasibility of an ST-based super compact neutron source.

The first experiments on noninductive current drive (CD) using lower hybrid waves in a spherical tokamak are described. Waves at 2.45 GHz were launched by a 10 waveguide grill with 120° phase shift between neighbouring waveguides. The experimental results for a novel poloidal slowing-down scheme are described. The CD efficiency is found to be somewhat larger than that predicted theoretically whilst at the same time being somewhat less than that for standard tokamak lower hybrid CD. Geodesic acoustic modes (GAM) have been discovered in Globus-M. GAMs are localized 2-3 cm inside the separatrix. The GAM frequency agrees with theory. The mode structures of plasma density and magnetic field oscillation at the GAM frequency have been studied. Fast particle confinement during neutral beam injection has been investigated and numerically simulated. Alfvén instabilities excited by fast particles were detected by a toroidal Mirnov probe array. Their excitation conditions are discussed and the dynamics of fast ion losses induced by Alfvén eigenmodes is presented. Preliminary experiments on the isotopic effect influence on global confinement in the ohmic Nuclear Fusion

Experiments and simulations to achieve high values of plasma parameters at the Globus-M spherical tokamak (ST) at moderate auxiliary heating power (0.2-0.8 MW) are described. Important distinguishing features are the low edge safety factor range, which is unusual for spherical tokamaks, 2.7 < q < 5 and small plasma-outer wall space (3-5 cm). High ion heating efficiency with NB injection was demonstrated. Results of numerical simulation of fast ion trajectories are described and fast ion generation during NB injection and ICR heating is discussed. Also results on their confinement and slowing down processes investigation are presented. Reasons for achievement of high IC heating efficiency are outlined. Reliable H-mode regime achievement is described. Transport ASTRA modeling demonstrated that during NB heated H-mode ion heat diffusivity remains neoclassical and the particle diffusion coefficient inside transport barrier decreases significantly. RGTi divertor tile analysis was performed after irradiation by plasma during big number of shots (10000 shots in average). Mixed layer composition is measured and deuterium retention in different tokamak first wall area is estimated. Plasma jet injection experiments with upgraded plasma jet are described. Jet penetration to the plasma center with immense increase of density and temperature drop is proved and analogy with pellet injection is outlined.