Fusion product losses due to fishbone instabilities in deuterium JET plasmas

During development of a high-performance hybrid scenario for future deuterium–tritium experiments on the Joint European Torus, an increased level of fast ion losses in the MeV energy range was observed during the instability of high-frequency n  =  1 fishbones. The fishbones are excited during deuterium neutral beam injection combined with ion cyclotron heating. The frequency range of the fishbones, 10–25 kHz, indicates that they are driven by a resonant interaction with the NBI-produced deuterium beam ions in the energy range  ≤120 keV. The fast particle losses in a much higher energy range are measured with a fast ion loss detector, and the data show an expulsion of deuterium plasma fusion products, 1 MeV tritons and 3 MeV protons, during the fishbone bursts. An MHD mode analysis with the MISHKA code combined with the nonlinear wave-particle interaction code HAGIS shows that the loss of toroidal symmetry caused by the n  =  1 fishbones affects strongly the confinement of non-resonant high energy fusion-born tritons and protons by perturbing their orbits and expelling them. This modelling is in a good agreement with the experimental data.This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053 and from the RCUK Energy Programme [grant No EP/P012450/1]. To obtain further information on the data and models underlying this paper please contact [email protected] . The views and opinions expressed herein do not necessarily reflect those of the European CommissionPeer ReviewedPostprint (author's final draft

Effect of the divertor configuration on the JET edge radial electric field

The role of the divertor configuration and divertor plasma physics on the L???H transition is poorly understood, leading to large uncertainties in predicting the L???H power threshold in future devices. This contribution reports on edge perpendicular plasma flow measurements by Doppler backscattering in JET L???H transition experiments with the outer divertor strike-point at different positions: horizontal target (HT), vertical target (VT), and in the corner configuration (between the HT and VTs). The edge perpendicular flow was found to be significantly affected by changes in the divertor configuration in the region inside the separatrix. Our results do not show evidence for the existence of a critical edge flow shear needed to achieve H-mode for different divertor configurations, with a larger shear observed for the VT configuration. No significant change in the shear flow and in the density fluctuation level is measured preceding the L???H transition in the region just inside the separatrix. The dynamics of the L???H transitions are also influenced by the divertor configuration with divertor oscillations (DOs) observed only in the HT configuration. Interestingly, DOs are associated with marked changes in the edge perpendicular flow around the separatrix

Structure of the JET edge radial electric field in He and D plasmas

El primer autor es Carlos Silva, experto en medidas del plasma por reflectometr??a. Soy la segunda autora, ya que se estudian mis experimentos. Se acababa de instalar en JET un sistema de reflectometr??a Doppler que permite medir la velocidad de las fluctuaciones de densidad del plasma en la direcci??n perpendicular a la l??nea de campo magn??tico loca, v_perp. Tal velocidad tiene una componente dominante debida al campo el??ctrico radial local (E_r) del plasma. En el borde del plasma se observa que el perfil de E_r o v_perp a menudo tiene un pozo muy localizado en la zona del gradiente de densidad, dentro de la separatriz que delimita al plasma, y una colina centrada en la separatriz. Hay muchos modelos de transici??n L-H que se basan en la suposici??n de que el gradiente de la rotaci??n perpendicular rasga los remolinos de turbulencia que dan lugar al transporte elevado en el modo L (bajo confinamiento). Se ha llegado a decir que la transici??n L-H requiere un valor critico del pozo de E_r. En este art??culo se estudia el perfil de v_perp en plasmas de Deuterio y de Helio justo antes de la transici??n L-H (experimentos descritos en la aportaci??n 1). En primer lugar se estudian plasmas con calentamiento por inyecci??n de part??culas r??pidas neutras (Neutral Beam Injection, NBI). Tanto en plasmas de Deuterio como de Helio se observa que para valores de densidad bajos el perfil radial de v_perp apenas tiene pozo, y tiene una colina elevada. Seg??n sube la densidad la colina decrece hasta desaparecer y se forma un pozo cada vez m??s profundo. Plasmas de Deuterio calentados con ondas (Ion Cyclotron Resonant Heating, ICRH) muestran perfiles cualitativamente semejantes a los de NBI, pero con una variaci??n de la profundidad del pozo mucho m??s d??bil en funci??n de la densidad. Se concluye que no hay un valor cr??tico del pozo, ni de la colina, necesarios para la transici??n L-H. El resultado m??s interesante que se presenta es un estudio de la evoluci??n del perfil de v_perp seg??n aumenta la potencia de calentamiento hasta que ocurre la transici??n. La mayor??a de los modelos de transici??n L-H suponen que el pozo de v_perp se hace paulatinamente m??s profundo seg??n aumenta el calentamiento, hasta llegar al valor cr??tico. Aqu?? se muestra (Fig. 7) que ??ste no es el caso. En cu??nto se entra en modo L (con calentamiento por ondas) el plasma tiene b??sicamente el mismo perfil de v_perp. Si acaso la profundidad del pozo decrece muy despacio. La ??nica carencia de este estudio es que la resoluci??n temporal es limitada: se necesitan 300 ms para medir un perfil de v_perp. El tiempo de confinamiento del plasma es de ??se orden, y la rampa de potencia es muy lenta, del orden de 1MW/s. Por lo tanto no podemos medir c??mo cambia v_perp justo antes de la transici??n a escalas de tiempo m??s cortas. A pesar de eso es evidente que el modelo convencional de la transici??n no se ajusta a nuestras observaciones.Perpendicular velocity, v???, measurements have been obtained in JET experiments designed to investigate the underlying mechanisms influencing the L???H power threshold. L???H transitions were induced by using both NBI and ICRH in deuterium and helium plasmas. The v??? profile in the low density branch of the L???H transition has a modest or even no well and a marked peak near the separatrix in NBI heated discharges for both D and He plasmas, with a sharper SOL peak for He plasmas. As the line-averaged density increases, the SOL v??? peak decreases, in agreement with the modifications in the electron temperature profile at the divertor target, while the v??? well becomes deeper. Nevertheless, even in the high density branch, a shallow v??? well is found at the L???H transition, v??? ??? 1???2 km s???1, which is lower by a factor of about two than the contribution from the diamagnetic term. No evidence for the existence of a critical value in v??? is found at JET, particularly for helium plasmas. This may be explained by the existence of an edge toroidal flow relevant mainly at low density where the power threshold is high. In addition, no significant change in the edge v??? is measured preceding the L???H transition

Power balance analysis at the L-H transition in JET-ILW NBI-heated deuterium plasmas

The understanding of the physics underlying the L-H transition has strong implications for ITER experimental reactor and demonstration power plant (DEMO). In many tokamaks, including JET, it has been observed that, at a particular plasma density, ne,min, the power necessary to access H-mode PL-H is minimum. In the present work, L-H transitions of JET deuterium plasmas heated by neutral beam injection (NBI) are studied for the first time by means of a power balance analysis to characterize the main contributions in the transition, through integrated transport modelling. In the pulses analysed, we do observe a minimum of the L-H power threshold in density, indicating the presence of density branches and of ne,min. Electron and ion heat fluxes at the transition are estimated separately. The electron/ion equipartition power results in favour of the ions, as shown by QuaLiKiz quasilinear gyrokinetic simulations, which predict a larger ion transport that causes Te > Ti. The resulting edge ion heat flux also shows a clear change of slope below ne,min, similarly to ASDEX-Upgrade (AUG) NBI pulses (Ryter et al 2014 Nucl. Fusion 54 083003). JET NBI data are compared to radio-frequency heated AUG and Alcator C-mod pulses (Schmidtmayr et al 2018 Nucl. Fusion 58 056003), showing a different trend of the power, coupled to ions at the L-H transition with respect to the linearity observed in the radio-frequency heated plasmas. The presence of ne,min and the role of the ion heat flux is discussed in the paper, although it seems it is not possible to explain the presence of a PL-H minimum in density by a critical ion heat flux and by the equipartition power for the JET NBI-heated plasmas analysed

Recent progress in L–H transition studies at JET : tritium, helium, hydrogen and deuterium

We present an overview of results from a series of L–H transition experiments undertaken at JET since the installation of the ITER-like-wall (JET-ILW), with beryllium wall tiles and a tungsten divertor. Tritium, helium and deuterium plasmas have been investigated. Initial results in tritium show ohmic L–H transitions at low density and the power threshold for the L–H transition (PLH) is lower in tritium plasmas than in deuterium ones at low densities, while we still lack contrasted data to provide a scaling at high densities. In helium plasmas there is a notable shift of the density at which the power threshold is minimum () to higher values relative to deuterium and hydrogen references. Above (He) the L–H power threshold at high densities is similar for D and He plasmas. Transport modelling in slab geometry shows that in helium neoclassical transport competes with interchange-driven transport, unlike in hydrogen isotopes. Measurements of the radial electric field in deuterium plasmas show that Er shear is not a good indicator of proximity to the L–H transition. Transport analysis of ion heat flux in deuterium plasmas show a non-linearity as density is decreased below . Lastly, a regression of the JET-ILW deuterium data is compared to the 2008 ITPA scaling law

An Approach to Interfacing the Brain with Quantum Computers: Practical Steps and Caveats

We report on the first proof-of-concept system demonstrating how one can control a qubit with mental activity. We developed a method to encode neural correlates of mental activity as instructions for a quantum computer. Brain signals are detected utilising electrodes placed on the scalp of a person, who learns how to produce the required mental activity to issue instructions to rotate and measure a qubit. Currently, our proof-of-concept runs on a software simulation of a quantum computer. At the time of writing, available quantum computing hardware and brain activity sensing technology are not sufficiently developed for real-time control of quantum states with the brain. But we are one step closer to interfacing the brain with real quantum machines, as improvements in hardware technology at both fronts become available in time to come. The paper ends with a discussion on some of the challenging problems that need to be addressed before we can interface the brain with quantum hardware.Ministerio de Ciencia, Innovación y Universidades PID2019-104002GB-C21Junta de Andalucía P20-00617Shanghai’s Municipality, China 2019SHZDZX01-ZX04 and 20DZ229090

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR

On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Magnetic Phase transitions in Plasmas and Transport Barriers

A model of magnetic phase transitions in plasmas is presented: plasma blobs with pressure excess or defect are dia- or para-magnets and move radially under the influence of the background plasma magnetisation. It is found that magnetic phase separation could be the underlying mechanism of L to H transitions and drive transport barrier formation. Magnetic phase separation and associated pedestal build up, as described here, can be explained by the well known interchange mechanism, now reinterpreted as a magnetisation interchange which remains relevant even when stable or saturated. A testable necessary criterion for the L to H transition is presented.Comment: 3 figures, 9 pages, equations created with MathType To be published in Nuclear Fusion, accepted August 201