Thomson scattering near the high-fluence target surface of the Magnum-PSI linear plasma generator

In the quest to long-term operation of high-power magnetically confined fusion devices, it is crucial to control the particle and heat loads on the wall. In order to predict these loads, understanding of the plasma-wall interaction is important. Near the wall surface, the plasma is accelerated towards the Debye sheath edge. In plasma conditions with high density and low temperature, the interaction between the incoming plasma and recycled neutrals can become important. In this paper, we present incoherent Thomson Scattering (TS) measurements in the near-surface region of the Magnum-PSI linear plasma generator. To enable TS measurements close to the plasma target of Magnum-PSI, a stray light suppression up to a factor 104 was achieved, while retaining high transmission. By incrementally moving the target along the magnetic field, this adapted system was used down to 1.9 mm from the target. In the last 10–15 mm in front of the surface, the electron density as well as temperature were observed to decrease significantly. Under the assumption of constant particle flux in this region, the density drop indicates plasma acceleration. In that case, the measurements can be interpreted to show the plasma presheath, and its lengthscale: ~ 1 cm. The electron cooling indicates an energy loss channel for the electrons near the wall. A reduced electron temperature near the sheath entrance leads to lower estimates of particle and energy flux, as well a

Velocimetry analysis of type-I edgelocalized mode precursors in ASDEXUpgrade

When the electron transport barrier remains in its final shape before a type-I edge localized mode (ELM) crash in ASDEX Upgrade, ELM precursors appear as electron temperature fluctuations. In order to relate these precursors to an instability, spatial scales, parity and the cross-phase between electron temperature and radial velocity fluctuations are evaluated by means of velocimetry of measured 2D electron temperature fluctuations. A comprehensive comparison with properties of different instabilities points to microtearing modes. Bispectral analysis indicates a nonlinear coupling of these precursors to a ballooning-type mode prior to the ELM onset

Numerical modelling of detached plasma experiments with differential pumping in Magnum-PSI

Sufficient decrease of plasma pressure, ion and heat flux along the scrape-off layer of a tokamak fusion reactor are imperative to ensure the survival of the divertor tiles. This specific condition, defined as detachment, occurs within low temperature, high density, highly recycling\u3cbr/\u3eplasma that can also be realized in linear plasma devices such as Magnum-PSI [1]. In MagnumPSI, high recycling is achieved by utilizing differential pumping across three vacuum chambers. Experiments have been conducted to mimic detachment in the Magnum-PSI linear device by\u3cbr/\u3evarying the neutral background pressure at the chamber of the recycling target via H2 gas puffing [2], while the hydrogen plasma source parameters are kept constant across the experiments. Numerical modelling is carried out to help gain further insights regarding the physics behind detachment. The experiments are benchmarked with a coupled fluid-kinetic approach using the B2.5- EUNOMIA code package [3]. EUNOMIA is a Monte Carlo neutral simulation optimized for linear\u3cbr/\u3egeometry. Thomson scattering measurements without gas puffing are used as a plasma boundary condition at the source in the simulation, and the gas pressure in the target chamber will be varied. The resulting electron density and temperature simulated profiles near the target are\u3cbr/\u3ecompared with profiles measured in experiments. This paper presents the result of the benchmark tests, and identifies the collisional processes and other physical effects relevant to the detached plasma stat

Sawtooth precursor oscillations on DIII-D

The sawtooth oscillation, observed in tokamak plasmas with a central safety factor of less than unity, is a periodic disruptive instability characterized by a slow ramping of central plasma density and temperature, followed by a fast relaxation resulting in flattening of both profiles. Elongated neutral-beamheated discharges on the DIII-D tokamak exhibit multiple precursor oscillations with mode number m/n = 1/1. The dominant m/n = 1/1 mode oscillates at the plasma rotation frequency. A downshifted mode also appears early in the sawtooth ramp. A normalization of electron cyclotron emission imaging data that removes the contribution of slow electron temperature profile evolution reveals that both modes are consistent with an underlying quasi-interchange plasma displacement

Measurement of turbulent electron temperature fluctuations on the ASDEX Upgrade tokamak using correlated electron cyclotron emission

\u3cp\u3eTurbulent temperature fluctuations are measured on the ASDEX Upgrade tokamak using pairs of closely spaced, narrow-band heterodyne radiometer channels and a standard correlation technique. The pre-detection spacing and bandwidth of the radiometer channel pairs is chosen such that they are physically separated less than a turbulent correlation length, but do not overlap. The radiometer has 4 fixed filter frequency channels and two tunable filter channels for added flexibility in the measurement position. Relative temperature fluctuation amplitudes are observed in a helium plasma to be δT/T = (0.76 ± 0.02)%, (0.67 ± 0.02)%, and (0.59 ± 0.03)% at normalised toroidal flux radius of ρ\u3csub\u3etor\u3c/sub\u3e = 0.82, 0.75, and 0.68, respectively.\u3c/p\u3

Experimental evidence of enhanced recombination of a hydrogen plasma induced by nitrogen seeding in linear device Magnum-PSI

\u3cp\u3e In this work we investigate the effects induced by the presence of nitrogen in a detached-like hydrogen plasmas in linear plasma machine Magnum-PSI. Detachment has been achieved by increasing the background neutral pressure in the target chamber by means of H \u3csub\u3e2\u3c/sub\u3e /N \u3csub\u3e2\u3c/sub\u3e puffing and two cases of study have been set up, i.e. at 2 and 4 Pa. Achieved n \u3csub\u3ee\u3c/sub\u3e are ITER-relevant i.e. above 10 \u3csup\u3e20\u3c/sup\u3e m \u3csup\u3e−3\u3c/sup\u3e and electron temperatures are in the range 0.8–2 eV. A scan among five different N \u3csub\u3e2\u3c/sub\u3e /H \u3csub\u3e2\u3c/sub\u3e +N \u3csub\u3e2\u3c/sub\u3e flux ratios seeded have been carried out, at values of 0, 5, 10, 15 and 20%. A n \u3csub\u3ee\u3c/sub\u3e decrease while increasing the fraction of N \u3csub\u3e2\u3c/sub\u3e has been observed for both background pressures, resulting in a plasma pressure drop of ̴ 30%. T \u3csub\u3ee\u3c/sub\u3e remains constant among all scans. The peak intensity of NH*(A \u3csup\u3e3\u3c/sup\u3e ∏->X \u3csup\u3e3\u3c/sup\u3e ∑ \u3csup\u3e−\u3c/sup\u3e , ∆v = 0) at 336 nm measured with optical emission spectroscopy increases linearly with the N \u3csub\u3e2\u3c/sub\u3e content, together with the NH \u3csub\u3e3\u3c/sub\u3e signal in the RGA. A further dedicated experiment has been carried out by puffing separately H \u3csub\u3e2\u3c/sub\u3e /N \u3csub\u3e2\u3c/sub\u3e and H \u3csub\u3e2\u3c/sub\u3e /He mixtures, being helium a poorly-reactive atomic species, hence excluding a priori nitrogen-induced molecular assisted recombination. Interestingly, plasma pressure and heat loads to the surface are enhanced when increasing the content of He in the injected gas mixture. In the case of N \u3csub\u3e2\u3c/sub\u3e , we observe an opposite behavior, indicating that N–H species actively contribute to convert ions to neutrals. Recombination is enhanced by the presence of nitrogen. Numerical simulations with two different codes, a global plasma-chemical model and a spatially-resolved Monte Carlo code, address the role of NH \u3csub\u3ex\u3c/sub\u3e species behaving as electron donor in the ion conversion with H \u3csup\u3e+\u3c/sup\u3e by means of what we define here to be N-MAR i.e. NH \u3csub\u3ex\u3c/sub\u3e + H \u3csup\u3e+\u3c/sup\u3e → NH \u3csub\u3ex\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e + H, followed by NH \u3csub\u3ex\u3c/sub\u3e \u3csup\u3e+\u3c/sup\u3e + e \u3csup\u3e−\u3c/sup\u3e → NH \u3csub\u3ex-\u3c/sub\u3e \u3csub\u3e1\u3c/sub\u3e + H. Considering the experimental findings and the qualitative results obtained by modelling, N-MAR process is considered to be a possible plasma-chemical mechanism responsible for the observed plasma pressure drop and heat flux reduction. Further studies with a coupled code B2.5-Eunomia are currently ongoing and may provide quantitative insights on the scenarios examined in this paper. \u3c/p\u3

Studying the influence of nitrogen seeding in a detached-like hydrogen plasma by means of numerical simulations

\u3cp\u3eThe leading candidate for impurity seeding in ITER is currently nitrogen. To date, there have only been a few studies on the plasma chemistry driven by N\u3csub\u3e2\u3c/sub\u3e/H\u3csub\u3e2\u3c/sub\u3e seeding and its effect on the molecular-activated recombination of incoming atomic hydrogen ions in a detached-like scenario. Numerical simulations are needed to provide insights into such mechanisms. The numerous plasma chemical reactions that may occur in such an environment cannot be entirely included in a 2- or 3-dimensional code such as Eirene. A complete global plasma model, implemented with more than 100 plasma chemical equations and 20 species, has been set up on the basis of the Plasimo code. This study shows that two main nitrogen-including recombination reaction paths are dominant, i.e. the ion conversion of NH followed by dissociative recombination, and the proton transfer between and N\u3csub\u3e2\u3c/sub\u3e, producing N\u3csub\u3e2\u3c/sub\u3eH\u3csup\u3e+\u3c/sup\u3e. These two processes are referred to as N-MAR (nitrogen molecular-activated recombination) and have subsequently been implemented in Eunomia, which is a spatially resolved Monte Carlo code designed to simulate the neutral inventory in linear plasma machines such as Pilot-PSI and Magnum-PSI. To study the effect of N\u3csub\u3e2\u3c/sub\u3e on the overall recombination, three studies have been set up, and from a defined puffing location with a constant total seeding rate of H\u3csub\u3e2\u3c/sub\u3e + N\u3csub\u3e2\u3c/sub\u3e, three N\u3csub\u3e2\u3c/sub\u3e ratios were simulated, i.e. 0%, 5% and 10%. The parameter monitored is the density of atomic hydrogen, being the final hydrogenic product of any recombination mechanism in the scenario considered. The difference in H density between the 0% case and the 10% case is about a factor of three. The importance of NH as an electron donor is highlighted, and the N-MAR reaction routes are confirmed to enhance the conversion of ions to neutrals, making the heat loads to the divertor plate more tolerable. This work is a further step towards a full understanding of the role of N\u3csub\u3e2\u3c/sub\u3e-H\u3csub\u3e2\u3c/sub\u3e molecules in a detached divertor plasma.\u3c/p\u3