Gas Puff Research Articles

Overview of the preliminary design of the ITER plasma control system

An overview of the preliminary design of the ITER plasma control system (PCS) is described here, which focusses on the needs for 1st plasma and early plasma operation in hydrogen/helium (H/He) up to a plasma current of 15 MA with moderate auxiliary heating power in low confinement mode (L-mode). Candidate control schemes for basic magnetic control, including divertor operation and kinetic control of the electron density with gas puffing and pellet injection, were developed. Commissioning of the auxiliary heating systems is included as well as support functions for stray field topology and real-time plasma boundary reconstruction. Initial exception handling schemes for faults of essential plant systems and for disruption protection were developed. The PCS architecture was also developed to be capable of handling basic control for early commissioning and the advanced control functions that will be needed for future high performance operation. A plasma control simulator is also being developed to test and validate control schemes. To handle the complexity of the ITER PCS, a systems engineering approach has been adopted with the development of a plasma control database to keep track of all control requirements.

Relationship between frequency power spectra and intermittent, large-amplitude bursts in the Alcator C-Mod scrape-off layer

Fluctuations in the boundary region of the Alcator C-Mod tokamak have been analyzed using gas puff imaging data from a set of Ohmically heated plasma density scan experiments. It is found that the relative fluctuation amplitudes are modest and close to normally distributed at the separatrix but become increasingly larger and skewed towards the main chamber wall. The frequency power spectra are nevertheless similar for all radial positions and line-averaged densities. Predictions of a stochastic model, describing the plasma fluctuations as a super-position of uncorrelated pulses, are shown to be in excellent agreement with the measurements. This implies that the pulse duration is the same, while the degree of pulse overlap decreases radially outwards in the scrape-off layer. The universal frequency power spectral density is thus determined by the shape and duration of the large-amplitude bursts associated with blob-like structures. The model also describes the rate of threshold level crossings, for which the exponential tails underline the intermittency of the fluctuations in the far scarpe-off layer.

Impact of neutral density fluctuations on gas puff imaging diagnostics

A three-dimensional turbulence simulation of the SOL and edge regions of a toroidally limited tokamak is carried out. The simulation couples self-consistently the drift-reduced two-fluid Braginskii equations to a kinetic equation for neutral atoms. A diagnostic neutral gas puff on the low-field side midplane is included and the impact of neutral density fluctuations on light emission investigated. We find that neutral density fluctuations affect the emission. In particular, at a radial distance from the gas puff smaller than the neutral mean free path, neutral density fluctuations are anti-correlated with plasma density, electron temperature, and fluctuations. It follows that the neutral fluctuations reduce the emission in most of the observed region and, therefore, have to be taken into account when interpreting the amplitude of the emission. On the other hand, higher order statistical moments (skewness, kurtosis) and turbulence characteristics (such as correlation length, or the autocorrelation time) are not significantly affected by the neutral fluctuations. At distances from the gas puff larger than the neutral mean free path, a non-local shadowing effect influences the neutral density fluctuations. There, the fluctuations are correlated with the neutral density fluctuations, and the high-order statistical moments and measurements of other turbulence properties are strongly affected by the neutral density fluctuations.

Investigation of the transition of multicycle AC operation in ISTTOK under edge electrode biasing

In this paper we present recent results obtained on plasma edge electrode biasing during AC discharges. The goal is to obtain experimental evidence on a number of plasma parameters that can play a role during the AC transition on the repeatability and reproducibility of AC operation. The control of the plasma density in the quiescent phase is made just before the AC transition by means of positive edge biasing leading to a transitory improved of density (30%–40%). Gas puff experiments show that the increase of background gas pressure during discharge led to a better success of the AC transition. The experimental results indicate that the increase of density during the AC transition induced by edge biasing is followed by an electron temperature drop. The drop in electron temperature leads in most cases the formation of runaway electrons. It has been observed that the runaway population during discharge flattop depends on the interplay between gas content and plasma density and temperature. The results also confirm that the correct balance of external magnetic fields is crucial during the AC transition phase where drift electron currents are formed. The results from the heavy ion beam diagnostic show that the formation of plasma current during consecutive AC transitions is asymmetric. Numerical simulations indicate that for some particular conditions this result could be reproduced from assuming the presence of two counter-currents during AC transition.

Preliminary study of divertor particle exhaust in the EAST superconducting tokamak

The particle exhaust of the upper tungsten and lower carbon divertors in EAST has been preliminarily studied during the 2016 experimental campaign. The density decay time during terminating gas puffing has been employed as a key parameter to evaluate the divertor particle exhaust performance. Comparative plasma discharges have been carried out on the particle exhaust performance between two toroidal field directions in the upper single null and lower single null divertor configurations. This work has enhanced the understanding of the effects of the in–out asymmetry and divertor geometry on the efficiency of the divertor particle exhaust. In addition, the sensitivity of the particle exhaust capability on different strike point locations has been analyzed. The experimental results are expected to provide important information on the future upgrade of EAST bottom divertor and facilitate the realization of longer pulse operation.

Spatial coherence measurements of the EUV emission from laser-plasma source based on xenon/helium gas puff target

In this paper, we present the first measurements of the partial spatial coherence of the EUV emission from xenon plasma in laser-plasma source, based on a double stream gas puff target. The Young double slit approach was employed to measure complex coherence factor of the EUV Xe emission at 13.5-nm wavelength in two orthogonal directions. The radius of coherence of ~60 μm was estimated at the distance of 2.1 m from the source. The number of coherently emitted photons was sufficient to demonstrate coherent imaging. Using partially coherent radiation from such source Gabor EUV holography was successfully demonstrated.

Review of recent experiments on the T-10 tokamak with all metal wall

Transition to an all-metal wall was realized on T-10 by replacement of graphite limiters with tungsten ones. Light impurity levels remained high and W accumulation in the plasma core was revealed. A movable lithium (Li) limiter was added to investigate the possibility of the limitation of tungsten and light impurity levels in plasma. For the first time, tokamak results on tungsten protection with Li were obtained in OH and ECRH regimes. After lithization the tungsten density in the core dropped more than an order of magnitude, while W influx into plasma decreased 2–4 times. Drastic drops of light impurities in plasma were observed together with improvement of energy confinement time and density limit values. Nevertheless, Li levels in plasma remained low in both OH and ECRH regimes. Li density in the core as low as 0.5% of ne was obtained. Tungsten transport in T-10 plasma was investigated and results on prevention of W accumulation with central ECRH were obtained. Effects of plasma exposure on ITER-grade tungsten plates from limiters were studied. Investigations of density fluctuation with correlation reflectometry confirmed a decrease of fluctuation amplitude on high field side. Modeling showed that this effect can be, to a great extent, explained by the non-locality of reflectometry. Toroidal correlations at a distance of 2.5 m along field lines were studied. Three-wave interaction between geodesic acoustic modes and broad-band turbulence was found by analysis of heavy ion beam probe diagnostics data. The possibility of plasma current control and the prevention of non-thermal electron beams formation at density limit disruption by means of ECR heating and the controlled operation of OH power supply system has been demonstrated. The study of plasma density decay after gas puff switch off during density ramp-up phase in OH regimes and the effect of ‘density pump out’ during ECRH showed that both effects can be explained by the assumption regarding electron transport growth upon reaching marginal pressure profile. A new type of internal transport barrier in the narrow zone near q = 1 surface has been found.

The impact of three dimensional MHD instabilities on the generation of warm dense matter using a MA-class linear transformer driver

Warm dense matter is difficult to generate since it corresponds to a state of matter which pressure is order of magnitude larger than can be handled by natural materials. A diamond anvil can be used to pressurize matter up to one Gbar, this matter is at high density but at room temperature. High power lasers and heavy ion beams can generate warm dense matter on time scales where measuring quasi-static transport coefficients such as viscosity or heat conduction proves difficult since both experimental techniques relies on inertial confinement. We present here a third method to generate warm dense matter. It uses a pulsed-power driver which current rise time is substantially shortened by using a plasma opening switch, limiting the development of electrothermal instabilities. The switch relies on the implosion of a gas puff Z-pinch which carries most of the discharge current until the pinch reaches the sample. After that, the sample is compressed until it reaches the warm dense matter regime. Three-dimensional magnetohydrodynamics computations show that if the density of the gas is low enough no detectable instabilities (e.g. kinks and sausages modes) impede the remainder of the implosion.

Note: Infrared laser diagnostics for deuterium gas puff Z pinches.

Deuterium gas puff Z pinches have been used for generation of strong neutron fluxes on the MA class pulse power machines. Due to the low electron density of deuterium Z-pinch plasma, regular laser diagnostics in the visible range cannot be used for observation and study of the pinch. Laser probing at the wavelength of 1064 nm was used for visualization of deuterium plasma. Infrared schlieren and interferometry diagnostics showed the deuterium gas puff plasma dynamics, instabilities, and allowed for the reconstruction of the profile of the plasma density.

Overview of ASDEX Upgrade results

The ASDEX Upgrade (AUG) programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. Since 2015, AUG is equipped with a new pair of 3-strap ICRF antennas, which were designed for a reduction of tungsten release during ICRF operation. As predicted, a factor two reduction on the ICRF-induced W plasma content could be achieved by the reduction of the sheath voltage at the antenna limiters via the compensation of the image currents of the central and side straps in the antenna frame. There are two main operational scenario lines in AUG. Experiments with low collisionality, which comprise current drive, ELM mitigation/suppression and fast ion physics, are mainly done with freshly boronized walls to reduce the tungsten influx at these high edge temperature conditions. Full ELM suppression and non-inductive operation up to a plasma current of MA could be obtained at low plasma density. Plasma exhaust is studied under conditions of high neutral divertor pressure and separatrix electron density, where a fresh boronization is not required. Substantial progress could be achieved for the understanding of the confinement degradation by strong D puffing and the improvement with nitrogen or carbon seeding. Inward/outward shifts of the electron density profile relative to the temperature profile effect the edge stability via the pressure profile changes and lead to improved/decreased pedestal performance. Seeding and D gas puffing are found to effect the core fueling via changes in a region of high density on the high field side (HFSHD).The integration of all above mentioned operational scenarios will be feasible and naturally obtained in a large device where the edge is more opaque for neutrals and higher plasma temperatures provide a lower collisionality. The combination of exhaust control with pellet fueling has been successfully demonstrated. High divertor enrichment values of nitrogen have been obtained during pellet injection, which is a prerequisite for the simultaneous achievement of good core plasma purity and high divertor radiation levels. Impurity accumulation observed in the all-metal AUG device caused by the strong neoclassical inward transport of tungsten in the pedestal is expected to be relieved by the higher neoclassical temperature screening in larger devices.

Spectral lines and characteristic of temporal variations in photoionized plasmas induced with laser-produced plasma extreme ultraviolet source

Abstract Spectral lines for Kr/Ne/H2 photoionized plasma in the ultraviolet and visible (UV/Vis) wavelength ranges have been created using a laser-produced plasma (LPP) EUV source. The source is based on a double-stream gas puff target irradiated with a commercial Nd:YAG laser. The laser pulses were focused onto a gas stream, injected into a vacuum chamber synchronously with the EUV pulses. Spectral lines from photoionization in neutral Kr/Ne/H2 and up to few charged states were observed. The intense emission lines were associated with the Kr transition lines. Experimental and theoretical investigations on intensity variations for some ionic lines are presented. A decrease in the intensity with the delay time between the laser pulse and the spectrum acquisition was revealed. Electron temperature and electron density in the photoionized plasma have been estimated from the characteristic emission lines. Temperature was obtained using Boltzmann plot method, assuming that the population density of atoms and ions are considered in a local thermodynamic equilibrium (LTE). Electron density was calculated from the Stark broadening profile. The temporal evaluation of the plasma and the way of optimizing the radiation intensity of LPP EUV sources is discussed.

Overview of recent experimental results from the Aditya tokamak

Several experiments, related to controlled thermonuclear fusion research and highly relevant for large size tokamaks, including ITER, have been carried out in ADITYA, an ohmically heated circular limiter tokamak. Repeatable plasma discharges of a maximum plasma current of ~160 kA and discharge duration beyond ~250 ms with a plasma current flattop duration of ~140 ms have been obtained for the first time in ADITYA. The reproducibility of the discharge reproducibility has been improved considerably with lithium wall conditioning, and improved plasma discharges are obtained by precisely controlling the position of the plasma. In these discharges, chord-averaged electron density ~3.0–4.0 × 1019 m−3 using multiple hydrogen gas puffs, with a temperature of the order of ~500–700 eV, have been achieved. Novel experiments related to disruption control are carried out and disruptions, induced by hydrogen gas puffing, are successfully mitigated using the biased electrode and ion cyclotron resonance pulse techniques. Runaway electrons are successfully mitigated by applying a short local vertical field (LVF) pulse. A thorough disruption database has been generated by identifying the different categories of disruption. Detailed analysis of several hundred disrupted discharges showed that the current quench time is inversely proportional to the q edge. Apart from this, for volt–sec recovery during the plasma formation phase, low loop voltage start-up and current ramp-up experiments have been carried out using electron cyclotron resonance heating (ECRH). Successful recovery of volt–sec leads to the achievement of longer plasma discharge durations. In addition, the neon gas puff assisted radiative improved confinement mode has also been achieved in ADITYA. All of the above mentioned experiments will be discussed in this paper.

Favourable scenarios established by SMBI for the realization of the ELMy H-mode at HL-2A**Project supported by the National Natural Science Foundation of China (Grant Nos. 11375057 and 11175061) and the National Magnetic Confinement Fusion Science Program, China (Grant Nos. 2010GB102003 and 2014GB108003).

The ELMy H-mode plasmas realized with the supersonic molecular beam injection (SMBI) are studied in relation to the energy confinement and the heating power for the L–H transition ( in the HL-2A tokamak. A database is assembled for this study based on the ELMy H-mode discharges during the experimental campaigns in the period 2009–2013 at the HL-2A tokamak. The statistical results show that the SMBI is favourable for reaching the H-mode by reducing the heating power at the L–H transition and for the H-mode performance by improving the energy confinement compared with the ordinary gas puffing (GP). The reduction of is about 20% when the density is low, and the energy confinement enhancement factor of 1.5 is achieved with the SMBI. Note that in the database the density dependence of is non-monotonic with the at which the is minimum. Most of data are on the low density branch where the increases with the decrease in density. The minimum of the in HL-2A is comparable to the ITPA multi-machine threshold power scaling . The physics behind the reduction of the with the SMBI is also investigated in relation to the change of the density gradient at the plasma edge, the gas fuelling efficiency, and the recycling.

Argon impurity transport studies at Wendelstein 7-X using x-ray imaging spectrometer measurements

In the first operational phase of the stellarator Wendelstein 7-X (W7-X), the x-ray imaging crystal spectrometer (XICS) system has been commissioned for measuring radial profiles of ion and electron temperature, T i and T e, plasma rotation velocities, v P, and selected impurity densities, n Z . This paper shows the first measurements of the spectrometer and gives an initial calculation of impurity transport parameters derived from an Ar impurity transport study. Using Bayesian analysis, the temporal evolution of Ar impurity density profiles after an Ar gas puff could be observed with a time resolution of up to 5 ms, yielding a maximum value for the diffusion coefficient of D = 1.5 m2 s−1 at ρ ~ 0.5 and small pinch velocities in the inner plasma region.

Changes in divertor conditions in response to changing core density with RMPs

The effects of changes in core density on divertor electron temperature, density and heat flux when resonant magnetic perturbations (RMPs) are applied are presented, notably a reduction in RMP induced secondary radial peaks in the electron temperature profile at the target plate is observed when the core density is increased, which is consistent with modeling. RMPs is used here to indicate non-axisymmetric magnetic field perturbations, created using in-vessel control coils, which have at least one but typically many resonances with the rotational transform of the plasma (Evans et al 2006 Phys. Plasmas 13 056121). RMPs are found to alter inter-ELM heat flux to the divertor by modifying the core plasma density. It is shown that applying RMPs reduces the core density and increases the inter-ELM heat flux to both the inner and outer targets. Using gas puffing to return the core density to the pre-RMP levels more than eliminates the increase in inter-ELM heat flux, but a broadening of the heat flux to the outer target remains. These measurements were made at a single toroidal location, but the peak in the heat flux profile was found near the outer strike point where simulations indicate little toroidal variation should exist and tangentially viewing diagnostics showed no evidence of strong asymmetries. In experiments where divertor Thomson scattering measurements were available it is shown that local secondary peaks in the divertor electron temperature profile near the target plate are reduced as the core density is increased, while peaks in the divertor electron density profile near the target are increased. These trends observed in the divertor electron temperature and density are qualitatively reproduced by scanning the upstream density in EMC3-Eirene modeling. Measurements are presented showing that higher densities are needed to induce detachment of the outer strike point in a case where an increase in electron temperature, likely due to a change in MHD activity, is seen after RMPs are applied.

Simulation of burn control for DEMO using ASTRA coupled with Simulink

DEMO is a proposed demonstration fusion power plant which is currently being designed. Fusion power, Pfus, has to be controlled to stay above a minimum level to produce sufficient net electricity. However, this has an impact on the power through the separatrix, Psep, and thus can produce excessive heat flux to the divertor which can lead to damage. This gives a limit for the maximum allowable Psep.The aim of this work is to develop a framework which will give the possibility to design and to test controllers by simulating DEMO plasma transport. Therefore, we coupled the ASTRA transport modelling code for fusion devices with the Simulink simulation framework. ASTRA is equipped with equilibrium, transport, fuelling, heating and current drive modules. Simulink is a powerful tool to model and to simulate different dynamic systems. It allows fast and simple development of controllers using its built-in blocks. Therefore, coupling of ASTRA with Simulink gives the advantage of fast development of controllers for the power plant modelled with sophisticated physics based on the transport codes.This coupling was used to run the first basic simulations, where we simulated feedback control of fusion power, power through separatrix and pedestal top Greenwald density fraction, nGWped top. nGWped top is controlled via the pellet frequency to keep it below one. Fusion power is controlled with external heating (NBI). To prevent increasing Psep and heat load on the divertor while increasing Pfus, xenon gas can be puffed into the vessel. However, Psep has to be kept above a threshold to stay in H-mode. Therefore, feedback control of the xenon gas puff is modelled. These controllers were used to control fusion power, power through separatrix and pedestal top Greenwald density fraction with two different external disturbances. In the first one, we mimicked changes of the H-factor and in the second one we mimicked the fall of a tungsten flake. Results show that we can control these parameters already with a combination of simple controllers.

Nanoscale imaging applications of soft X-ray microscope based on a gas-puff target source

A compact microscope based on nitrogen double stream gas puff target soft X-ray source, which emits radiation in the water-window spectral range, at the wavelength of λ = 2.88 nm, is presented. The microscope, employing ellipsoidal grazing incidence condenser and a Fresnel zone plate objective, is capable of capturing images with a 60 nm spatial resolution and exposure time as low as a few seconds. Examples of different applications of the SXR microscopy, and its applicability for various fields of science, are presented and discussed.

Bioimaging Using Full Field and Contact EUV and SXR Microscopes with Nanometer Spatial Resolution

We present our recent results, related to nanoscale imaging in the extreme ultraviolet (EUV) and soft X-ray (SXR) spectral ranges and demonstrate three novel imaging systems recently developed for the purpose of obtaining high spatial resolution images of nanoscale objects with the EUV and SXR radiations. All the systems are based on laser-plasma EUV and SXR sources, employing a double stream gas puff target. The EUV and SXR full field microscopes—operating at 13.8 nm and 2.88 nm wavelengths, respectively—are currently capable of imaging nanostructures with a sub-50 nm spatial resolution with relatively short (seconds) exposure times. The third system is a SXR contact microscope, operating in the “water-window” spectral range (2.3–4.4 nm wavelength), to produce an imprint of the internal structure of the investigated object in a thin surface layer of SXR light sensitive poly(methyl methacrylate) photoresist. The development of such compact imaging systems is essential to the new research related to biological science, material science, and nanotechnology applications in the near future. Applications of all the microscopes for studies of biological samples including carcinoma cells, diatoms, and neurons are presented. Details about the sources, the microscopes, as well as the imaging results for various objects will be shown and discussed.

Helium neutral beam injection into ASDEX Upgrade

ASDEX Upgrade's (AUG) neutral beam injection (NBI) system is primarily designed for deuterium injection and delivers 20MW heating power from two injectors with four beams each at 60 and 93keV, respectively. As opposed to the cryo pumps of the JET NBI, the Ti getter pumps of the AUG NBI with a pumping speed of 3×106ℓ/s for hydrogen do not pump helium at all, leaving only the conventional pumping system with <6×103ℓ/s for He. This imposes constraints on the possible operation in helium.In order to prepare for AUG He plasma campaigns, serious trials to operate the AUG NBI with He began in 2014. It was found that despite the lack of high speed pumping up to two beams per injector could be operated simultaneously at reduced feed gas flow without particular restrictions on the beam-on time. For injector 1 the power per beam was limited to 550kW at 40keV by the required filament current in its arc sources, while for injector 2 the limitation came from the bending magnet's power supply that restricted the beam energy to 68keV and the NBI power to 730kW per beam. Thus the maximum available NBI heating power in He amounts to 2.6MW for 10s, the maximum discharge duration of AUG. Helium neutral beam injection into plasma was first tried out in 2014 for two discharges. In 2015 a dedicated He campaign used He NBI in a total of 44 discharges. As He is almost not pumped in the injectors the neutral gas flow into the torus is comparable with the total He gas puff.

Effects of outer top gas injection on ICRF coupling in ASDEX Upgrade: towards modelling of ITER gas injection

The influence of outer top gas injection on the scrape-off layer (SOL) density and ion cyclotron range of frequency (ICRF) coupling has been studied in ASDEX Upgrade (AUG) L-mode plasmas for the first time. The three-dimensional (3D) edge plasma fluid and neutral transport code EMC3-EIRENE is used to simulate the SOL plasma density, and the 3D wave code RAPLICASOL is used to compute the ICRF coupling resistance with the calculated density. Improvements have been made in the EMC3-EIRENE simulations by fitting transport parameters separately for each gas puffing case. It is found that the calculated local density profiles and coupling resistances are in good agreement with the experimental ones. The results indicate that the SOL density increase depends sensitively on the spreading of the injected outer top gas. If more gas enters into the main chamber through the paths near the top of vessel, the SOL density increase will be more toroidally uniform; if more gas chooses the paths closer to the mid-plane, then the SOL density increase will be more local and more significant. Among the various local gas puffing methods, the mid-plane gas valve close to the antenna is still the best option in terms of improving ICRF coupling. Differences between the outer top gas puffing in AUG and the outer top gas puffing in ITER are briefly summarized. Instructive suggestions for ITER and future plans for ITER gas injection simulations are discussed.