Pellet Ablation Cloud Research Articles

Study of the pellet ablation cloud using the tomography technique for two-directional simultaneous photography in GAMMA 10/PDX

The pellet ablation mechanism is an interesting subject for plasma fuelling in fusion plasmas. In GAMMA 10/PDX, pellet injection experiments for higher density plasma production are planned to conduct detached plasma experiments in the higher density plasma condition. We measured the pellet ablation cloud by using the two-directional simultaneous photography system in GAMMA 10/PDX. The tomography reconstruction technique was used for considering the pellet trajectory in the plasma and pellet ablation. The three-dimensional pellet trajectory and pellet ablation images in the plasma were clearly obtained for the first time, to the best of our knowledge.

Better Understanding of Hydrogen Pellet Ablation Cloud Spectra through the Occupation Probability Formalism in LHD

We have recently incorporated the occupation probability formalism (OPF) in the simulation model [C. Stehlé and S. Jacquemot, Astron. Astrophys. 271, 348 (1993)] to have a smooth transition from discrete lines to continuum spectrum in the wavelength range near the Balmer series limit. We have analyzed spectra measured for the hydrogen pellet ablation cloud in the Large Helical Device with the revised model, and have found that the electron density in the ablation cloud has a close correlation with the electron temperature of the background plasma. This type of correlation is first confirmed in the present analysis and should give a new insight in the simulation studies of pellet ablation for the magnetically confined fusion plasma.

Measurement of Paα line from pellet ablation cloud in Heliotron J.

The Paα line (1875.13nm) in the near-infrared (NIR) region was evaluated to apply Stark broadening of the line spectrum to the electron density measurement of the small-pellet ablation cloud in Heliotron J, a medium-sized helical-axis heliotron device. Paα is three-to-four times broader than the visible Hβ line (486.13nm) for the same electron density. Using a portable NIR spectrometer, preliminary proof-of-concept experiments determined the marginal density, below which the broadening was undetectable. The lower detection density limit can be decreased using a narrower entrance slit or a denser grating.

First Observation of Pellet Ablation Clouds Using Two-Directional Simultaneous Photography in GAMMA 10/PDX

The study of pellet ablation mechanisms is an important subject for plasma fueling in fusion plasmas. In GAMMA 10/PDX, sub-millimeter hydrogen pellet injection experiments are conducted in the higher electron density plasma for detached plasma experiments. We observed the pellet ablation cloud by two-directional simultaneous photography during the GAMMA 10/PDX pellet injection experiments. The three-dimensional pellet ablation cloud and its trajectory were clearly obtained for the first time.

Non-CLTE spectral modeling approach for hydrogen pellet ablation cloud in the Large Helical Device

Spatially-resolved spectral measurement of a hydrogen pellet ablation cloud has been conducted in the Large Helical Device. Least square fittings of one of the obtained spectrum are inconsistent when the complete local thermodynamic equilibrium (CLTE) condition is supposed. A better agreement is found when a more general spectral model is used. The electron density of 1.4 × 1023 m−3 obtained is similar to that obtained in a previous study [M. Goto et al., Plasma Phys. Control. Fusion 49, 1163 (2007)]. The electron temperature of 0.4 eV is however significantly lower than the value of 1 eV derived in the same previous study.

Spatially-Resolved Electron Density Measurement in Hydrogen Pellet Ablation Cloud

A spectroscopic method for spatial resolution measurement in fuel pellet ablation clouds is being developed in the Large Helical Device (LHD). Spatial resolution is obtained thanks to optics that have a narrow, band-shaped field-of-view. The Stark-broadened Hβ emission line of a deuterium pellet ablation cloud is isolated and analyzed with a spectral lineshape code. The electron density profile of the ablation cloud along its direction of elongation is derived through least squares fitting. The obtained profile is peaked and has a dip at its center which confirms what can be found in simulations. Moreover, the order of magnitudes for the derived electron densities are in agreement with what has already been found in the LHD.

29a-ZC-10 ヘリオトロンEにおけるPellet Ablation Cloudの挙動(プラズマ物理・核融合)

29a-ZC-10 ヘリオトロンEにおけるPellet Ablation Cloudの挙動(プラズマ物理・核融合)

Self-absorption of Hβ line in pellet ablation clouds in devices with magnetic confinement of high-temperature plasma

We have determined the optical absorption depth for Hβ line in clouds of secondary plasma near pellets ablating in high-temperature plasma of devices with magnetic confinement. It is established that, for all values of the electron concentration (1016–3 × 1017 cm−3) and electron temperature (2–5 eV) measured in typical impurity pellet clouds, the absorption is negligibly small. At the same time, it can be significant in clouds of higher density (1017–5 × 1017 cm−3) with a temperature of 1–2 eV, which are typical of cryogenic fuel pellets.

Spectroscopic study of a carbon pellet ablation cloud

Carbon pellets were injected into high-temperature plasmas produced in the Large Helical Device (LHD), a heliotron-type fusion experimental device. Radiation from a high-density plasma formed around the pellet core, the so-called ablation cloud, was observed and its spectrum in the UV–visible wavelength range was obtained. The observed spectrum is found to be dominated by emission lines of CII and CIII ions, and their level populations are determined from the measured line intensities. The result suggests that LTE (local thermodynamic equilibrium) is established over the measured excited levels for both ions, and the electron temperature is determined to be 2.5 eV and 3.0 eV for the CII and CIII ions, respectively, on the assumption that level populations follow a Boltzmann distribution. For the plasma dominated by the CII lines, the electron density, ne = 6.5 × 1022 m−3, and plasma volume, V = 5.3 × 10−6 m3, are simultaneously derived from a fit of the CII λ 723 nm ([1s22s2]3p 2Po–3d 2D) line profile which is subject to various broadening effects such as Stark broadening. For the plasma dominated by CIII lines, V = 4.5 × 10−2 m3 is derived from an analysis of the reabsorption effect appearing in fine structure lines of the CIII λ 117 nm ([1s2]2s2p 3Po–2p2 3P) transition. The assumption of electric charge neutrality in the ablation cloud yields ne = 4.7 × 1020 m−3. These results suggest such a structure in the ablation cloud that a dense plasma dominated by the CII lines is surrounded by a larger and lower density plasma which is responsible for the CIII lines. The assumption of complete LTE for CIII levels, which is used in the analysis, is shown to be approximately true with the help of collisional-radiative model calculations, for which the reabsorption effect is taken into account. Thus, in this study various plasma parameters in the ablation cloud are determined and information about the cloud structure is obtained.

Calculation of low-Z impurity pellet induced fluxes of charge exchange neutral particles escaping from magnetically confined toroidal plasmas

Measurements of energy- and time-resolved neutral hydrogen and helium fluxes from an impurity pellet ablation cloud, referred to as pellet charge exchange or PCX experiments, can be used to study local fast ion energy distributions in fusion plasmas. The estimation of the local distribution function f(i)(E) of fast ions entering the cloud requires knowledge of both the fraction F(0)(E) of incident ions exiting the cloud as neutral atoms and the attenuation factor A(E,rho) describing the loss of fast atoms in the plasma. Determination of A(E,rho), in turn, requires the total stopping cross section sigma(loss) of neutral atoms in the plasma and the Jacobian reflecting the measurement geometry and the magnetic surface shape. The obtained functions F(0)(E) and A(E,rho) enter multiplicatively into the probability density for escaping neutral particle kinetic energy. A general calculation scheme has been developed and realized as a FORTRAN code, which is to be applied for the calculation of f(i)(E) from PCX experimental results obtained with low-Z impurity pellets.

Active Neutral Particle Diagnostics on LHD by Locally Enhanced Charge Exchange on an Impurity Pellet Ablation Cloud

First radially resolved local data on the ion distribution function have been obtained on the Large Helical Device (LHD) from measurements of escaping neutral particle energy spectra by using an impurity pellet ablation cloud as a spatially localized target for the charge exchange with plasma ions. The active diagnostic employs a transversely directed injector of solid pellets and a compact high-resolution neutral particle energy spectrometer capable of observing the ablation cloud throughout the pellet flight across the plasma column. The paper describes the experimental method and presents the initial measurement results. The data analysis is discussed with emphasis on the local ion distribution calculation from the pellet charge-exchange neutral spectra, taking into account the pellet trajectory, the cloud parameters, and the relevant electron capture and stripping cross sections. The estimated carbon/hydrogen ablation cloud neutralization fraction is used for the interpretation of the neutral spectra from tangential neutral beam injection–heated plasmas and from ion cyclotron heating–sustained plasmas.

Modelling of the pellet cloud evolution and mass deposition with an account of ∇B induced drift

Modelling of the pellet ablation cloud evolution including the ∇B induced drift motion is performed. The model includes cloud heating, expansion and ionization, acceleration in the low-field side (LFS) direction, Alfven conductivity of the background plasma, compensation of ∇B currents in different parts of the cloud during its propagation along the magnetic field, cooling of the background plasma and simulation of the pellet ablation rate in a self-consistent manner. The time evolution of cloud density and temperature profiles and the mass deposition after the pellet injection are calculated. The LFS and high-field side injection scenarios with plasma and pellet parameters typical for the ASDEX-Upgrade tokamak are compared. An effect of pre-cooling on the pellet penetration depth is studied. The calculated size of the neutral part of the cloud, the characteristic values of cloud density and temperature far from the pellet and the fuelling efficiency (for LFS pellets) are in reasonable agreement with those observed in experiments on the ASDEX-Upgrade.

Studies of the structure of C pellet ablation clouds in W7-AS

The structure of the ablation clouds surrounding carbon pellets injected into the ECR-heated Wendelstein 7-AS plasma has been studied. Snapshot and integrated photographs obtained in the spectral ranges containing the CII (720 ± 5 nm and 723 ± 1 nm) and CIII (770 ± 5 nm) spectral lines were analyzed over a wide range of the bulk plasma parameters. It is found that the cloud luminosity profile along the magnetic field is exponential with either one or two characteristic decay lengths of about a few millimeters and a few centimeters. The smaller length corresponds to the zone closer to the pellet. There is good agreement between the characteristic decay lengths deduced from snapshot and integrated photographs. The characteristic decay lengths were obtained along the entire pellet trajectory and were found to change weakly in the central region and to grow at the plasma periphery (generally, in inverse proportion to the plasma electron density). In the central region, the characteristic decay lengths are about a few millimeters and 1 cm. They depend weakly on the bulk plasma temperature and decrease with increasing bulk plasma density. These lengths agree fairly well with estimates of the ionization length of carbon ions into the C2+, C3+, and C4+ charge states, respectively, assuming that ionization is provided by the hot electrons of the bulk plasma and that the cloud expands with the ion-acoustic velocity at a temperature of ∼1 eV. The results obtained prove that the cloud structure in the vicinity of the pellet is mainly determined by the bulk plasma electrons.

Modeling of the pellet cloud structure in the presence of ∇B induced drift

Modeling of the pellet ablation cloud evolution with account of the ∇B induced drift motion is performed. The model includes acceleration in the low-field-side direction, Alfven conductivity of the background plasma, compensation of ∇B currents in different parts of the cloud during its propagation along the magnetic field, cloud heating, expansion and ionization and simulation of the pellet ablation rate in a self-consistent manner. The time evolution of density and temperature profiles and the mass deposition after the pellet injection are calculated. The size of neutral part of the cloud, the characteristic values of cloud density and temperature far from the pellet and the fuelling efficiency are in a reasonable agreement with those observed in experiments on ASDEX-Upgrade tokamak.

Digital processing of solid state detector signals in pellet charge exchange measurements on LHD

Radially resolved measurements of the plasma ion distribution function by detecting charge exchange neutrals from an impurity pellet ablation cloud require a fast operating energy analyzer working at high count rates to build several spectra during the pellet flight. Currently a solid state detector in the pulse height analysis (PHA) mode is used for such measurements on the Large Helical Device. Traditional PHA techniques cannot provide the operating speed required for a good spatial resolution. An algorithm has been proposed based on digital processing of noisy data series containing charge-sensitive preamplifier signals with discontinuities corresponding to incident particles. The algorithm employs the modified Tikhonov regularization and the successive detection–estimation of signal increments at discontinuity points. Such an approach allows one to realize an ultrafast particle energy spectroscopy by taking advantage of detector/preamplifier capabilities without limiting the system throughput by subsequent electronics.

Development and initial operation of the pellet charge exchange diagnostic on LHD heliotron

An active corpuscular diagnostic with an artificially created localized target for the charge-exchange process has been developed and tested on a large helical device. The diagnostic is a combination of an impurity pellet injector and a natural diamond detector-based energy analyzer. High-energy particles neutralized at the pellet ablation cloud are detected while the pellet travels across the plasma column. Time-resolved atomic energy spectra translate into local measurements along the pellet trajectory. Thus, local parameters are obtained in the toroidally non-axis-symmetrical configuration. Physical basis, data interpretation, and the technical description of the diagnostic are given. The requirements to the analyzer geometry, electronics, and data acquisition needed to provide the desired spatial resolution are described. The initial experimental results are presented along with the other diagnostic data. The scope of experiments, possible comparisons with multichord passive charge-exchange measurements and future work on the diagnostic are also discussed.

Pellet fuelling experiments in the HL-1M tokamak

An eight shot pellet injector is used to fuel the HL-1M tokamak from the low field side. The pellet ablation process is investigated witha CCD camera and an array of Hα emission detectors. It isobserved that the velocity of the pellet ablation cloud is obviouslyslowed down after it enters the plasma. The ablation clouds areelongated along the magnetic field direction. By means of multi-exposures with theCCD camera during pellet ablation, the safety factor profile isestimated with the ablation cloud inclination angles with respectto the torus on HL-1M. The pellet penetration depth is analysed withAbel inversion of the Hα emissions. The result is compared with theprediction of ablation models. Kuteev's model is more suitable fordescribing pellet ablation processes in the HL-1M plasma than the neutral gasshielding model.

Photographing of a Multi-Pellet Ablation Process in HL-1M Tokamak with CCD Camera

The 2D CCD camera has been used to take photos during hydrogen multi-pellet injection in HL-1M tokamak. The hydrogen multi-pellet (2 × ϕ 1.0 mm, 3 × ϕ 1.2 mm, 3 × ϕ 1.2–1.3 mm) is horizontally injected into plasma. The observation is performed above the injection path at a sight angle 13.4°. As the shape of cloud ablation varies so quickly, the key points of the experiment have to be the high temporal resolution of CCD and the determination of pellet radial location in plasma. A series of improvements have been made with the experiment setup, including camera parameter, control (NA, ROI) and trigger mode, so as to satisfy the experiment requirements. Thus very nice photos along with the satisfying experimental results are obtained such as: (1) single exposure time reduced to 100 ns (2) multi-frame in one discharge (FPS ⩾ 40) (3)multi-exposure for one pellet so that further observation of the temporal process of pellet ablation may be possible. Through the data analysis on the spatial distribution of pellet ablation clouds in photos taken, the pellet dimensions, trajectory of the cloud and pellet velocity are obtained, and the physical mechanism of pellet-plasma interactions also analyzed. In particular, it is possible to provide an effective means for measuring q-profile of HL-1M plasma.

Three-dimensional time-resolved H pellet trajectory reconstruction in RFX by position sensitive detector Hα diagnostic

Hydrogen pellets injected in reversed field pinches (RFPs) experience large poloidal and toroidal accelerations along magnetic field lines due to the rocket effect caused by asymmetric ablation processes. The 3D pellet trajectory and the global ablation rate is diagnosed in RFX by an absolutely calibrated multiple position sensitive detector (PSD) system looking at the Hα radiation coming from the pellet ablation cloud. The design of the diagnostic is presented. Then the performance is highlighted by cross-comparison between different PSD measurements and with time-integrated images from a CCD camera. Overall accuracy of the order of a few millimeters is achieved, which makes the instrument well-suited to assist transport studies (particle deposition), to estimate the radial profile of fast electron tails (derived from the modulus of pellet acceleration) and to obtain information on internal magnetic field pitch (from the direction of pellet acceleration).

Equilibrium pellet and liquid jet shape under high ablation pressures

Owing to the nonspherical nature of the heat deposition in the pellet ablation cloud by energy loss of incident plasma electrons streaming parallel to the uniform magnetic field, a nonuniform pressure distribution develops at the pellet surface. This can lead to deformation of “soft” cryogenic pellets exposed to high temperature and high density magnetized plasmas. The effect of deformation on the burning rate and stability of the condensed phase is evaluated for pellets and liquid jets.