Pellet Trajectory Research Articles

Investigation of electron-distribution function and dynamo mechanisms in a reversed-field pinch by analysis of hydrogen-pellet deflection

In reversed-field pinches, two different mechanisms have been proposed to explain the dynamo process which drives the poloidal current needed to sustain the magnetic configuration: the kinetic dynamo theory and the magnetohydrodynamic (MHD) dynamo theory. Experimentally, they can be distinguished by the radial behavior of the electron distribution function. In this Letter the trajectory deflection of frozen hydrogen pellets has been used as a diagnostic of suprathermal electrons in the plasma. The classical Spitzer-Harm distortion of the electron distribution function consistent with the MHD dynamo electric field is found to give a better modeling of the pellet trajectory.

Transport process of high-speed impurity pellets through curved guide tubes

Curved guide tubes to transport high-speed pellets are used for pellet injection into plasma devices. This article describes the transport process of the high-speed pellets through curved guide tubes. The pellets pass through the curved tubes with repeating reflections. Using the equation of friction at each reflection point, the velocity and the direction of the output pellet are calculated by following the pellet trajectory. The analytic expressions, with respect to the velocity and the direction of the output pellet, are also derived. It has been confirmed that there is good correspondence between the results obtained by following the pellet trajectories, those by the analytic expressions and those by the experiments. The analytic expressions are particularly useful for the design and making of the curved guide tubes used in the pellet injection.

Fast Backward Drift of Pellet Ablatant in Tokamak Plasmas

Pellet ablation in the Rijnhuizen Tokamak Project has been studied with an array of 16 fibers viewing the pellet trajectory along very narrow lines of sight. Rapid backward drift of the ablation cloud away from the pellet was observed. This has implications for the determination of the fueling profile and the penetration of the pellet in the plasma. Furthermore, this movement of the ablation cloud offers a new explanation for the often observed striations in the light emission. In contrast to the picture of the pellet periodically leaving the ablation cloud, the ablation cloud moves periodically away from the pellet.

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).

Pellet imaging techniques in the ASDEX tokamak

As part of a USDOE/ASDEX collaboration, a detailed examination of pellet ablation in ASDEX with a variety of diagnostics has allowed a better understanding of a number of features of hydrogen ice pellet ablation in a plasma. In particular, fast-gated photos with an intensified Xybion CCD video camera allow in situ velocity measurements of the pellet as it penetrates the plasma. With time resolution of typically 100 ns and exposures every 50 μs, the evolution of each pellet in a multipellet ASDEX tokamak plasma discharge can be followed. When the pellet cloud track has striations, the light intensity profile through the cloud is hollow (dark near the pellet), whereas at the beginning or near the end of the pellet trajectory the track is typically smooth (without striations) and has a gaussian-peaked light emission profile. New, single pellet Stark broadened Dα, Dβ, and Dγ spectra, obtained with a tangentially viewing scanning mirror/spectrometer with Reticon array readout, are consistent with cloud densities of 2×1017 cm−3 or higher in the regions of strongest light emission. A spatially resolved array of Dα detectors shows that the light variations during the pellet ablation are not caused solely by a modulation of the incoming energy flux as the pellet crosses rational q surfaces, but instead are a result of dynamic, nonstationary, ablation process.

Pellet imaging techniques in the ASDEX tokamak (abstract)

As part of a USDOE/ASDEX collaboration, a detailed examination of pellet ablation in ASDEX with a variety of diagnostics has allowed a better understanding of a number of features of hydrogen ice pellet ablation in a plasma. In particular, fast-gated photos with an intensified Xybion CCD video camera allow in situ velocity measurements of the pellet as it penetrates the plasma. With time resolution of typically 100 ns and exposures every 50 μs, the evolution of each pellet in a multipellet ASDEX tokamak plasma discharge can be followed. When the pellet cloud track has striations, the light intensity profile through the cloud is hollow (dark near the pellet), whereas at the beginning or near the end of the pellet trajectory the track is typically smooth (without striations) and has a gaussian-peaked light emission profile. New, single pellet Stark broadened Dα, Dβ, and Dγ spectra, obtained with a tangentially viewing scanning mirror/spectrometer with Reticon array readout, are consistent with cloud densities of 2×1017 cm−3 or higher in the regions of strongest light emission. A spatially resolved array of Dα detectors shows that the light variations during the pellet ablation are not caused solely by a modulation of the incoming energy flux as the pellet crosses rational q surfaces, but instead are a result of dynamic, nonstationary, ablation process.

Diagnostic of the current density profile in the vicinity of q = 1: The Hα emission of ablated pellets

The strong striation observed in large tokamaks when a pellet crosses the q=l surface has been shown to be due to an almost shearless annular region and to be extremely sensitive to the details of the magnetic structure. Therefore, if this q-profile exists before pellet injection, analysis of the shape of the striation can be a powerful diagnostic of the local current density distribution. The paper presents a method of precisely calculating the width, depth and shape of the striation for an arbitrary pellet trajectory throughout a discharge. Different mechanisms that are possibly responsible for the flattening of q in the vicinity of q=l (e.g. turbulence or magnetic reconnection) are discussed and it is shown that pellet induced perturbation is not likely to produce such a feature (which confirms its interest as a diagnostic). A simple parameterization of plasma rotation and of a magnetic island is introduced and the model is applied in a simulation of striations displaying important profile distortions even for shallow current perturbations, which could therefore be detected, demonstrating the strong sensitivity of the method. At present, there are no sufficiently documented experimental data, but the available diagnostics on TORE SUPRA, JET, TFTR and JT-60 could be used to study systematically the q=l structure with this method.

Pellet injection on the ZT-40M reversed field pinch

The four barrel pneumatic pellet injector originally used on the Alcator C tokamak at MIT has been reconfigured for use on the ZT-40M reversed field pinch (RFP) at Los Alamos National Laboratory. The lower temperature and density in this device require operation at velocities significantly lower than on Alcator. Allowing a 2-s warmup prior to firing the injector has enabled operation with pressures as low as 40 psig and velocities ranging as low as 200 m/s with H2 pellets, without major cryostat modification. Initial injection experiments in the ZT-40M plasma have verified that pellet refueling is effective for density control, but the net particle inventory increase is less than might be expected based on simple models. Indications of nonoptimal penetration and curvature of the pellet trajectory, in addition to comparable time scales for particle confinement and pellet transit, may account for this.

Laser-guided pellets for inertial fusion

A new scheme for accurately targeting fuel pellets in inertial-fusion reactors is described. The concept uses a feedback-controlled hollow laser as both the pellet carrier and trajectory control effector. The laser must be capable of intensity modulation at frequencies of a few kHz, and power levels of 50 to 100 kW. This scheme eliminates the need for adaptive optics and axial pellet tracking and greatly simplifies the injection system design.