RESUMEN
For the first time, details of the spatial and temporal acceptable evolution of the merging process of co-rotating electron vortices in a potential well are successfully captured using a "single-shot method" with a high temporal resolution of 10 µs. Four-electron filaments are trapped inside the Beam eXperiment-Upgrade linear trap [H. Himura, Nucl. Instrum. Methods Phys. Res. A 811, 100 (2016)] with a uniform axial magnetic field and co-axial multi-ring electrodes. Images of non-emitting electron filaments are captured using a high-speed camera with up to 1 000 000 fps, a microchannel plate, a fast-decay phosphor screen of which fluorescence duration is 0.15 µs, and a super fine metallic mesh with an open area ratio of 89%. Images captured every 10 µs clearly show the growth of multiple short-wave instabilities in the wing trailing electron vortices. The experimental methods and measurement techniques presented in this paper can contribute to revealing exactly how small vortices evolve into a large structure or turbulence in a potential well through complex processes.
RESUMEN
This paper experimentally investigates the applicability of a micro-channel plate (MCP) followed by a phosphor screen to charged particles along with a calibration method for estimating the acceptable limit of input particle flux and appropriate operation parameters of a particular MCP. For the first time, plasmas consisting of only lithium ions are injected into the MCP. Despite large ion numbers (Ni) on the order of ≃10(7), no deterioration in the effective gain (αG) of the MCP owing to an excess amount of the extracted charge occurs in a certain range of the amplifier voltage (ΔUM) applied to the MCP. The measured αG nearly agrees with the expected value. However, once ΔUM exceeds a limit value, αG eventually begins to saturate. This is also verified in experiments using pure electron plasmas. An appropriate range of ΔUM is presented to avoid saturation and, finally, derive Ni directly from the secondary electron current outputted from the MCP only after the indispensable calibration.
RESUMEN
A system to detect ultraviolet emissions from plasma is developed for multi-point measurement, the target of which is the imaging of turbulence with a high temporal resolution. A fluorescent glass, which converts ultraviolet emissions to visible light with a wavelength of approximately 540 nm, is utilized in the system. Following the conversion, the fluorescent light is transferred with fibre optics, and is converted to electric signals in a location that is sufficiently distant from an electrically noisy environment around the plasma device. This paper describes a prototype system of this diagnostic method and discusses the spectral analysis obtained using a low-aspect-ratio reversed field pinch RELAX.
RESUMEN
We have developed a two-dimensional (2D) electron temperature (T(e)) diagnostic system for thermal structure studies in a low-aspect-ratio reversed field pinch (RFP). The system consists of a soft x-ray (SXR) camera with two pin holes for two-kinds of absorber foils, combined with a high-speed camera. Two SXR images with almost the same viewing area are formed through different absorber foils on a single micro-channel plate (MCP). A 2D Te image can then be obtained by calculating the intensity ratio for each element of the images. We have succeeded in distinguishing T(e) image in quasi-single helicity (QSH) from that in multi-helicity (MH) RFP states, where the former is characterized by concentrated magnetic fluctuation spectrum and the latter, by broad spectrum of edge magnetic fluctuations.
RESUMEN
Tangential soft-x ray (SXR) imaging diagnostic has been developed and three-dimensional (3D) structure of the internal magnetic surface has been deduced by comparing the experimental and calculated two-dimensional SXR images in a reversed field pinch. The SXR imaging system, consisting of a MCP, a fluorescent plate, and an intensified charge coupled device camera, has been installed in REversed field pinch of Low-Aspect-ratio eXperiment (RELAX) machine. Major characteristics of an experimental SXR image could be reproduced by numerical calculations of the image using a single island model, suggesting a helical hot core in RELAX. The SXR imaging system could be useful for 3D structural studies when tangential and vertical simultaneous imaging systems would be installed, with appropriate numerical modeling of 3D structure of the magnetic surfaces.
RESUMEN
A vortex crystal is a quasistationary, symmetric array of intense vortices (clumps). A low level of background vorticity is experimentally observed to assist three clumps in forming an equilateral triangle starting from initial positions on a linear array. The triangle constitutes a unit cell of a crystal in a many-vortex system. The background vortex curbs the orbital motion of the clumps with unequal strengths to arrest them at the vertices of an equilateral triangle by wrapping them with different sized belts of depleted vorticity (ring holes). We characterize the contributions of a low-level background vorticity distribution on the formation of ordered states of clumps in light of the experimental results and existing theories.
RESUMEN
Dynamics of a point vortex in interaction with a broad profile of background vorticity is studied experimentally by using an electron plasma. The observed motion of the vortex compares favorably with a recently proposed theoretical model [D. A. Schecter and D. H. E. Dubin, Phys. Rev. Lett. 83, 2191 (1999)]. Perturbations in the background distribution in the wake of the spiral orbit of the vortex amount to several tens of percent and are considered to be a major reason for deviations of the observation from the linear theoretical model.
