Heavy Ion Beam Probe System Research Articles

Feasibility study of a Heavy Ion Beam Probe for the Thailand Tokamak-1

A Heavy Ion Beam Probe (HIBP) diagnostic system can offer non-disturbing, local measurement of plasma electric potential and various plasma parameters. This paper presents results of a feasibility study for implementing a HIBP system on the Thailand Tokamak-1 (TT-1). With successful plasma discharges achieved in early 2023, TT-1, is planed for upgrades and expansion of diagnostic capabilities to conduct experiments in high confinement modes. Sector K is designated for the installation of the HIBP diagnostic system, where primary beam ions are injected through the top port and secondary beam ions emerge from the horizontal port. The HIBP system consists of two main components: (1) the primary beam injection, and (2) the detection of the secondary beam. In the injection system, the incident direction of the primary beam into the plasma is controlled by two pairs of electrostatic sweepers. For the detection of the secondary beam, the electric field produced by electrostatic sweepers, each with a length of 0.25 m and inclined at 30 degrees, ensures that the secondary beam ions enter the energy analyzer entrance at the center with normal angles. Candidate ions, such as Cs+, Rb+, and K+, with varying energies, are evaluated for their suitability based on beam attenuation and plasma density conditions. Cesium ions are suitable at lower densities (n̄e≤1×1019 m−3), while Rb+ and K+ are preferable at higher plasma densities (n̄e≥1×1019 m−3). The analysis indicates that the ratio of the attenuation current is approximately in the range of 10−3 to 10−2. This study provides valuable insights for the integration of the HIBP diagnostic system into TT-1, enabling investigations of H-mode plasma physics phenomena.

Measurements of 2D poloidal plasma profiles and fluctuations in ECRH plasmas using the heavy ion beam probe system in the TJ-II stellarator

2D poloidal contour plots of plasma potential, plasma density, and their fluctuations have been measured in low density plasmas sustained by Electron Cyclotron Resonance Heating using a heavy ion beam probe (HIBP) system in the TJ-II stellarator. A HIBP has been used in the new energy scanning mode to obtain the measurements for a 2D poloidal cross section of the stellarator. The 2D map for the absolute plasma potential shows a local maximum in the plasma core as expected in low density plasma scenarios. Fluctuations in the HIBP secondary ion current, as a proxy of plasma density fluctuations, appear both in positive and negative density gradient regions, with a normalized level of density fluctuations higher in the negative density gradient region. The TJ-II innovative experimental setup developed using a dual HIBP diagnostic paves the way for model validation on core plasma potential asymmetries and particle transport and fluctuations under positive and negative density gradient scenarios.

Feasibility study of a compact heavy ion source for investigation of laboratory magnetospheric plasma.

We are developing a laser ion source to provide a high brightness multi-charged heavy ion beam as a part of the heavy ion beam probe system, which will be used to diagnose plasma potential in the Ring Trap 1 device at the University of Tokyo. As a probe beam, Nb2+ was selected, and a detailed laser irradiation condition was explored. It was found that the laser power density of 1.2 × 109 W/cm2 gives the maximum particle number of Nb2+ per laser energy from a niobium foil target. Essential ablation plasma parameters to design the laser ion source were also obtained. The expected beam current was more than 12 mA/cm2, with a pulse width of 3.1 µs at 200 mm away from the target.

The impact of fast electrons on pellet injection in the stellarator TJ-II

Efficient plasma core fuelling is a critical issue for achieving steady-state scenarios in magnetically confined fusion devices. The current preferred method to achieve this goal is cryogenic pellet injection (PI). PI’s are now installed in most medium- and large-sized fusion devices. In this paper, recent results from PI experiments in the stellarator TJ-II are reported. Of particular interest is the influence of fast electrons. Whilst edge populations of such electrons do not affect PI in the TJ-II, fast electrons residing in its hot core have significant influence on both pellet ablation and fuelling efficiency. If present in the core, their radial location can be determined from the light emission profile produced by an ablating pellet as it traverses the plasma and confirmed from the power spectral distribution spectrograph of the total secondary beam current collected by a scanning heavy ion beam probe system. Moreover, it is found that if a pellet is subject to excess ablation due to core fast electron impacts, the resultant pellet efficiency (deposited particles/delivered particles) increases by up to 50% with respect to injections into similar, fast-electron free, plasmas. This is consistently found for microwave-heated plasmas, where pellet penetration is shallower than the magnetic axis in TJ-II, when such a population is present, as well as for neutral beam created and heated plasmas in which fast-electrons, generated during magnetic field ramp-up, persist in the core along a discharge. Similarly, when polystyrene pellets (TESPEL) are injected into the same machine sector, an analogous increase in post-injection particle deposition is seen. It is postulated that vaporization of the pellet through heating by fast electrons modifies normal neutral cloud and plasmoid development and hence affects outwards drifting that is inherent to magnetic fusion devices.

Potential measurements with heavy ion beam probe system on LHD

The heavy ion beam probe system in the Large Helical Device (LHD) was improved as follows. At first, the additional new sweeper was installed into the diagnostic port to extend the observable region. By using this sweeper, the potential profile was measured in a wider minor radius range than in previous experiments, in the case of outward shifted magnetic configuration of LHD. Next, the real time control system was installed to control the probe beam orbit for measuring the potential in plasma with large plasma current. In this system, a digital signal processor was used to control the probe beam in real time. The system worked well in the fixed position observation mode. In the sweeping mode for profile measurement, this control system became unstable. The details of this system and the experimental results are reported in this article.

Potential Measurement with the 6-MeV Heavy Ion Beam Probe of LHD

The potential in the plasma core of the Large Helical Device (LHD) was measured with the 6 MeV Heavy Ion Beam Probe (HIBP). The radial profile of the potential in the region where the normalized minor radius, ρ, is less than 0.5 was measured and the electric field obtained from the fitting function of the experimental data was compared with that calculated from neoclassical ambipolarity. The experimental results coincide well with the theoretical calculations. Negative pulses were observed in the potential signal in the inward shifted magnetic configuration. The time constants of these pulses were less than the energy confinement time. Potential fluctuations of the coherent modes were also observed, and one of their frequencies coincided with the geodesic acoustic mode (GAM). In this report, the present status of potential measurements with the HIBP system in LHD is described. c

Magnetic field fluctuation measurement with a heavy ion beam probe in the Large Helical Device

The measurements of magnetic field fluctuation with a heavy ion beam probe (HIBP) in the Large Helical Device (LHD) are considered in this article. Measured value reflects not only the fluctuation at the observation point (sample volume) but also the fluctuation on the beam path. These effects (called as the path integral effects) are considered and calculated for variations of fluctuations in LHD. The distortion of the measured profile by the path integral effect is not significant at the peak of the fluctuation amplitude. In contrast, in the region far from the peak, phony signals due to path integral effects are observed. Finally, the resolution of magnetic field fluctuation measurements with our HIBP system is estimated. Estimated results show that the magnitude of 4.1x10(-5) T can be measured with our system.

Status of the heavy ion beam probe system in the Large Helical Device

A heavy ion beam probe (HIBP) system has been installed into the Large Helical Device (LHD) to measure the spatial profile of the plasma potential and density fluctuations. The optimization of the HIBP system, especially the beam injector, is described. The negative ion beam is required for the MeV beam production in a tandem accelerator. A sputter-type heavy negative ion source has been developed as an intense Au(-) beam source to produce Au(+) beams with energy in the MeV range. The extraction electrodes and the Einzel lens system of the ion source have been designed taking into account the beam optics, and installed into the real machine. Throughout the plasma diagnostics on LHD experiments, the consumptions of vaporized caesium and gold target are being characterized for practical operations. In addition, the experimental charge fractions are compared with the theoretical fractions for understanding the charge-changing behavior of Au(-) ions and optimizing the fraction of Au(+) ions at the exit of the tandem accelerator of the HIBP system.

Observation of the Effects of Radially Sheared Electric Fields by the Use of a Gold Neutral Beam Probe

Intermittent turbulent potential fluctuations in ion-cyclotron wave heated plasmas and effects of their suppression due to strongly sheared plasma rotation with electron-cyclotron heating (ECH) for potential formation are observed in GAMMA 10. Such a shear effect on the central-cell potential data is demonstrated by the use of a heavy ion beam probe (HIBP) for the first time in GAMMA 10. The HIBP system with a gold neutral beam is employed for measuring profiles of radially formed potential as well as the associated radially sheared Er in the central cell. During the application of ECH, the produced stronger Er shear results in reduction of the turbulent fluctuations. The results are consistent with reported X-ray tomography data on the effect of Er shear on turbulence suppression.

Development of mesh probe for the calibration of the HIBP diagnostic system in the JFT-2M tokamak

A direct calibration technique for the heavy ion beam probe system using the mesh probe is established. The mesh probe makes it possible to calibrate the diagnostic system with the same condition as a real plasma measurement. The location of the sample volume could be determined with the spatial resolution of about 7.5 mm. It shows good agreement with this expected result by the trajectory code with an accuracy of about 1%.

Suppression of plasma electrons in the diagnostic ports of the MST

The recent application of a heavy ion beam probe (HIBP) to the Madison Symmetric Torus (MST) has motivated the development of permanent magnet plasma suppression structures. Unconfined plasma at the MST diagnostic ports is free to flow out the ports and into adjoining diagnostic chambers. The HIBP system incorporates seven pairs of high voltage, electrostatic steering plates. Stray charged particles that exit the MST-HIBP ports are attracted to these biased steering plates, loading down the power supplies, and detrimentally affecting the desired operation of the plates. A second source of loading is electron current generated by UV light emitted from the MST plasma. Structures comprised of steel keepers and nickel plated magnets were designed to conform to the walls of the two HIBP diagnostic ports. The magnetic fields in the keeper aperture are able to suppress most of the plasma that would otherwise flow into the HIBP chambers. The fields external to the keeper structure are sufficiently small to avoid perturbing the confining fields at the plasma edge. Analysis indicates that electron current from UV radiation dominates the remaining loading of the HIBP steering plates.The recent application of a heavy ion beam probe (HIBP) to the Madison Symmetric Torus (MST) has motivated the development of permanent magnet plasma suppression structures. Unconfined plasma at the MST diagnostic ports is free to flow out the ports and into adjoining diagnostic chambers. The HIBP system incorporates seven pairs of high voltage, electrostatic steering plates. Stray charged particles that exit the MST-HIBP ports are attracted to these biased steering plates, loading down the power supplies, and detrimentally affecting the desired operation of the plates. A second source of loading is electron current generated by UV light emitted from the MST plasma. Structures comprised of steel keepers and nickel plated magnets were designed to conform to the walls of the two HIBP diagnostic ports. The magnetic fields in the keeper aperture are able to suppress most of the plasma that would otherwise flow into the HIBP chambers. The fields external to the keeper structure are sufficiently small to avoid ...

Potential profile measurements on compact helical system (CHS) using a 200 keV heavy ion beam probe

A 200 keV heavy ion beam probe system has been prepared to measure potential profiles in the compact helical system. Probing beam trajectory is successfully controlled by a secondary beam sweep system to keep the injection angle of the secondary beam constant during the radial scan. As a result, it was observed that the potential profile changes after the neutral beam heating is applied on electron cyclotron heated plasmas.

Active trajectory control for a heavy ion beam probe on the compact helical system

A 200 keV heavy ion beam probe (HIBP) on the compact helical system heliotron/torsatron uses a newly proposed method in order to control complicated beam trajectories in nonaxisymmetrical devices. As a result, the HIBP has successfully measured potential profiles of the toroidal helical plasma. This article will describe the results of the potential profile measurements, together with the HIBP system and procedures to realize the method.

Active control of beam trajectories for heavy ion beam probe on helical magnetic configurations

Active control of beam trajectories using four sets of sweep plates is proposed for a heavy ion beam probe (HIBP) system on helical plasma, such as torsatron, stellarator, etc., in order to solve problems arising from the complicated beam trajectories in its nonaxisymmetrical magnetic field. By this method, a series of the probing beam trajectories giving a whole potential distribution can be managed to pass at a particular point (detector aperture) with the same angle. An equation is introduced as a guiding principle to simplify the optimization process of geometrical configuration of the HIBP system, which will be adjusted for different magnetic configurations by only changing the sweep plate voltages.

Ion trajectory simulations for the TEXT upgrade 2 MeV heavy ion beam probe (abstract)

Ion trajectory calculations play a key role in the design of a heavy ion beam probe system and the interpretation of its measurements. In the case of the HIBP for TEXT Upgrade presently under construction, ion trajectory simulations showed that ions with energies up to 2 MeV have to be injected and detected at two different locations in order to make measurements over the whole plasma cross section. Trajectory calculations also provide essential guidelines for the construction of system components such as energy analyzers. In order to measure fluctuation-induced transport (ñ, φ̃, phase, k), the analyzers will have multiple slits. The distance of the apertures that gives reasonable distances of the sample volumes in the plasma was determined, and examples for sample volumes sizes and shapes, which determine the quality of the measurements, are presented. Another issue involving the quality and the understanding of HIBP measurements is ion beam attenuation. The trajectory code has been used to estimate the signal strength that can be expected from the 2 MeV HIBP. This work was supported by DOE.

Text-upgrade 2 MeV heavy ion beam probe (abstract)

A unique 2 MeV heavy ion beam probe diagnostic system, presently under construction, with two injection lines and two energy analyzers will be used to measure the space potential as well as density, electrostatic (and possibly magnetic) fluctuations, and average wave numbers in the TEXT upgrade tokamak. The capabilities of the 2 MeV beam include a good penetration of the TEXT-U magnetic fields (up to 2.2 T) and access to most of the plasma cross section (90%) including all null points (during divertor operations), the top, bottom, and outside edges. We plan to use parallel plate electrostatic energy analyzers which require that 400 kV be held on a large electrode across a 20 cm vacuum gap. Feasibility tests of such analyzers are under way. Progress of construction and testing of different parts of the system are reported. A description of the 2 MeV heavy ion beam probe system and a discussion on the measurements to be performed will be presented. This work was supported by DOE.

Potential Formation in the Plasma Confinement Region of a Radio-Frequency Plugged Linear Device

Plasma potential formation in an open-ended plasma confinement system with RF plugging (the RFC-XX-M device) is investigated. The plasma potential in the central confinement region is measured with a heavy ion beam probe system and potentials at the RF plug section are measured with multi-grid energy analyzers. The measured plasma potential is compared with that deduced from the generalized Pastukhov formula. Results show that the plasma potential develops as an ambipolar potential to equate ion and electron end losses. During RF plugging, electrons are heated by Landau damping, while ions are not heated since adiabatic conditions are satisfied during ion plugging in this experiment.

Design of a Wien Filter Type Heavy Ion Beam Probe System for the Measurement of Plasma Potential

This paper describes the design of a Heavy Ion Beam Probe (HIBP) system to measure the two-dimensional plasma potential profile in the central mirror region of an open ended plasma confinement device (RFC-XX-M). This HIBP system incorporates three novel features. The first is a Wien filter-type guide field that reduces the required beam acceleraltion voltage. The second is a set of deflecting electrodes located at the exit position of the secondary ion beam that steers all secondary ions into the analyzer at the same angle. And the third is a new method of absolute calibration.