Intense Neutral Beam Research Articles

Hyperons in neutron stars: studies of hyperon spectroscopy and the hyperon-nucleon interaction with the K-long Facility.

Elucidating the role of strange baryons (hyperons) in neutron stars requires detailed knowledge of hyperon-nucleon interactions in the light (u,d,s) quark sector. The structure of the hyperons and their excitation spectra also directly impact, and are an input to, models of big-bang nucleosynthesis. The upcoming K-long Facility will provide a much-needed intense and clean neutral strange meson beam, from which hyperons can be produced at rates where hyperon structure, hyperon-nucleon interactions and higher-order interactions can be studied with a new level of accuracy and for hitherto unreachable measurements. The new facility has the potential to address long-standing questions surrounding the strange sector of the strong force and its relevance to the structure of atomic nuclei, neutron stars and the cosmos at large. This article is part of the theme issue 'The liminal position of Nuclear Physics: from hadrons to neutron stars'.

Некоторые особенности модификации поверхности ионными пучками очень большого размера

Ion beam surface modification is typically carried out in a laboratory environment where the processing rate is not a big concern. Nevertheless, it is interesting to consider the possibilities that might allow substantial increase (orders of magnitude) in the processing rate, possibly for industrial application. Here we consider two possible approaches to very large scale ion beam surface modification – the use of very broad beam vacuum arc ion sources and the use of ion sources that have been developed to provide intense neutral beams as part of the world controlled fusion research effort. These sources can provide beam intensities in the tens of amperes range, and quasi-dc operation has been demonstrated. A third possibility is scale-up of the Plasma Immersion Ion Implantation approach, and some relevant work has been done along these lines. These approaches to giant ion beam surface modification systems, and the implied parameters and features of such systems, as well as the highly related concern of the necessary accompanying electrical systems, are briefly considered here.

Resistive Interchange Modes Destabilized by Helically Trapped Energetic Ions in a Helical Plasma

A new bursting m=1/n=1 instability (m,n: poloidal and toroidal mode numbers) with rapid frequency chirping down has been observed for the first time in a helical plasma with intense perpendicular neutral beam injection. This is destabilized in the plasma peripheral region by resonant interaction between helically trapped energetic ions and the resistive interchange mode. A large radial electric field is induced near the edge due to enhanced radial transport of the trapped energetic ions by the mode, and leads to clear change in toroidal plasma flow, suppression of microturbulence, and triggering an improvement of bulk plasma confinement.

A 2D Particle in Cell model for ion extraction and focusing in electrostatic accelerators

Negative ions are fundamental to produce intense and high energy neutral beams used to heat the plasma in fusion devices. The processes regulating the ion extraction involve the formation of a sheath on a scale comparable to the Debye length of the plasma. On the other hand, the ion acceleration as a beam is obtained on distances greater than λD. The paper presents a model for both the phases of ion extraction and acceleration of the ions and its implementation in a numerical code. The space charge of particles is deposited following usual Particle in Cell codes technique, while the field is solved with finite element methods. Some hypotheses on the beam plasma transition are described, allowing to model both regions at the same time. The code was tested with the geometry of the NIO1 negative ions source, and the results are compared with existing ray tracing codes and discussed.

Novosibirsk Project of Gas-Dynamic Multiple-Mirror Trap

Development of a new linear device for confinement of fusion plasmas is under way in the Budker Institute of Nuclear Physics, Novosibirsk. The new device combines features of existing GOL-3 and GDT traps, namely, the central GDT-like cell with sloshing ions produced by intense neutral beam injection, and the multiple-mirror end sections for suppression of axial plasma losses. It is designed as a prototype of an energy-efficient neutron source and a testbed for development of mirror-based fusion reactors.

Development of a Neutral Beam Profile Monitor

We developed a novel beam profile monitor for measuring the shape of an intense neutral beam for particle physics. This monitor is based on a detector technology of scintillating fibers, photomultiplier tubes, and sampling ADCs. It has an advantage for its simplicity, reliability, flexibility, and versatility. This monitor was installed in the small-diameter neutral beam for the KL0 →π0 νν experiment at J-PARC. We report in this article the development and performance of this monitor.

Contribution of the Large Helical Device Plasmas to Alfvén Eigenmode Physics in Toroidal Plasmas

In the large helical device (LHD) having three dimensional configuration, Alfvén eigenmodes (AEs) destabilized by energetic ions are widely investigated using neutral beam heated plasmas with monotonic and non-monotonic rotational transform (ι/2π) profiles. In a plasma with monotonic ι/2π-profile, core-localized toroidicity-induced Alfvén eigenmode (TAE) as well as global one are often observed. With the increase in the averaged toroidal beta value, defined as the ratio of total plasma pressure to toroidal magnetic pressure, core-localized TAE with low toroidal mode number becomes global. In a relatively high beta plasma with monotonic ι/2π-profile, two TAEs with different toroidal mode number often interact nonlinearly and generate another modes through three wave coupling. In a plasma with non-monotonic ι/2π-profile generated by intense counter neutral beam current drive, reversed shear Alfvén eigenmode (RSAE) and geodesic acoustic mode (GAM) excited by energetic ions were observed for the first time in a helical plasma. Nonlinear coupling was also observed between RSAE and GAM.

Proof of principle of SAPIS, a new type of polarized-ion source

A new type of polarized negative-ion source, based on the combination of an intense neutral Cs beam colliding with the polarized atomic H or D beam from a Stern–Gerlach device, and using a storage cell (SC) as ionization region, has been developed and tested. H- and D- ions were extracted from the SC. Their polarization was not yet measured. Extrapolation from this prototype source using results from most modern tested components such as the H/D atomic–beam and the Cs0 source shows an appreciable potential of this type of source with intensities comparable to or exceeding those of existing sources.

Precise formation of geometrically focused ion beams

Geometrically focused intense neutral beams for plasma diagnostic consist of many elementary beams formed by a multiaperture ion-optical system and aimed at the focal point. In real conditions, some of the elementary beams may have increased angular divergence and/or deviate from the intended direction, thus diminishing the neutral beam density at the focus. Several improvements to the geometrical focusing are considered in the article including flattening of the plasma profile across the emission surface, using of quasi-Pierce electrodes at the beam periphery, and minimizing the deviation of the electrodes from the spherical form. Application of these measures to the neutral beam Russian diagnostic injector developed in Budker Institute of Nuclear Physics allows an increase of neutral beam current density in the focus by ∼50%.

MHD Instabilities and Their Effects on Plasma Confinement in Large Helical Device Plasmas with Intense Neutral Beam Injection

MHD stability of the Large Helical Device (LHD) plasmas produced with intense neutral beam injection is experimentally studied. When the steep pressure gradient near the edge is produced through L-H transition or linear density ramp experiment, interchange-like MHD modes whose rational surface is located very close to the last closed flux surface are strongly excited in a certain discharge condition and affect the plasma transport appreciably. In NBI-heated plasmas produced at low toroidal field, various Alfvén eigenmodes are often excited. Bursting toroidal Alfvén egenmodes excited by the presence of energetic ions induce appreciable amount of energetic ion loss, but also trigger the formation of internal and edge transport barriers.

Beam optics of the SINGAP 1 MeV 100 mA D− direct current accelerator for thermonuclear fusion

Injection of intense neutral beams (e.g., ITER up to 50 MW of neutral power) based on the neutralization of negative deuterium ions up to energies of 1 MeV is an important option for plasma heating and noninductive current drive in future thermonuclear fusion machines. The objective of the SINGAP experiment is to demonstrate the acceleration of 100 mA of D− to 1 MeV in an electrostatic accelerator concept. It is composed of only two acceleration stages: a preacceleration to 60 keV in a multiaperture structure, and a postacceleration to 1 MeV. The particular feature of the SINGAP accelerator is that the postaccelerator by means of an electrostatic lens merges the 60 keV beamlets into a single beam and accelerates the merged beam to 1 MeV in a single gap. The present article is devoted to the SINGAP optics study (simulation and experiment) for two geometric configurations: cylindrical and rectangular (more ITER relevant). Despite a higher measured divergence (8 mrad) than expected (4–5 mrad calculated) which is due to the magnetic field of the electron suppression system, this study has proved that the SINGAP concept can meet the optical requirement (divergence <5 mrad) for multiamperes injectors.

Simulation of neutral beam tomography using maximum entropy method

A tomography algorithm to maximize the entropy of an image was used for the reconstruction of 2D spatial distribution of intense neutral beams, such as triple beams of KSTAR NBI (Korea Superconducting Tokamak Advanced Research Neutral Beam Injector). A possible detection system was modeled by system geometry parameters and numerical simulations were done to test the reconstruction fidelity of simulated beam profiles. First of all, we simulated with changes of number of cameras and 11 cameras were proposed from this simulation. The simulations with changes of noise level were also done and their relationships with reconstruction fidelity were shown.

Study of Electron Temperature Profiles in GDT during Neutral Beam Heating by Thomson Scattering System

Thomson scattering system based on a ruby laser was applied in the GDT to measure the electron temperature during intense neutral beam injection. The system is capable of measuring the radial profile of electron temperature in the midplane over plasma radii varying from 0 to 12 cm. For this purpose the focusing lens is moved from shot to shot along the laser beam. The scattered light condenser with the polychromator is correspondingly rotated. Angle of scattering is in the range of 86–97 degrees. Thomson scattering system enables measurement of the electron temperature at minimal plasma density of ∼1013 cm−3. The time between shots (about 3 minutes) is long enough to collect, store the data and subsequently calculate the electron temperature. In the paper, the data on radial profiles and temporal behaviour of electron temperature during neutral beam heating at GDT are presented. In the experiments, electron temperatures of the target exceeding 100 eV was observed.

Fragmentation, charge transfer and chemical reactions in C60+/C70+–SF6 collisions

Collisions of C60+ and C70+ with SF6 were investigated in a time of flight mass spectrometer and compared with the results of collisions with Ar and Xe. Reaction cross sections were determined as a function of the center of mass collision energy between 25 and 300 eV. The influence of thermal excitation on fragmentation was examined. The effects of additional vibrational degrees of freedom in the case of SF6 on the fragmentation cross sections are discussed. Charge transfer reactions producing C602+ or C702+ were observed only with SF6. Thermal electron emission from highly excited C60− was used for the formation of an intense neutral C60 beam with 1 keV kinetic energy which was used to investigate charge transfer in neutral C60+SF6 collisions. Reaction products CnF+, n=1–11, were found in SF6 collisions.

Visualization of density structures in tokamak edge plasmas using lithium beam fluorescence

To study the local behavior of the edge plasma density in the DIII-D tokamak, we image the collisionally induced fluorescence from an injected neutral lithium beam using a gated intensified charge-coupled device (CCD) camera coupled to a UNIX workstation. The goal of these experiments is to search for spatially coherent structures in the plasma edge density which may be transiently illuminated by the beam. In our imaging studies to date we have been able to follow the evolution of the edge density as the main plasma discharge progresses through various confinement states under intense neutral beam injection (NBI) heating.

Summary of recent studies of beam-driven BAE and chirping modes in DIII-D

In high beta DIII-D plasmas with intense neutral beam injection, beta induced Alfven eigenmodes (BAE modes) are observed. These instabilities cause concentrated losses of >50% of the fast ions and thus are of concern for future devices. The authors have now observed BAE modes and resultant fast ion loss in full field (2.0 T) discharges where the ratio of parallel velocity to Alfven speed is v||/vA ≈ 0.3. In a few discharges, they have also observed a new instability, a `chirping' mode. These modes have frequencies between 50 and 200 kHz that `whistle' down a factor of two in a single 2 ms burst. They occur in plasmas with relatively large values of fast ion β((βf) ⩾ 1%), Alfven speed (v||/vA ⩽ 0.5) and plasma rotation (frot > 20 kHz). In contrast to the usual Alfven modes, which are fluid modes of the background plasma, the chirping instabilities seem to be beam modes that are nearly stationary in the plasma frame

High harmonic fast waves in high beta plasmas

High harmonic fast magnetosonic waves in high beta plasmas are investigated. In the high beta regime, a combination of reduced group velocity and a high beta enhancement of magnetic pumping lead to such large values of electron absorption that one can expect strong (≊100%) single pass absorption. In particular, by controlling the wave spectrum, the prospect of localized electron heating and current drive appears to be feasible in high beta low-aspect-ratio tokamak regimes. Inclusion of finite-Larmor-radius terms reveals an accessibility limit in the high ion beta regime (βi≊50% for a deuterium plasma) due to mode conversion into an ion-Bernstein-wave-like mode. No similar beta limit is expected for electrons. With increasing ion beta, the ion damping can increase significantly, particularly near the mode-conversion accessibility limit. The presence of an energetic ion component expected during intense neutral beam injection and α heating does not appear to modify the accessibility condition nor cause excessive wave absorption by the energetic ions.

Alfvén wave heating and stability

Alfvén waves in fusion plasmas play an important role in a number of situations. First, in Alfvén Wave Heating (AWH) schemes. Second, both theory and experiment have demonstrated the existence of Global Alfvén Eigenmodes (GAEs). GAEs have been observed in different tokamaks (PRETEXT, TCA, TEXTOR, etc.) and, more recently, in a stellarator (Wendelstein 7-AS) where they were shown to become unstable under intense Neutral Beam injection. Third, the existence and possible destabilization by fast ions of Toroidicity induced Alfvén Eigenmodes (TAEs) has been evidenced both theoretically and experimentally. This destabilization could hamper the operation of a magnetically confined fusion reactor by setting a limit on the number of fusion alpha particles in the plasma. It is therefore crucial to understand the mechanisms leading to the occurrence of the instability and also those that can stabilize the TAEs by increasing the strength of the damping. The aim is to be able to devise possible ways to avoid the instability of Alfvén eigenmodes in a region of parameter space that is compatible with the functioning of a fusion reactor. A global perturbative approach is presented to tackle the problem of the linear stability of TAEs. Our model computes the overall wave particle power transfers to the different species and thus could also be applied to the study of alpha power extraction in the presence of Alfvén waves. We indicate also how to go beyond the perturbative approach.

Ion-sputtering atomic beam source for high-resolution laser spectroscopy of refractory elements

A high-resolution laser spectroscopy system for refractory elements with an argon-ion- sputtering atomic beam source was constructed, and it has been demonstrated that the sputtering method is extremely useful as an atomic beam source for high-resolution laser spectroscopy to measure hyperfine structures and isotope shifts for refractory elements, such as Hf, Ta, and W. This source produced intense neutral atomic beams of more than 1010 atoms/s for these elements, and the yield was easily controlled by adjusting the argon-ion current and the acceleration voltage. Resonance linewidths were 32, 66, and 48 MHz for Hf, Ta, and W, respectively, which are sufficient to measure the hyperfine structures and the isotope shifts in optical transitions. With this system, the signal-to-noise ratio reached more than 106. In addition, the temperature of the sputtered atoms found was rather low compared to the kinetic energy given to the atoms.

Scattering of swift, neutral molecular hydrogen from Ni(110)

The scattering of hydrogen from metal surfaces is of interest in nuclear fusion research for two reasons: (i) the plasma-wall interaction and (ii) the production of intense neutral hydrogen beams for refuelling and heating. The Ni-H system is also of basic interest in surface chemistry and physics for reasons of hydrogen storage, catalysis and fuel cells. Studies of hydrogen interaction with surfaces are proceeding and a discussion follows as to the mechanism of dissociation of the incident molecules.