Magnetic Beach Research Articles

Structure of intermittent variation of floating potential in a converging field following a magnetic beach ECR plasma source

In the converging field configuration of cylindrical plasma following an ECR plasma source, intermittent variation of floating potential, which is a negative spike with about a ten microsecond duration, is observed. The event is ubiquitous; independent of measurement position and magnetic field. Cross-correlation analysis for two floating potentials at separate positions reveals that a strong correlation exists within the event timescale. The correlation along the magnetic field line suggests an axially elongated structure. The radial extent of the structure is clarified by the radial correlation profile. Different FWHM of the radial profile at the plasma production region and the downstream converging field region is essentially explained by field line tracing.

The Material Plasma Exposure eXperiment: Mission and conceptual design

• New linear plasma device MPEX will address plasma material interaction gaps for fusion. • MPEX will have increased capabilities over existing linear plasma devices and will be world-leading when completed. • MPEX will expose apriori neutron irradiated material samples for fusion reactor divertor relevant heat and particle fluxes. • The conceptual design of MPEX is finished. Mastering Plasma Material Interactions (PMI) is key for obtaining a high performance, high duty-cycle and safe operating fusion reactor. Numerous gaps exist in PMI which have to be addressed before a reactor can be built. In particular the lack of data at high ion fluence, fusion reactor divertor relevant plasma conditions and neutron displacement damage requires new experimental devices to be able to develop plasma facing materials and components. This has been recognized in the community and the U.S. fusion program is addressing this need with a new linear plasma device—the Material Plasma Exposure eXperiment (MPEX). MPEX will be a superconducting linear plasma device with magnetic fields of up to 2.5 T. The plasma source is a high-power helicon source (200 kW, 13.56 MHz). The electrons will be heated via Electron Bernstein Waves with microwaves using multiple 70 GHz gyrotrons (up to 600 kW in total). Ions will be heated via ion cyclotron heating in the so-called “magnetic beach heating” scheme in the frequency range of 6−9 MHz (up to 400 kW in total). An overview of the conceptual design and the project/design requirements is given.

On the Theory of ICR Heating by the Magnetic Beach Method

Current interest in ion cyclotron resonance (ICR) heating by the magnetic beach method is motivated by the experiments carried out with the VASIMR plasma thruster. The method implies excitation of Alfven eigenmodes of a plasma column (or a plasma jet in the case of plasma thruster) outside the ICR zone. In the present work, it is shown that, under conditions typical of, e.g., plasma thrusters, most of the electromagnetic energy deposited in the plasma can be transferred to the so-called Alfven continuum, rather than to the discrete spectrum of Alfven eigenmodes. Due to the phenomenon of the Alfven resonance, continuous-spectrum oscillations transform into short-wavelength lower hybrid oscillations, which differ from the Alfven oscillations in their polarization. These oscillations efficiently interact with electrons, thereby heating them.

High-Density Plasma Production in Converging Field Following a Magnetic Beach Plasma Source

High-Density Plasma Production in Converging Field Following a Magnetic Beach Plasma Source

Preliminary computation of the gap eigenmode of shear Alfvén waves on the LAPD** Project supported by the National Natural Science Foundation of China (Grant No. 11405271), the China Postdoctoral Science Foundation (Grant No. 2017M612901), the Fund from Chongqing Science and Technology Commission (Grant No. cstc2017jcyjAX0047), Chongqing Postdoctoral Special Foundation (Grant No. Xm2017109), the Fundamental Research Funds for Central Universities, China

Characterizing the gap eigenmode of shear Alfvén waves (SAWs) and its interaction with energetic ions is important to the success of magnetically confined fusion. Previous studies have reported an experimental observation of the spectral gap of SAW on the on Large Plasma Device (LAPD) (Zhang et al. 2008 Phys. Plasmas 15 012103), a linear large plasma device (Gekelman et al. 1991 Rev. Sci. Instrum. 62 2875) possessing easier diagnostic access and lower cost compared with traditional fusion devices, and analytical theory and numerical gap eigenmode using ideal conditions (Chang 2014 Ph.D Thesis at Australian National University). To guide experimental implementation, the present work models the gap eigenmode of SAWs using exact LAPD parameters. A full picture of the wave field for previous experiment reveals that the previously observed spectral gap is not global but an axially local result. To form a global spectral gap, the number of magnetic mirrors has to be increased and stronger static magnetic field makes it clearer. Such a spectral gap is obtained for the magnetic field of B0(z) = 1.2 + 0.6 cos[2π (z − 33.68)/3.63] with 7.74-m magnetic beach. By introducing two types of local defects (corresponding to Eθ(z0) = 0 and respectively), odd-parity and even-parity discrete eigenmodes are formed clearly inside the gap. The strength of these gap eigenmodes decreases significantly with collision frequency, which is consistent with previous studies. Parameter scans show that these gap eigenmodes can be even formed successfully for the field strength of B0(z) = 0.2 + 0.1 cos[2π (z − 33.68)/3.63] and with only four magnetic mirrors, which are achievable by the LAPD at its present status. This work can serve as a strong motivation and direct reference for the experimental implementation of the gap eigenmode of SAWs on the LAPD and other linear plasma devices.

Ion cyclotron resonant heating in the central cell of the Keda Mirror with AXisymmetricity (KMAX)

In this paper, we report the results of ion cyclotron resonance frequency (ICRF) heating in the central cell of a fully axisymmetric tandem mirror. With a total power of 100 kW radiated by double half-turn and half-turn antennas, the plasma diamagnetism increases by 15-fold, with a corresponding peak β⊥ ∼2%, density ∼1.5×1018m−3, and total temperature ∼60 eV. The effects of the magnetic configuration on resonance heating and wave emission are studied by varying the magnetic fields at the midplane and at the location of the antennas, respectively; the results confirm that the magnetic beach configuration is key to successful ICRF heating. The axial phase speed measurements suggest that the excited wave is a slow wave in the plasma core and a fast wave at the edge.

Ion cyclotron resonance heating (ICRH) systems for the Keda Mirror with AXisymmetry (KMAX).

In this paper, we describe the engineering work involved in constructing two ion cyclotron resonance heating (ICRH) systems for use in the Keda Mirror with AXisymmetry tandem mirror experiment. Because they offer an effective and robust heating method, ICRH systems have been widely used in a variety of plasma experiments. The goal of our system is to heat the hydrogen plasma contained in the central cell using the fundamental ion cyclotron frequency. Both systems can deliver a radiofrequency power of ∼120 kW with adjustable operating frequencies that are tuned to be slightly lower than their local ion cyclotron frequencies. Two types of antennas are installed in the central cell in an attempt to launch both slow and fast waves. The heating mechanism is reliant on the magnetic beach effect for slow waves.

Plasma source development for fusion-relevant material testing

Plasma-facing materials in the divertor of a magnetic fusion reactor have to tolerate steady state plasma heat fluxes in the range of 10 MW/m2 for ∼107 s, in addition to fusion neutron fluences, which can damage the plasma-facing materials to high displacements per atom (dpa) of ∼50 dpa. Materials solutions needed for the plasma-facing components are yet to be developed and tested. The material plasma exposure experiment (MPEX) is a newly proposed steady state linear plasma device designed to deliver the necessary plasma heat flux to a target for testing, including the capability to expose a priori neutron-damaged material samples to those plasmas. The requirements of the plasma source needed to deliver the required heat flux are being developed on the Proto-MPEX device which is a linear high-intensity radio-frequency (RF) plasma source that combines a high-density helicon plasma generator with electron- and ion-heating sections. The device is being used to study the physics of heating overdense plasmas in a linear configuration. The helicon plasma is operated at 13.56 MHz with RF power levels up to 120 kW. Microwaves at 28 GHz (∼30 kW) are coupled to the electrons in the overdense helicon plasma via electron Bernstein waves and ion cyclotron heating at 7–9 MHz (∼30 kW) is via a magnetic beach approach. High plasma densities >6 × 1019/m3 have been produced in deuterium, with electron temperatures that can range from 2 to >10 eV. Operation with on-axis magnetic field strengths between 0.6 and 1.4 T is typical. The plasma heat flux delivered to a target can be >10 MW/m2, depending on the operating conditions. An initial plasma material interaction experiment with a thin tungsten target exposed to this high heat flux in a predominantly helium plasma showed helium bubble formation near the surface, with no indication of source impurity contamination on the target.

On RF heating of inhomogeneous collisional plasma under ion-cyclotron resonance conditions

During ion-cyclotron resonance (ICR) heating of plasma by the magnetic beach method, as well as in some other versions of ICR heating, it is necessary to excite Alfven oscillations. In this case, it is difficult to avoid the phenomenon of the Alfven resonance, in which Alfven oscillations transform into lower hybrid oscillations. The latter efficiently interact with electrons, due to which most of the deposited RF energy is spent on electron (rather than ion) heating. The Alfven resonance takes place due to plasma inhomogeneity across the external magnetic field. Therefore, it could be expected that variations in the plasma density profile would substantially affect the efficiency of the interaction of RF fields with charged particles. However, the results obtained for different plasma density profiles proved to be nearly the same. In the present work, a plasma is considered the parameters of which correspond to those planned in future ICR plasma heating experiments on the PS-1 facility at the Kurchatov Institute. When analyzing the interaction of RF fields with charged particles, both the collisionless resonance interaction and the interaction caused by Coulomb collisions are taken into account, because, in those experiments, the Coulomb collision frequency will be comparable with the frequency of the heating field. Antennas used for ICR heating excite RF oscillations with a wide spectrum of wavenumbers along the magnetic field. After averaging over the spectrum, the absorbed RF energy calculated with allowance for collisions turns out to be close to that absorbed in collisionless plasma, the energy fraction absorbed by electrons being substantially larger than that absorbed by ions.

On the influence of Alfvén resonance on ion cyclotron resonance heating

Physical processes determining the excitation of RF electromagnetic fields in a plasma column in a magnetic field are analyzed. The Alfven resonance plays an important role at frequencies close to the ion cyclotron frequency. It leads to the enhancement of the RF electric field and transformation of Alfven oscillations with a predominantly transverse polarization of the electric field into lower hybrid ones, which have a significant longitudinal component of the electric field. Lower hybrid oscillations efficiently interact with electrons causing their heating. Difficulties in the implementation of ion cyclotron resonance heating by the magnetic beach method are outlined. The processes considered in this work can be important for the VASIMR plasma engine.

Fast wave heating in mirror traps

The magnetic beach, minority and second harmonic ion cyclotron heating schemes are addressed. Their specific features in application to mirror devices and the results of computations are discussed.

Generation of supersonic plasma flows using an applied-field MPD arcjet and ICRF heating

A quasi-steady, supersonic plasma flow was produced by using a magneto-plasma-dynamic arcjet (MPDA) in a divergent magnetic nozzle. The ion acoustic Mach number, Mi, evaluated by spectroscopy and Mach probe measurement, was unity in the uniform field near the MPDA and reached almost 3 in the divergent magnetic nozzle. In the case of a subsonic flow near the exit of the MPDA, a magnetic Laval nozzle was effective in converting it to a supersonic flow with Mi = 1 at the throat. We heated ions of the fast-flowing plasma by ICRF (ion-cyclotron-range of frequency) in a magnetic beach field. The increased thermal energy was converted to flow energy as it passed through the divergent magnetic nozzle in accordance with the constant magnetic moment. We were able to successfully control the plasma flow with a wide range of densities for basic magnetohydrodynamic (MHD) studies and space thruster applications.

Second harmonic ion cyclotron heating of sloshing ions in a straight field line mirror

A qualitative analysis of second harmonic heating is carried out, in which a fast magnetosonic wave is launched from a location near the magnetic mirror (where the magnetic field is stronger than the second harmonic resonance field) and directed to the midplane of the open trap. The analysis shows that there is no “magnetic beach” heating in contrast to the case with minority heating on the fundamental harmonic. Conversion to the ion Bernstein wave would distort the heating pattern, and the condition for this conversion is estimated. The scenario of second harmonic heavy ion heating is examined numerically. In the scenario chosen, the regime of global resonance overlapping is achieved that provides good heating performance. The computations show that the power deposition is core, the amount of deposited power does not depend sensitively on the parameters of the discharge, and the range of plasma beta at which the heating is efficient is not narrow. The estimated antenna Q is noticeably low and, therefore, the antenna performance is high.

Alfvén wave excitation and single-pass ion cyclotron heating in a fast-flowing plasma

Alfvén wave excitation and ion heating experiments were performed in a fast flowing plasma. When rf waves in the ion cyclotron range of frequency were excited by right- and left-handed helically wound antennas, shear and compressional Alfvén waves with azimuthal mode number m=−1 and m=+1, respectively were excited. The dispersion relations of the propagating waves were obtained experimentally and compared with the theoretical ones, including the Doppler effect of the plasma flow. Strong ion heating was observed in the fast-flowing plasma when rf waves were launched by the right-handed helically wound antenna in a magnetic beach configuration. The plasma thermal energy W⊥ and the ion temperature Ti drastically increased during the rf pulse. This large increase was observed under lower-density conditions, where the ratio of the ion cyclotron frequency to the ion-ion collision frequency becomes high. The resonance magnetic field was affected by the Doppler shift due to the fast-flowing plasma.

Improvement of Flow Characteristics for an Advanced Plasma Thruster

A higher specific impulse and a larger thrust are required for a manned interplanetary space thruster. Until the realization of a fusion-plasma thruster, a magneto-plasma-dynamic arcjet (MPDA) powered by a fission reactor is one of the promising candidates for a manned Mars space thruster. The MPDA plasma is accelerated axially by a self-induced j × B force. Thrust performance of the MPDA is expected to increase by applying a magnetic nozzle instead of a solid nozzle. In order to get a much higher thruster performance, two methods have been investigated in the HITOP device, Tohoku University. One is to use a magnetic Laval nozzle in the vicinity of the MPDA muzzle for converting the high ion thermal energy to the axial flow energy. The other is to heat ions by use of an ICRF antenna in the divergent magnetic nozzle. It is found that by use of a small-sized Laval-type magnetic nozzle, the subsonic flow near the muzzle is converted to be supersonic through the magnetic Laval nozzle. A fast-flowing plasma is successfully heated by use of an ICRF antenna in the magnetic beach configuration.

Production of a High-Mach-Number Plasma Flow for an Advanced Plasma Space Thruster

A higher specific impulse and a larger thrust are required for a manned interplanetary space thruster. Prior to a realization of a fusion-plasma thruster, a magneto-plasma-dynamic arcjet (MPDA) powered by a fission reactor is one of the promising candidates for a manned Mars space thruster. The MPDA plasma is accelerated axially by a self-induced j × B force. Thrust performance of the MPDA is expected to increase by applying a magnetic nozzle instead of a solid nozzle. In order to get a much higher thruster performance, two methods have been investigated in the HITOP device, Tohoku University. One is to use a magnetic Laval nozzle in the vicinity of the MPDA muzzle for converting the high ion thermal energy to the axial flow energy. The other is to heat ions by use of an ICRF antenna in the divergent magnetic nozzle. It is found that by use of a small-sized Laval-type magnetic nozzle, the subsonic flow near the muzzle is converted to be supersonic through the magnetic Laval nozzle. A fast-flowing plasma is successfully heated by use of an ICRF antenna in the magnetic beach configuration.

Shear Alfvén waves in a magnetic beach and the roles of electron and ion damping

Experiments are performed in the Large Plasma Device (LaPD) [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles to study the propagation of the shear Alfvén wave in a parallel gradient of the background magnetic field. The waves are excited by modulating a field-aligned electron current drawn to a disk antenna with a radius on the order of the electron skin-depth, δ=c/ωpe. The resulting shear waves have a nonzero parallel electric field and propagate both parallel and perpendicular to the background magnetic field. In this experiment, the wave is launched in a region where its frequency, ω equals one-half the local ion-cyclotron frequency, ωci and the local Alfvén speed, vA, is approximately equal to the electron thermal speed, v̄e. The wave propagates along a slowly decreasing background field to where ω=ωci and vA≈v̄e/2. The wave thus propagates from a region where Landau damping is significant to where ion-cyclotron damping dominates. Detailed two dimensional measurements of the wave magnetic field morphology are presented. The measured wavelength decreases in accord with WKB solutions of a modified wave equation. Wave damping is also observed and dissipation by both ions and electrons is required in the WKB model to fit the data. Suppression of the damping via electrons in the model results in a predicted wave magnetic field amplitude twenty times larger at the ion-cyclotron resonance point than observed.

Strong narrow-band electron cyclotron emission from a mirror plasma heated by electron cyclotron waves

Strong electromagnetic radiation is observed in a narrow band slightly above the frequency of a heating wave that is absorbed by electrons near the electron cyclotron resonance layer in a magnetic beach. The frequency spectrum consists of a sharp component and a broad background, which is enhanced by more than 30 dB and 10 dB, respectively, above the radiation of thermal electrons. This observation is explained in terms of cyclotron radiation emitted by electrons, localized in a magnetic mirror, that are resonantly heated and bunched by a strong monochromatic wave.

ICRF Plasma Heating in Axisymmetric Mirrors

The problems of Alfven and ion cyclotron heating in axicymmetrical mirrors are addressed.For the Alfven resonance heating scheme, the properties of continuum waves and kinetic (modified) Alfven waves are considered. The numerical modeling and experimental results for Alfven resonance heating with triple loop antenna are discussed.The specific features of magnetic beach ion cyclotron heating scenario in axisymmetric mirror are considered from the point of view of propagation condition and accessibility of the fast Alfven wave for excitation with external currents. The magnetic beach for continuum Alfven wave is analyzed. Using ray tracing calculations, the significant cyclotron damping of kinetic Alfven wave is predicted for the magnetic beach case.For fast magnetosonic wave cyclotron heating, the mechanisms of wave energy damping are described. The structure of global resonances is discussed. Heating scenarios with single global resonance and resonance overlapping are compared.The applicability of the scenarios mentioned for mirror based neutron source is addressed.

Modeling of electron-cyclotron-resonance ion source and scaling laws

The heating and confinement of electrons in an electron-cyclotron-resonance ion source (ECRIS) plasma is described: the single particle motion in a magnetic beach is presented, showing the role of the rf wave magnetic field in the electron losses. This effect can be approximately modeled by a Fokker–Planck quasilinear equation in velocity space. This modeling is discussed and numerical results are presented. The behavior of the wave close to resonance is analyzed, taking into account the thermal motion of the electrons. The role of the different design parameters (including rf frequency) is shown and the major ingredients for the design of an efficient ECRIS for multiply charged ions production are summarized.