Magnetic Cusp Research Articles

Beam Formation Studies on the CERN IS03b H Source

An H- ion source is being operated at the new 160 MeV linear injector (Linac4) of the CERN accelerator complex. The source’s plasma is of the Radio Frequency Inductively Coupled Plasma type (RF-ICP), without magnetic cusp and runs with Cs-loss compensation [1]. Vertical downward oriented filter- and electron dump-dipolar magnetic fields expand over the plasma chamber, beam-formation, beam-extraction and electron dump regions and generate horizontal asymmetry and beam angular deflection partially compensated by mechanical alignment of the front-end. The H- beam is generated via volume and caesiated plasma surface modes, the latter inducing a radial asymmetry characterized by an increased current density close to the plasma electrode surface [2]. Asymmetries affecting the meniscus shape, or its current density have to be simulated via 3D Particle In Cell Monte Carlo (PIC-MC) solvers, such as the Orsay Negative Ion eXtraction code (ONIX) [3]. Validation of these simulations require dedicated measurements. This contribution describes experimental methods newly implemented at CERN’s ion source test stand and initial results for Optical and Beam Emission Spectroscopy (OES, BES), emittance and beam profile measurements. In a later stage, the gathered data sets can be compared to source plasma parameters and extracted beam parameters from PIC-MC simulations, once coupled to the Ion Beam Simulation (IBSimu) [4] beam transport code. The experimental parameter space includes RF-power, density of neutrals, position of the RF coil and extraction field. Beams of H-, D- and protons were produced; examples of measured data are presented in this contribution.

Numerical analysis of RF discharge in a nonuniform magnetic field

The radio-frequency (RF) plasma thruster has been studied to overcome the issue of a reduced lifetime of plasma thrusters due to electrode erosion, and it has been reported that a nonuniform magnetic field near the RF antenna improves the thrust performance. In this study, to obtain knowledge for optimizing the configuration of the thruster and the magnetic field, the RF discharge in a nonuniform magnetic field is numerically investigated focusing on RF power absorption and plasma generation, and the effect of the positional relation between the magnetic cusp and the RF antenna on the RF discharge is discussed. We adopt the two-fluid plasma model considering ions and electrons and confirm the qualitative validity of the simulation by comparing the simulation results with the results of a laboratory experiment. The simulation reveals that the energy transport due to the Ey×B drift from the region of RF power absorption near the RF antenna determines the distributions of the plasma generation and temperature. Therefore, the plasma distribution changes depending on the RF antenna position and the phase of the RF period because the Ey×B drift direction changes depending on the directions of the electric and magnetic fields. In addition, we find that it is important to place the antenna in a strong magnetic field for effective power absorption because this increases the azimuthal diamagnetic current.

Ionized Molecular Hydrogen Build-Up and Confinement Inside the Space Charge of a Continually Resupplied Electron Cloud

A cylindrically symmetric electromagnetic ion trap has been constructed that creates a potential well suitable for confining ions with the space charge of a continually resupplied electron cloud. The trap uses permanent magnets to form a linear array of magnetic field ring cusps in a cylindrical geometry and includes electrostatic plugging of the cusping magnetic fields. The ion source is electron bombardment ionization of a controlled leak of thermal molecular hydrogen directly inside the trap, and a potential well created by those selfsame electrons is the mechanism of ion confinement. The ion trap is unique in that the confinement volume has relatively minimal external static electric and magnetic field intrusion into the regions intended to confine ions and no overall axial magnetic field. Another standout feature is the inclusion of electrostatic elements in two locations inaccessible to electrons inside the trap that provide axial ion confinement in conjunction with electrons in the trap. Build-up of ions, and therefore confinement, was demonstrated via an ion signal that increased in intensity over a period of several dozen expected axial ion transit times while injecting electrons. A study has been conducted for a range of incident electron currents from the electron flood gun for several fixed partial pressures of molecular hydrogen and interpreted using a full particle-in-cell simulation of the trap. Data were acquired via a microchannel plate and phosphor screen assembly biased in a manner that could only be excited by positively charged particles exiting the trap. An order of magnitude estimate for mean lifetime is made using a combination of simulated charged particle population estimates and measured data.

Design and Experiments of the Sheet Electron Beam Transport with Periodic Cusped Magnetic Focusing for Terahertz Traveling-Wave Tubes

The successful transport of a sheet electron beam under the periodic cusped magnet (PCM) focusing at the terahertz frequencies is reported. The sheet beam with a current density of 285 A/cm2 is intended for the developing G-band sheet-beam traveling-wave tube (TWT) whose operating voltage is nominally 24.5 kV. A beamstick was developed to validate the design of the electron optics system, which is considered as the most challenging part for developing a sheet-beam device. A beam transmission ratio of 81% is achieved over a distance of 37.5 mm at a cathode voltage of −25.0 kV. The total current and the collector current were measured to be 125 and 102 mA, respectively. The experimental results are promising, demonstrating that the PCM scheme is capable of focusing a high-current-density sheet beam and hence can find use in the terahertz TWTs, offering the advantages of compact size and light weight.

Development of a Magnetic Cusp Gun for a 1-THz Harmonic Gyrotron

High-quality axis-encircling electron beam is essential to drive a high-harmonic gyrotron. A magnetic cusp gun that can generate a low-spread axis-encircling electron beam at an extremely high magnetic-compression ratio was developed for a 1-THz fourth-harmonic gyrotron toward continuous-wave operation. The optimized gun could generate an axis-encircling electron beam with 80-kV voltage, 1-A current, and a nearly constant pitch factor of about 1.5 in a wide range of magnetic field strength. The transverse velocity spread was less than 2.1% and the guiding center radius was <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$26.6 ~\mu \text{m}$ </tex-math></inline-formula> . The design was verified by using MAGIC, EGUN, and CST simulations and good agreements were achieved.

On the horizontal currents over the Martian magnetic cusp

Localized crustal magnetization over heavily cratered southern hemisphere at Mars gives rise to open magnetic field configurations which interact with the solar wind magnetic field to form magnetic cusp. The downward acceleration of energetic electrons in these cusps can produce aurora and an extended topside ionospheric structure over regions of magnetic anomalies. We report plasma collisions with the neutral atmosphere at one of the Martian cusps located at 82 o S and 180 o E, where the crustal field is strong with a radial component ~35° from the local zenith. We find that the dynamo region in the upper ionosphere of Mars is located between altitudes of 102 km and 210 km. The electrons in this region are constrained to gyrate along magnetic field lines while ions are dragged by neutrals and move along the direction of applied force. In the absence of the electric field, the horizontal current in the Martian dynamo is generated by the differential motion of ions and electrons. We find that the bulk of the current density is equatorward and confined within the Martian dynamo near the ionospheric peak with a magnitude of ~3.5 µA/m 2 . We also find that the eastward current density of magnitude ~0.4 µA/m 2 peaking near the upper boundary of the Martian dynamo is generated by magnetized ions in the F × B direction.

Continuous Wave Operation of a Ka-Band Broadband High-Power Sheet Beam Traveling-Wave Tube

To meet the need to transmit increasingly massive volumes of data, a more than 3 kW continuous wave (CW) Ka-band sheet beam traveling-wave tube (SB-TWT) is reported for the first time. Working under unsaturated drive, the maximum gain obtained is 29 dB with a sheet beam of 30.9 kV, 0.82 A, and the efficiency exceeds 24% with a single-stage collector depressed to -15 kV. The 1-dB bandwidth reaches 6 GHz from 26 GHz to 32 GHz and the bandwidth with output power greater than 2 kW can even reach 7GHz from 26 GHz to 33 GHz. The realization of 99.8% static transmission as well as over 97% actual CW transmission of sheet beam is verified successfully, which benefits from adopting periodic cusp magnet (PCM) focusing, and thereby total weight is less than 15 kg. The relevant experimental details are introduced in this letter.

Double‐Peak Structures of Martian Nightside Total Electron Content in Strong Crustal Magnetic Cusp Regions

AbstractWith the help of the high‐spatial‐resolution total electron content (TEC) data obtained by Mars Advanced Radar for Subsurface and Ionospheric Sounding on board the Mars Express (MEx) during June 2005 to September 2007, the detailed configurations of the Martian nightside ionosphere in crustal magnetic field cusp regions are researched in this paper. It is found that double‐peak structures of orbital observed TEC values frequently appeared when MEx was crossing through the strong crustal magnetic field cusp regions. This observational result implies that the Martian nightside TECs in the periphery of the cusp regions are generally higher than those in the central areas of the cusp regions, a phenomenon never reported before. By combining the TEC data with the in‐situ electron flux data obtained by the Analyzer of Space Plasma and Energetic Atoms‐3 (ASPERA‐3), it is found that the difference between the fluxes of the electrons precipitating in the central and peripheral areas of the cusp regions can be responsible for the observed TEC double‐peak structures.

A Magnetically Filtered Atomic Oxygen Plasma Source for Low-Earth-Orbit Simulation

A magnetically filtered oxygen plasma source has been developed and characterized for the purpose of simulating the low-Earth-orbit environment. The source uses a magnetic ring cusp geometry coupled with a transverse magnetic filter to produce a plasma stream consisting of streaming atomic oxygen ions with energy, density, and ion species composition corresponding to orbital conditions at altitudes ranging from 275 to 550 km. Plasma properties downstream of the source are dependent on the operational settings for source flow rate and discharge current. The properties are examined as a function of these operating parameters to optimize the production of ions with energy and density relevant for low-Earth-orbit satellite drag applications. At each source operational setting presented in this work, the ion energy distributions are measured 0.55 m downstream of the source using a retarding potential analyzer, and the streaming ions were shown to have most probable energies ranging from 2.8 to 7 eV or 5800 to . In addition, a Wein filter () and a residual gas analyzer are used to measure the ion species percent composition as a function of the operational conditions. Source flow rate and discharge current settings are presented that allow for the composition of atomic oxygen ions to be adjusted from 63 to 90% of the total plasma ion population.

Particle‐In‐Cell Modeling of Martian Magnetic Cusps and Their Role in Enhancing Nightside Ionospheric Ion Escape

AbstractAmongst various escape channels, ion outflow is a major contributor to atmospheric loss at Mars over geologic time. On Mars' nightside, observations have indicated that cusp regions within crustal magnetic fields are associated with phenomena such as accelerated particle populations, discrete auroral emissions, and ionospheric outflow; however, the kinetic physics occurring within crustal magnetic cusps is poorly understood. Here, we present 1.5‐dimensional particle‐in‐cell simulations of magnetospheric‐ionospheric interactions within martian crustal magnetic cusp regions of varying strength. Simulation results demonstrate the formation of quasi‐static, field‐aligned potentials pointing away from Mars that accelerate electrons into the martian atmosphere while accelerating ions away, thereby enhancing ionospheric escape. Escaping ionospheric flux scales with crustal field strength, with 160 nT crustal fields yielding &gt;2× the ion escape flux than in the case with no crustal fields. We discuss these results and conclude that magnetic cusp regions may be significant sources of ion loss at Mars.

Magnetic cusp confinement in low-β plasmas revisited

Magnetic cusps have been used for more than 50 years to limit charged particle losses to the walls and confine the plasma in a large variety of plasma sources or ion sources. Quantification of the effective loss area has been the subject of many experimental as well as theoretical investigations in the 1970s–1990s. In spite of this effort, there is no fully reliable expression of the effective wall loss as a function of cusp magnetic field, electron temperature, ion mass, gas pressure, etc. We describe in this paper a first attempt at obtaining scaling laws for the effective loss width of magnetic cusps based on two-dimensional particle-in-cell Monte Carlo collision simulations. The results show that the calculated leak width follows a 1/B scaling in the collisionless low B limit, is approximately proportional to the hybrid gyroradius with an ion velocity equal to the Bohm velocity, and is proportional to the square root of gas pressure in the collisional limit.

Development of a compact coaxial cusped periodic permanent magnet focusing system.

To achieve the application of a periodic permanent magnet in high power microwave, a compact coaxial cusped periodic permanent magnet (CPPM) focusing system is constructed. The system consists of permanent magnets with different magnetization directions and soft magnets. Taking the required magnetic field performance and the effect of demagnetization into account, NdFeB and FeCoV are selected as the permanent and soft magnet materials. After the system is constructed, the magnetic field is measured. The results show that the guiding magnetic field strength and period of the CPPM are about 0.29 T and 26 mm, respectively, and the magnetic field distribution of measurement shows good agreement with the simulation results. However, there are some differences between the measurement and simulation results, and the differences are compared and analyzed here.

Simulation study of influence of exit magnetic separatrix angle on plume divergence control

Cusped field thruster is a novel electrostatic thruster suitable for space exploration, but with high plume divergence angle. In this article, a 2D Particle-in-cell plus Monte Carlo (PIC-MCC) model is built to study the influence of exit magnetic separatrix angle on controlling plume divergence of cusped field thruster. The electric potential contour line distribution characteristics near exit magnetic separatrix with different angle are obtained and their formation mechanism is explained by magnetic mirror effect and electron pressure term. Simulation results show laying additional magnetic ring at the exit of channel can transfer exit magnetic separatrix towards inward of channel, which can decrease angle between electric potential contour line and central axis and increase ion axial speed. It is mainly due to change of magnetic field line direction and enhancement of magnetic mirror effect. However, simulation results show that ionization intensity in the channel decreases after adopting the plume control method. The plume divergence control method increases the channel length and causes the exit magnetic cusp inside the channel. Therefore the influence of exit magnetic separatrix angle on the characteristics of electron and ion energy deposition distribution on wall is further investigated.

A compact coaxial cusped periodic permanent magnet for a coaxial relativistic Čerenkov generator

A prototype of an integrated coaxial cusped periodic permanent magnet (CPPM) system is put forward and optimized. The guiding magnetic field strength and period of the CPPM are 0.32 T and 30 mm, respectively, and the whole focusing system is 240 mm long, with a maximum diameter of 170 mm and total weight of under 30 kg. Using this focusing system, an oversized Ku-band coaxial relativistic Čerenkov generator is proposed and investigated. Particle-in-cell simulation shows that the microwave with an output power of 830 MW at 15.29 GHz is achieved, yielding a conversion efficiency of 37%. The feasibility of CPPM applications in high power microwave (HPM) generation is confirmed, providing a new technical route for small, lightweight HPM generators.

The Rapid Imaging Planetary Spectrograph: Observations of Mercury’s Sodium Exosphere in Twilight

Abstract Ground-based observations of Mercury’s exosphere are intrinsically difficult due to its proximity to the Sun and must be made in daylight or during brief windows at twilight. While the dimmer twilight background is far preferred, high airmass seeing and haze through Earth’s atmosphere, windshake, and guiding all present formidable challenges toward spatially resolving the exosphere’s structure. This study explores how such effects can be mitigated using results from a new instrument for high cadence spectroscopy, the Rapid Imaging Planetary Spectrograph. While high cadence observations do not significantly improve upon the resolution floor imposed by atmospheric seeing, the method does mitigate obstacles such as telescope tracking inaccuracy, windshake, and flux calibration. Whereas daytime observing has been the predominant methodology in past exosphere studies, the twilight observations performed here easily resolve distinct brightness enhancements near 50°–60° latitude, just equatorward of magnetic cusp regions. The exosphere in these locations is diagnostic of space weather effects such as charged particle precipitation. The structure in the sodium exosphere generally appears both more extended and brighter over the southern cusp, which has a broader open magnetic field line region. However, a northern enhancement during one observation confirms that the exosphere responds dynamically to environmental drivers, presumably changes in the solar wind dynamic pressure and/or interplanetary magnetic field.

Discovery of an Electron Gyroradius Scale Current Layer: Its Relevance to Magnetic Fusion Energy, Earth's Magnetosphere, and Sunspots

In the Earth’s magnetosphere, sunspots and magnetic cusp fusion devices, the boundary between the plasma and the magnetic field is marked by a diamagnetic current layer with a rapid change in plasma pressure and magnetic field strength. First principles numerical simulations were conducted to investigate this boundary layer with a spatial resolution beyond electron gyroradius while incorporating a global equilibrium structure. The boundary layer thickness is discovered to be on the order of electron gyroradius scale due to a self-consistent electric field suppressing ion gyromotion at the boundary. Formed at the scale of the electron gyroradius, the electric field plays a critical role in determining equilibrium structure and plasma transport. The discovery highlights the necessity to incorporate electron gyroradius scale physics in studies aimed at advancing our understanding of fusion devices, the magnetosphere and sunspots.

Investigation of magnetism and magnetic structure of anti-ThCr2Si2-type Tb2O2Bi by magnetization and neutron diffraction measurements

In this study, magnetization and neutron diffraction of anti-ThCr2Si2-type Tb2O2Bi polycrystals were measured at various temperatures. The magnetization cusp at 11.1 K was confirmed to correspond to antiferromagnetic ordering with a propagation vector of [0.5 0.5 0]. In addition, a metamagnetic behavior was observed below 5.0 K. The metamagnetic behavior could be attributed to the incommensurate magnetic structure with an anomaly at 5.0 K.

Fluid simulation of ionization process in iodine cusped field thruster

In this paper, a one dimensional fluid model of iodine cusped field thruster is built along two typical electron paths. Simulation results show typical low frequency oscillations of plasma parameters. Density of I2+ is similar to that of I+, while density of I− is smaller than those of I+ by two orders of magnitude. Simulation results show that ionization rate and thrust of iodine cusped field thruster are a little higher than that of xenon cusped field thruster. However, electron density of main ionization region in iodine cusped field thruster is much higher than that in xenon cusped field thruster, which may be due to diversity of plasma components and complexity of ionization process. Compared with xenon cusped field thruster, ionization intensity is enhanced and main ionization region moves towards downstream of channel in iodine cusped field thruster, which is related to dissociation process of I2. With decrease of leak electron proportion, ionization strength of secondary ionization region located in the middle magnetic cusp is enhanced and ionization strength of main ionization region is weakened. The main ionization region moves towards upstream of channel. These phenomena are mainly because confinement of electrons by middle magnetic cusp is enhanced, which causes excessive consumption of I2 and decrease of I2 dissociation intensity.

Numerical modeling of high efficiency multistage plasma thrusters for space applications

The high efficiency multistage plasma-thruster (HEMP-T) represents an ion thruster technology that was developed by Thales Deutschland GmbH beginning in the early 2000s. It features a dielectric-coated discharge channel with an anode at the channel bottom. A magnetic field is applied by periodic permanent magnets (PPMs) that is mostly axially oriented in most channel regions, with radial orientation only at spatially confined magnetic cusps resulting in low particle fluxes towards the dielectric wall. The typical length scales in the description of the HEMP-T plasma range from microscopic over mesoscopic to macroscopic. Microscopic effects are introduced, e.g., by atomic interactions and surface interactions. Mesoscopic scales appear by the gyro radius of particles in the magnetic field and the formation of a transition zone between plasmas and walls on the length scale of some Debye lengths, respectively. Large-scale macroscopic length scales are introduced, e.g., by the plume expansion and interaction with test vessel walls. For a correct description of both thruster and plume plasma, one has to solve a kinetic problem for the whole region of interest, including all significant physical processes. This review offers an overview of modeling strategies and their results for the HEMP thruster. The axisymmetric 2D3v Particle-in-Cell (PIC) simulation method is used to identify the basic physics of the acceleration channel and the near-field plume region. Direct comparison with the observed plasma radiation and angular ion energy distributions, both for stationary and dynamic modes, is also presented. While good qualitative agreement with experimental data is achieved, even better agreement with experiments is necessary to predict thruster performance via numerical models in the future. The limited size of the simulation domain restricts the study of the coupled thruster-plume dynamics, which is an important aspect for the interaction of ion thruster and carrier spacecraft. Improvements in computer technology, the use of hierarchical models and the multigrid method discussed in this review can help overcome these limits and improve the quality of predictive thruster modeling.

Powerful continuous-wave sub-terahertz electron maser operating at the 3rd cyclotron harmonic

Coherent subterahertz radiation is achieved in an electron cyclotron maser (large-orbit gyrotron configuration) operating at a higher cyclotron harmonic in the continuous-wave generation regime. This auto-oscillator is based on the use of a gun with a magnetic field cusp and a section for adiabatic magnetic compression with a factor of 1000, forming a 30 keV/0.7 A axis-encircling electron beam in a magnetic field of 5 T. A stable single-mode generation is achieved in the case of operation at the third and second cyclotron harmonics at frequencies of 0.394 THz and 0.267 THz with radiation powers of 0.37 kW and 0.9 kW, respectively.