Multipolar Magnetic Field Research Articles

Coronal Flux Rope Catastrophe in Partly-Open Multipolar Magnetic Field

采用球坐标下二维三分量理想MHD模型,研究部分开放多极背景磁场中日冕磁绳的灾变现象.背景磁场由含3个闭合双极场的冕流和带赤道中性电流片的开放场构成,磁绳位于中心双极场的下方,其特性由环向磁通和轴向磁通表征.对给定的环向磁通,存在轴向磁通的一个临界值;对给定的轴向磁通,也存在环向磁通的一个临界值.在该临界值以下,磁绳附着于太阳表面,系统处于平衡状态;该临界值一旦被超越,磁绳将脱离太阳表面向上喷发,说明部分开放多极背景磁场中的日冕磁绳系统存在灾变现象.本文算例表明,灾变点对应的磁能阈值超过对应部分开放场(中心双极场开放,两侧的双极场仍维持闭合)能量约15%,其超过部分可为日冕物质抛射一类太阳爆发提供能源.

Neutral Beam Injection System for the SHIP Experiment

The injector ion source is based on an arcdischarge plasma box. The plasma emitter is produced by a 1 kA arc discharge in deuterium. A multipole magnetic field produced with permanent magnets at the periphery of the plasma box is used to increase its efficiency and improve homogeneity of the plasma emitter. The ion beam is extracted by a 4-electrodes ion optical system (IOS). Initial beam diameter is 200 mm. The grids of the IOS have a spherical curvature for geometrical focusing of the beam. The optimal IOS geometry and grid potentials were found by means of numerical simulation to provide precise beam formation. The measured angular divergence of the beam is 0.025 rad, which corresponds to a 4.7 cm Gaussian radius of the beam profile measured at focal point.

High Power Hydrogen Injector with Beam Focusing for Plasma Heating

High power neutral beam injector has been developed with the atom energy of 25 keV, a current of 60 A, and several milliseconds pulse duration. Six of these injectors will be used for upgrade of the atomic injection system at central cell of a Gas Dynamic Trap (GDT) device and 2 injectors are planned for SHIP experiment.The injector ion source is based on an arcdischarge plasma box. The plasma emitter is produced by a 1 kA arc discharge in hydrogen. A multipole magnetic field produced with permanent magnets at the periphery of the plasma box is used to increase its efficiency and improve homogeneity of the plasma emitter. The ion beam is extracted by a 4-electrodes ion optical system (IOS). Initial beam diameter is 200 mm. The grids of the IOS have a spherical curvature for geometrical focusing of the beam. The optimal IOS geometry and grid potentials were found with the numerical simulation to provide precise beam formation. The measured angular divergence of the beam is 0.02 rad, which corresponds to the 2.5 cm Gaussian radius of the beam profile measured at focal point.

An Injector for a Multibeam Proton Accelerator

Proton acceleration experiments in a 19-channel accelerator with alternating phase focusing are described. An ion source with a multipolar magnetic field that allows charged particle beams with a small phase volume and a high phase current density to be obtained at the injector output has been developed and tested. A beam with a current >40 mA is accelerated in the experimental multibeam proton accelerator for an energy of 530 keV with this ion source.

Particle modelling of plasma confinement by a multipolar magnetic field

Multipolar plasma confinement is known to result in enhanced plasma density, homogeneous plasma of a large volume, and quiescent plasmas. The effects of a multipolar magnetic field on the dc discharge are confirmed using the particle-in-cell/Monte Carlo (PIC/MC) method. The gas is argon. The present PIC/MC simulation made it possible to consider three roles: (1) magnetic mirror effect near the cusp of two magnetic poles, (2) suppressed electron diffusion in the direction normal to magnetic field line, and (3) electron repelling in the sheath on the chamber wall. Electron number density for the magnetic field |Bθ|max = 0.1 T is up to nine times higher than that for no field for our plasma dimensions. The multipolar confinement is effective when the mean free path of fast electrons is of the order or larger than the discharge size. In order to give design criteria of the magnet system, the effects of magnetic field, number of magnetic poles, and gas pressure on the plasma density are examined systematically.

Energy Buildup of Multipolar Magnetic Fields by Photospheric Shear Motion

Using an axisymmetrical ideal magnetohydrodynamic (MHD) model in spherical coordinates, we present a numerical study of the energy buildup of coronal magnetic fields by photospheric shear motions. The unsheared potential field consists of a dipolar and an octopolar component with a neutral point somewhere at the equator. The separatrix crossing the neutral point divides the field into four topologically disconnected regions: a central arcade astride the equator, a bipolar field on each side of the central arcade, and an overlying arcade above the separatrix. A specific type of photospheric shearing is then introduced at the base of the central arcade, and the energy of the resultant sheared field is calculated. It is found for this special case that the magnetic energy of the sheared force-free field cannot exceed that of the corresponding partly open field in which the central and overlying arcades become fully open but the bipolar fields at the flank remain closed. This inspires us to extend Aly's conjecture from fully open force-free fields to partly open ones in such a way that in the frame of ideal MHD, it is impossible to store more magnetic energy in the corona by photospheric shear motions at the base of any part of the closed flux of a force-free field than that of the field in which the sheared closed flux opens but the remainder remains closed.

An Optimal Design of Coaxial Coils With Constraints on Inner and Outer Multipole Magnetic Fields

An optimal design method for magnets with double-layered coaxial coils to generate a uniform central magnetic field and a small stray magnetic field has been proposed. To find the optimal current density distribution, we used linear programming with constraints on the multipole field strengths, computed with analytical formulae, of the central and stray fields. The final coil positions were optimized with a quasi-Newton method, again taking into account the inner and outer multipole field strengths.

A modified permanent magnet structure for a stronger multipole magnetic field

The performance of the electron cyclotron resonance ion source (ECRIS) increases proportionally to the microwave frequency squared. This behavior encourages the use of higher microwave frequencies. However, a higher frequency would require a stronger magnetic field for the efficient operation of ECRIS. A rather complicated magnetic field configuration results from the combination of solenoids for the axial confinement and a multipolar radial field usually provided by permanent magnets. These fields produce the so-called B-minimum structure which is required for a stable and efficient operation of ECRIS. The highest multipole field achieved so far in an ECR ion source by using permanent magnets is about 1.3 T. This makes the efficient operation at a microwave frequency of about 18 GHz possible. We introduce here a new approach to further increase the magnetic multipole field provided by permanent magnets. According to our two-dimensional (2D) simulations, a remarkable improvement in the radial magnetic field of the multipole can be achieved. The idea was tested using a simple construction in the plasma chamber of the JYFL 6.4 GHz ECRIS. The multipole field increased from 0.37 to 0.5 T while the solenoids for the axial magnetic field were excited. This result is consistent with our 2D simulations. Based on our simulations and the simple test with the JYFL 6.4 GHz ECRIS, we believe that a multipole field value of 2 T is feasible in ECRIS while still using permanent magnets.

High power hydrogen neutral beam injector with focusing for plasma heating

High power neutral beam injector has been developed with particle energy of 25 keV, a current of 60 A, and several milliseconds pulse duration. Six of these injectors are planned to be used for the upgrade of the neutral beam system of a gas dynamic trap device. The injector ion source is based on an arc-discharge plasma box. The plasma emitter is produced by a 1 kA arc discharge in hydrogen. A multipole magnetic field produced with permanent magnets at the periphery of the plasma box is used to increase its efficiency and improve homogeneity of the plasma emitter. The ion beam is extracted by a four-electrode ion optical system (IOS). Initial beam diameter is 200 mm. The grids of the IOS have a curvature for geometrical focusing of the beam. The optimal IOS geometry and grid potentials were calculated to provide precise beam formation. The measured angular divergence of the beam is 0.02 rad, which corresponds to the measured width of the beam profile at a focal point of 2.5 cm.

Effects of multipolar magnetic fields on the characteristics of plasma and photoresist etching in an internal linear inductively coupled plasma system

The development of a large-area plasma source with high-density plasmas is desired for a variety of plasma processes from microelectronics fabrication to flat panel display device fabrication. In this study, high density (approx. 10 11/cm 3) and stable plasmas could be obtained by applying multipolar magnetic fields to a linear inductively coupled plasma source having the size of 1020×830 mm. The application of the magnetic fields decreased r.f. antenna voltage by half times and increased photoresist etch rates by three times. The plasma uniformity, which is considered as one of the most important factors in the large area plasma processing, could be maintained lower than 9%.

Effects of multipolar magnetic fields on the characteristics of plasma and photoresist etching in an internal linear inductively coupled plasma system

The development of a large-area plasma source with high-density plasmas is desired for a variety of plasma processes from microelectronics fabrication to flat panel display device fabrication. In this study, high density (approx. 10 11 /cm 3 ) and stable plasmas could be obtained by applying multipolar magnetic fields to a linear inductively coupled plasma source having the size of 1020×830 mm. The application of the magnetic fields decreased r.f. antenna voltage by half times and increased photoresist etch rates by three times. The plasma uniformity, which is considered as one of the most important factors in the large area plasma processing, could be maintained lower than 9%.

High-order map treatment of superimposed cavities, absorbers, and magnetic multipole and solenoid fields

Various modern systems for the transport and manipulation of large acceptance beams of rare and short-lived particles require the treatment of nonlinear optics of acceleration, absorption, and focusing in a combined approach. We describe a differential algebraic method for the treatment of such nonlinear dynamics via high-order transfer maps. We include the processes of scattering and straggling through absorbing material, which are inherently non-deterministic and hence not representable in the map formalism, in a split operator approach. Some examples of simulations of muon beam ionization cooling channels are provided.

Improved planar radio frequency inductively coupled plasma configuration in plasma immersion ion implantation

Plasmas with higher density and better uniformity are produced using an improved planar radio frequency (rf) inductively coupled plasma configuration in plasma immersion ion implantation (PIII). An axial magnetic field is produced by external electromagnetic coils outside the discharge chamber. The rf power can be effectively absorbed by the plasma in the vicinity of the electron gyrofrequency due to the enhanced resonant absorption of electromagnetic waves in the whistler wave range, which can propagate nearly along the magnetic field lines thus greatly increases the plasma density. The plasma is confined by a longitudinal multipolar cusp magnetic field made of permanent magnets outside the process chamber. It can improve the plasma uniformity without significantly affecting the ion density. The plasma density can be increased from 3×109 to 1×1010 cm−3 employing an axial magnetic field of several Gauss at 1000 W rf power and 5×10−4 Torr gas pressure. The nonuniformity of the plasma density is less than 10% and can be achieved in a process chamber with a diameter of 600 mm. Since the plasma generation and process chambers are separate, plasma extinction due to the plasma sheath touching the chamber wall in high-energy PIII can be avoided. Hence, low-pressure, high-energy, and high-uniformity ion implantation can be accomplished using this setup.

Dust particles in a thermophoretic trap in a plasma

The feasibility of confining dust particles in a plasma by thermophoretic forces was demonstrated. An extended dust structure in a positive glow discharge column was experimentally obtained at liquid nitrogen temperature. The dust structure was confined in an electrostatic-thermal trap, in which vertical stability was provided by the summed action of longitudinal electrostatic field and thermophoretic forces. Traps of this kind can be analyzed in terms of the general principles developed for confining particles in traps with the use of electric and magnetic multipole fields. We were able to change the shape and volume of the structure and even separate it into parts by varying temperature fields.

X-ray and gamma-ray emission from millisecond pulsars

We present a self-consistent model to describe X-ray and γ-ray emission from millisecond pulsars (MSPs). The X-rays of MSPs are produced by the backflow of primary charged particles from the outer gap and most likely consist of three components, two thermal components and one power law component if there is a strong multipole magnetic field on the stellar surface. The backflow of ultra-relativistic particles emits photons with energies about several tens of GeV via curvature radiation. These photons cause an electromagnetic cascade about 2–3 stellar radii above the polar cap. The synchrotron radiation of these cascade e pairs produces hard X-rays with a power law index ~. Near 105 cm above the stellar surface, the primary charged particles encounter the strong surface magnetic field, which alters the local radius of curvature greatly, and they quickly loose more than half of their remaining energies to curvature radiation. These curvature photons heat up the polar cap area with a radius ~ 105 cm, which produce the softer thermal X-ray component. Finally, the primary charged particles deposit their remaining energies in a much smaller polar cap area, which corresponds to the footprints of outer gap and produce the medium hard X-ray component. γ-rays are produced in the outer gap through synchro-curvature radiation. We have applied this model to the MSPs which emit pulsed X-rays and likely γ-rays such as PSR J0437-4715, PSR J2124-3358, PSR J0218+4232 and PSR B1821-24. Our results give an agreement between predicted spectrum and the observed spectrum of MSP emission.

Coronal holes and the solar polar field reversal

The relationship between the solar polar magnetic field reversal and coronal hole evolution is investigated for the time interval from 1996 to 2001. The distribution of coronal holes shows some evolutionary changes in relation to the polarity reversals, and these changes appear to be coordinated with changes in the global magnetic field structure, suggesting that the polar reversal originates from global processes. There are periods when the character of the coronal hole distribution on the solar disk changes significantly, indicating that the magnetic field structure changes. These can be interpreted as periods of rearrangement of the multipole magnetic field structure. It is found that the evolution of the geometric structure of the photospheric magnetic field during these periods is not characterized by a continuous transition from one dominant structure to another, but by relatively sudden rearrangement of the dominant geometrical structure of the magnetic field. These coronal holes and photospheric magnetic field rearrangements coincide with the polar photospheric magnetic field strength variations. The minimum phase of the solar cycle is dominated by the dipole component of the global solar magnetic field. The zonal magnetic field structure and zonal non-polar coronal hole distribution existed at that time. The sectorial magnetic field structure appears when the polar field strength reaches 0.7 of its maximum value in the corresponding hemisphere. This structure was established at dierent times in each hemisphere. In the north hemisphere it has existed since November 1998 and in the south hemisphere it has existed since October 1997. The rearrangement from one configuration to another occurs during a short time period, about 1 2 solar rotations. The coronal hole number and area evolution indicates a redistribution of positive-polarity and negative-polarity photospheric magnetic fields inside these longitudinal sectors, which reflects the solar polar field reversal.

Concept of multipole magnetic field rotation in ECRIS

The conventional type of magnetic well is formed by superposition of two types of magnetic field, axial bumpy field and radial multipole field. It is used to contain plasma that consists of neutrals, ions and electrons. These particles are in constant motion in the well and energetic electrons create plasma by violent collisions with neutrals and ions. The confined electrons are constantly heated by ECR technique in the presence of magnetic field. In this paper it has been shown theoretically that how the electron motion is influenced in terms of heating, containment and azimuthal uniformity of plasma, by the axial rotation of the multipole magnetic field [1,2]. Afterwards, the feasibility of achieving a rotating magnetic multipole field is discussed to some extent. And it is seen that it is not beyond the capability of the scientific community in the present scenario of the advanced technology. Presently, it can be achieved for lesser field and slightly larger size of the multipole electromagnet and can be used for improvement of the ECR ion source (ECRIS).

The role of multipolar magnetic fields in pulsar magnetospheres

We explore the role of complex multipolar magnetic fields in determining physical processes near the surface of rotation powered pulsars. We model the actual magnetic field as the sum of global dipolar and star-centred multipolar fields. In configurations involving axisymmetric and uniform multipolar fields, ‘neutral points’ and ‘neutral lines’ exist close to the stellar surface. Also, the curvature radii of magnetic field lines near the stellar surface can never be smaller than the stellar radius, even for very high-order multipoles. Consequently, such configurations are unable to provide an efficient pair-creation process above pulsar polar caps, necessary for plasma mechanisms of generation of pulsar radiation. In configurations involving axisymmetric and non-uniform multipoles, the periphery of the pulsar polar cap becomes fragmented into symmetrically distributed narrow subregions where curvature radii of complex magnetic field lines are less than the radius of the star. The pair-production process is only possible just above these ‘favourable’ subregions. As a result, the pair plasma flow is confined within narrow filaments regularly distributed around the margin of the open magnetic flux tube. Such a magnetic topology allows us to model the system of 20 isolated subbeams observed in PSR B0943+10 by Deshpande & Rankin. We suggest a physical mechanism for the generation of pulsar radio emission in the ensemble of finite subbeams, based on specific instabilities. We propose an explanation for the subpulse drift phenomenon observed in some long-period pulsars.

1162 マルチポール磁場による電子捕捉

Recently, plasma reactor using multipole magnetic field is watched with keen interest. The multipole magnetic field makes plasma low pressure, high density and uniform. We present the effect of multipole-magnetic field for plasma reactor, and research the better way to confine electrons.

108 マルチポール磁場による電子捕捉

108 マルチポール磁場による電子捕捉