Mirror Coils Research Articles

An 18.3 MJ charging and discharging pulsed power supply system for the space plasma environment research facility (SPERF): the subsystem for the magnetic mirror coil

The magnetic reconnection process relevant to that at the magnetotail is one of the research contents of the Space Plasma Environment Research Facility, which is under construction at the Harbin Institute of Technology in China. Two magnetic mirror sub-coils placed symmetrically in the vertical direction and connected in series cooperate with a dipole coil to generate a magnetic field environment similar to the Earth’s magnetotail. A capacitor-based pulsed power supply (PPS) system with a modular design is developed to excite two magnetic mirror sub-coils to generate a magnetic field with a magnetic flux density of not less than 200 G at the center of the two sub-coils. The PPS should deliver a pulsed current with a peak of more than 8 kA, and the duration of the current not be less than 95% of the peak over 5 ms to two magnetic mirror sub-coils when the charging voltage is not less than 20 kV. In addition, the duration from the peak to 10% of the peak is not more than 130 ms. The detailed design of the PPS is discussed in this paper, and a test method is designed to reduce the risk of damage to the wires and the connection between the wires and the coaxial cables of the PPS when the PPS discharges at a higher charging voltage. Finally, the discharge test of the PPS is carried out to verify the design of the PPS.

Shielding design and neutronics calculation of the GDT based fusion neutron source ALIANCE

This paper presents the high flux neutron shielding design and extensive neutronics calculations of GDT based fusion neutron source ALIANCE. Neutron distribution of ALIANCE is strongly inhomogeneous along the axis: significant portion of the neutron flux is generated near the two mirrors, while the rest of it is spread over the remaining central volume of plasma. The shielding design includes 40 cm stainless steel as the main shielding layer and an additional 5 cm tungsten carbide shielding layer at mirror plugs to protect superconducting coils from neutron damage and reduce nuclear heating. The simulations have been carried out by using Monte Carlo transport code SuperMC with nuclear data library FENDL 3.1. Results show that the nuclear heating on the mirror coils can be reduced by more than two thirds with additional tungsten carbide shield, and fast neutron fluence by 30 %. The highest nuclear heating and the highest fast neutron fluence zones are located at the mirror coils, and the values are about 300 W/m3 and 9 × 1018 n/cm2 respectively, which meets the threshold of ITER superconducting coils. The specific activities of shielding layers are of order of 1012 Bq/kg. The structural materials’ specific activities will decrease to 4 × 1011 Bq/kg in one year after shutdown, and their decay heat will quickly drop below 2 kW/m3 after one day. Besides, all the structural materials of ALIANCE can be recycled by different recycling technologies. The modeling and calculations reported in this paper will be beneficial for the pre-conceptual engineering design of ALIANCE.

The General Appearance of the Superconducting Magnet System for the Gas-Dynamic Multimirror Trap

The paper presents the evaluation of basic parameters and general appearance of the superconducting magnet ic system of the Gas-Dynamic Multimirror Trap – the next-generation magnetic mirror research facility developed by the Budker Institute of Nuclear physics. The major requirements are that the magnetic field would be ramped up from 0.3 to 3 T within 5 seconds, which implies that a special low AC-loss superconducting cable must be chosen for this part of the magnet system. The mirror coils should have the field ∼12 T with an aperture exceeding typical of large-bore NMR magnets. In accordance with these requirements the optimized magnet system has been evaluated. The system can have modular design and consists of a set of 12 coils with NbTi superconductors and 4 coils made of Nb3Sn superconductor. Preliminary choice of superconducting cables and basic superconductors for the windings has been substantiated. The estimation of stability margins for all coils demonstrated that both NbTi and Nb3Sn coils have enough stability margins, including the operation of NbTi coils at fast changing current. The study demonstrated the feasibility of the optimized superconducting magnet system for the Gas-Dynamic Multimirror Trap with the use of modern low Tc superconductors and state-of-the-art superconducting magnet technologies.

A Multi-Cubic-Meter Ioffe Trap for Project 8

The Project 8 direct neutrino mass experiment requires a superconducting magnet of unusual scale and complexity. Project 8 will use atomic tritium held in a magnetic bottle of 10 to 100 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> to reach well beyond the sensitivity of the presentgeneration experiment. Project 8 is based on a new spectroscopic method, Cyclotron Radiation Emission Spectroscopy (CRES), that requires a highly uniform magnetic field in the fiducial volume at the center of the atom trap. The maximum fluctuation ΔB/B is nominally 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> . To meet these requirements, we propose a modified version of a Ioffe trap to confine the neutral tritium atoms and provide a large fiducial volume for CRES. The key change from the traditional topology is to replace the circular mirror coils that close the trap ends with pleated closures formed from the same conductors as the rest of the trap. This dramatically expands the fiducial volume of the trap. CRES achieves its best sensitivity with a particular particle density; for the highest event rate, all of these particles should be tritium atoms. Cooling is required to remove the atoms' radioactive decay heat and prevent them evaporating from the trap. We leave the magnetic bottle uncapped and maintain an equilibrium atom density by continuously supplying new, cold atoms to replace the warmer ones lost via the loading opening. Pleated trap ends, for certain choices of the number of Ioffe conductors, naturally transition to an extended quadrupole through which we load the trap. For a trapped atom population of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> atoms at a density of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , a trap volume of tens of cubic meters is required. Previous magnets have not simultaneously achieved the combination of high field strength, high uniformity, and large volume required for Project 8.

Optimal design of shear vertical wave electromagnetic acoustic transducers in resonant mode

In this paper, a new electromagnetic acoustic resonance (EMAR) transducer is proposed for precise thickness measurement in specimen. The new EMAR is composed of a mirror symmetric coil (MSC) and a pair of Nd-Fe-B permanent magnets with the different polarity for enhancing the generation and detection of resonant signals. Firstly, a finite element model was established to simulate the distributions of Lorentz force produced by new EMAR and the resonant process of shear waves. Furthermore, the relationship between the frequency response characteristic of the new EMAR and the common EMAR were explored. Finally, to verify the performance of the EMAR, several experiments were performed. Compared with the common EMAR transducer, the resonant amplitude of the new EMAR transducer was increased by 121.74% and the signal-to-noise ratio was increased by 28.35%, and the resonance frequency interval of the new EMAR was twice that of the common mode in the frequency domain simulation experiment, this advantage effectively reduced the error rate of measurement. The results show that the new EMAR transducer with mirror coil structure has higher accuracy in thickness detection of specimens.

Topology of the Magnetic Field and Resistivity of a Compact Torus Generated in a Mirrorless Theta Pinch

The mapping of the magnetic field and analysis of the plasma resistivity were used to study the change of magnetic field topology during the formation of a reversed field configuration. Using a theta pinch with straight coil, the formation of the torus was observed during the early time of the plasma radial implosion, in scale comparable to Alfven’s time and shorter than the resistive diffusion. The non-intrusive excluded flux probe also indicated the formation of the torus even in the absence of mirror coils in the system. The plasma at the end region of the coil has distributed in the entire cross section of the tube, slightly peaked at the null field region. The anomalous plasma resistivity at the reconnection site was close to the prediction given by numerical calculation using numerical hybrid code with anomalous collision frequency either calculated using Chodura’s algorithm or evolution of microinstabilities, particularly the lower hybrid drift instability.

First operation and effect of a new tandem-type ion source based on electron cyclotron resonance.

A new tandem type source has been constructed on the basis of electron cyclotron resonance plasma for producing synthesized ion beams in Osaka University. Magnetic field in the first stage consists of all permanent magnets, i.e., cylindrically comb shaped one, and that of the second stage consists of a pair of mirror coil, a supplemental coil and the octupole magnets. Both stage plasmas can be individually operated, and produced ions in which is energy controlled by large bore extractor also can be transported from the first to the second stage. We investigate the basic operation and effects of the tandem type electron cyclotron resonance ion source (ECRIS). Analysis of ion beams and investigation of plasma parameters are conducted on produced plasmas in dual plasmas operation as well as each single operation. We describe construction and initial experimental results of the new tandem type ion source based on ECRIS with wide operation window for aiming at producing synthesized ion beams as this new source can be a universal source in future.

Addressing Short Trapped-Flux Lifetime in High-Density Field-Reversed Configuration Plasmas in FRCHX

The objective of the field-reversed configuration heating experiment (FRCHX) is to obtain a better understanding of the fundamental scientific issues associated with high-energy density laboratory plasmas (HEDLPs) in strong, closed-field-line magnetic fields. These issues have relevance to such topics as magneto-inertial fusion, laboratory astrophysical research, and intense radiation sources, among others. To create HEDLP conditions, a field-reversed configuration (FRC) plasma of moderate density is first formed via reversed-field theta pinch. It is then translated into a cylindrical aluminum flux conserver (solid liner), where it is trapped between two magnetic mirrors and then compressed by the magnetically driven implosion of the solid liner. A requirement is that, once the FRC is stopped within the solid liner, the trapped flux inside the FRC must persist while the compression process is completed. With the present liner dimensions and implosion drive bank parameters, the total time required for implosion is ${\sim}{\rm 25}~\mu{\rm s}$ . Lifetime measurements of recent FRCHX FRCs indicate that trapped lifetimes following capture are now approaching ${\sim}{\rm 14}~\mu{\rm s}$ (and therefore, total lifetimes after formation are now approaching ${\sim}{\rm 19}~\mu{\rm s}$ ). By separating the mirror and translation coil banks into two so that the mirror fields can be set lower initially, the liner compression can now be initiated 7–9 $\mu{\rm s}$ before the FRC is formed. A discussion of FRC lifetime-limiting mechanisms and various experimental approaches to extending the FRC lifetime will be presented.

Antimatter Matters: Progress in Cold Antihydrogen Research

The purpose of cold antihydrogen research is briefly reviewed together with the latest developments of manipulating antihydrogen atoms. Two major progresses last year were the trapping of antihydrogen atoms in a magnetic bottle and synthesis of antihydrogen atoms in a cusp trap, where a spin-polarized antihydrogen beam can be extracted as an intensified beam. The magnetic bottle consists of an octupole coil and a pair of mirror coils, which improved the magnetic field uniformity near the axis, and so the stability of trapping antiprotons and positrons. Eventually, antihydrogen atoms were trapped for more than 1000s, which is ready to be testified with high precision laser spectroscopy. The cusp trap consists of a superconducting anti-Helmholtz coil and a stack of multiple ring electrodes. This success opens a new path to make a stringent test of the CPT symmetry via high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atoms.

Linear Connected Array of Non-Adiabatic Traps

An innovative confinement concept of a linearly connected array of non-adiabatic traps is examined. A non-adiabatic trap unit consists of a cylindrical vacuum chamber with external mirror coils and a pair of inner anti-parallel coils. Positions and currents on these coils are adjusted to achieve zero magnetic fields at the center. The plasma is trapped stably in this configuration due to the “Absolute Minimum B” magnetic configuration that is created. However, like a cusp field, energy confinement of plasma in a trap unit is poor, since particles suffer random pitch-angle scattering near the central zero field region and eventually fall into the loss-cone. However, once these non-adiabatic traps are linearly connected, plasma particles escaping a unit will be effectively re-trapped within the neighboring trap due to collision-free pitch-angle scattering in the zero field region. Since the transition of a charged plasma particle from unit to a next unit is stochastic, a connected array of non-adiabatic fusion core units and similar “leak suppressor” array units with low fusion rate at both ends of the core array improves the plasma confinement sufficient to achieve reactor-grade plasmas.

Simulation studies of non-neutral plasma equilibria in an electrostatic trap with a magnetic mirror

The equilibrium of an infinitely long, strongly magnetized, non-neutral plasma confined in a Penning-Malmberg trap with an additional mirror coil has been solved analytically [J. Fajans, Phys. Plasmas 10, 1209 (2003)] and shown to exhibit unusual features. Particles not only reflect near the mirror in the low field region, but also may be weakly trapped in part of the high field region. The plasma satisfies a Boltzmann distribution along field lines; however, the density and the potential vary along field lines. Some other simplifying assumptions were employed in order to analytically characterize the equilibrium; for example the interface region between the low and high field regions was not considered. The earlier results are confirmed in the present study, where two-dimensional particle-in-cell (PIC) simulations are performed with the Warp code in a more realistic configuration with an arbitrary (but physical) density profile, realistic trap geometry and magnetic field. A range of temperatures and radial plasma sizes are considered. Particle tracking is used to identify populations of trapped and untrapped particles. The present study also shows that it is possible to obtain local equilibria of non-neutral plasmas using a collisionless PIC code, by a scheme that uses the inherent numerical collisionality as a proxy for physical collisions.

Antimatter Plasmas in a Multipole Trap for Antihydrogen

We have demonstrated storage of plasmas of the charged constituents of the antihydrogen atom, antiprotons and positrons, in a Penning trap surrounded by a minimum-B magnetic trap designed for holding neutral antiatoms. The neutral trap comprises a superconducting octupole and two superconducting, solenoidal mirror coils. We have measured the storage lifetimes of antiproton and positron plasmas in the combined Penning-neutral trap, and compared these to lifetimes without the neutral trap fields. The magnetic well depth was 0.6 T, deep enough to trap ground state antihydrogen atoms of up to about 0.4 K in temperature. We have demonstrated that both particle species can be stored for times long enough to permit antihydrogen production and trapping studies.

Development of a HTS magnet and application to a beam scanner

A solenoid magnet using high-temperature superconductor tape was designed, fabricated, and tested for its suitability as beam scanner. After successful cooling tests, the magnet performance was studied using DC and AC currents. With DC current the magnet was successfully operated as mirror coils for a 2.45 GHz ECR ion source. The coils could be operated at 100 A and frequencies above 1 Hz. The installation of iron pole pieces and return yokes, enabled us to generate fields in excess of 2 T at 197 A DC. In AC mode this magnet can be operated in the ranges of 0.14–1.73 and 1.22–1.67 T at frequencies of 0.05 and 0.25 Hz, respectively.

Fullerenes in electron cyclotron resonance ion sources

Fullerene plasmas and beams have been produced in our electron cyclotron resonance ion sources (ECRIS) originally designed for other purposes. The ATOMKI-ECRIS is a traditional ion source with solenoid mirror coils to generate highly charged ions. The variable frequencies NIRS-KEI-1 and NIRS-KEI-2 are ECR ion sources built from permanent magnets and specialized for the production of carbon beams. The paper summarizes the experiments and results obtained by these facilities with fullerenes. Continuous effort has been made to get the highest C60 beam intensities. Surprisingly, the best result was obtained by moving the C60 oven deep inside the plasma chamber, very close to the resonance zone. Record intensity singly and doubly charged fullerene beams were obtained (600 and 1600nA, respectively) at lower C60 material consumption. Fullerene derivatives were also produced. We mixed fullerenes with other plasmas (N, Fe) with the aim of making new materials. Nitrogen encapsulated fullerenes (mass: 720+14=734) were successfully produced. In the case of iron, two methods (ferrocene, oven) were tested. Molecules with mass of 720+56=776 were detected in the extracted beam spectra.

Control of translating velocity using pulse field on field-reversed configuration plasmas

The translating velocity of a field-reversed configuration plasma is controlled using a pulse mirror field. Numerical calculations are carried out, using a flux-conserving moving mirror coil, to simulate the motion of the plasma subject to the pulse field. The calculations suggest that it is possible to stop the plasma smoothly without reflection under the pulse mirror coil when the strength of the mirror is changed depending on the velocity of the plasma. In the experiment, the pulse field is produced by a current flowing in a one-turn coil installed inside a theta-pinch coil. It is demonstrated that the translating plasma passing under the one-turn coil is reflected and stops depending on the strength of the pulse field. The optimum strength of the field to stop the plasma without reflection is in good agreement with the estimation of the calculations.

Ion acceleration in plasmas emerging from a helicon-heated magnetic-mirror device

Using laser-induced fluorescence, measurements have been made of metastable argon-ion, Ar+*(3d4F7/2), velocity distributions on the major axis of an axisymmetric magnetic-mirror device whose plasma is sustained by helicon wave absorption. Within the mirror, these ions have sub-eV temperature and, at most, a subthermal axial drift. In the region outside the mirror coils, conditions are found where these ions have a field-parallel velocity above the acoustic speed, to an axial energy of ∼30 eV, while the field-parallel ion temperature remains low. The supersonic Ar+*(3d4F7/2) are accelerated to one-third of their final energy within a short region in the plasma column, ⩽1 cm, and continue to accelerate over the next 5 cm. Neutral-gas density strongly affects the supersonic Ar+*(3d4F7/2) density.

Magnetic field modification of 18 GHz electron cyclotron resonance ion source at the Japan Atomic Energy Research Institute

An 18 GHz electron cyclotron resonance ion source under development since. 1994 has a solenoid coil between a pair of mirror coils to vary the mirror ratio over a wide range. The solenoid coil of the original design formed a small bump in the bottom of the mirror field profile when the source parameters were optimized for high charge states. Source performance in generating highly charged ions improved with a number of minor changes on the vacuum, the gas feed, and the beam extraction system. The maximum beam current of Ar16+ was 2 enA, which was much lower than that expected from the microwave frequency and the magnetic field strength. Little effect of a biased disk on intensities of highly charged ions and dependence of the charge state distribution on the solenoid coil current suggested that the bump divided plasma into two regions and limited the source performance. The bump was removed by halving the solenoid coil length, and the beam current of Ar16+ increased to 1.3 eμA as a result.

Axial compression of a field reversed configuration plasma

A new concept for plasma heating using axial magnetic compression of a field reversedconfiguration (FRC) plasma is proposed. In this concept, the FRC plasma is compressed only axially,keeping the magnetic flux between the separatrix and the confining chamber (flux conserver) wallunchanged, while allowing the plasma to expand radially. A simple model based on an empiricalscaling law of FRC confinement and on the assumption that the compression is done adiabatically predicts that, in addition to heating the plasma, improved confinement will also be accomplished with this concept. This compression is done by energizing segmented mirror coils successively in such a way as to decrease the length of the confinement region between the coils. The apparatus for this axial compression was developed and an experiment was carried out. In this experiment the plasma was compressed by about 30% and the plasma lifetime of about 500 μs was increased by about 50 μs.

Construction and development of the JAERI 18 GHz electron cyclotron resonance ion source

An 18 GHz electron cyclotron resonance ion source for multiply charged ions was constructed and is now in operation. A new distribution of the mirror field was adopted, and minimum strength is varied by a solenoid coil installed between the mirror coils. The measured mirror field distribution is close to the designed distribution and its maximum strength exceeds 1.4 T. The source is now being tuned by use of Ar ion generation. The relatively high base pressure in the plasma chamber has been improved by installing an additional vacuum pump. The source performance has been maturing gradually with the vacuum and Ar ions with charge states of up to 13+ have been observed so far.

Deposition of a-Si: H films by ECR plasma CVD using large diameter multi-slot antennae

A large diameter electron cyclotron resonance plasma was produced with a multi-slot antenna. The radial profile of the ion saturation current was examined as a function of input microwave power, gas pressure and magnetic mirror coil current to determine the experimental conditions necessary for a large diameter and uniform plasma at low pressure. Furthermore, the deposition of a-Si: H films was attempted onto 8 inch glass substrates using a gas mixture SiH 4-10% He.