Mirror Machine Research Articles

Applying alpha-channeling to mirror machines

The α-channeling effect entails the use of radio-frequency waves to expel and cool high-energetic α particles born in a fusion reactor; the device reactivity can then be increased even further by redirecting the extracted energy to fuel ions. Originally proposed for tokamaks, this technique has also been shown to benefit open-ended fusion devices. Here, the fundamental theory and practical aspects of α channeling in mirror machines are reviewed, including the influence of magnetic field inhomogeneity and the effect of a finite wave region on the α-channeling mechanism. For practical implementation of the α-channeling effect in mirror geometry, suitable contained weakly damped modes are identified. In addition, the parameter space of candidate waves for implementing the α-channeling effect can be significantly extended through the introduction of a suitable minority ion species that has the catalytic effect of moderating the transfer of power from the α-channeling wave to the fuel ions.

Divertor Simulation Study Using the GAMMA 10 End-Mirror Cell

In GAMMA 10 which is a tandem mirror machine in Plasma Research Center at University of Tsukuba, divertor simulation studies have been planned and started. This paper describes the recent results of divertor simulation experiment in the GAMMA 10 end-mirror cell. As a part of these experiments, characteristics of end-loss plasma have been investigated by using probes and calorimeters. The diagnostics are installed at two positions, which are separated by 40 cm in the axial direction. In the results, ion temperature of the end-loss flux in GAMMA 10 is much higher than that of other divertor simulators. It is found that heat-flux density can be controlled within the range from 0.4 to 0.8 MW/m 2 by changing the ICRF power. In addition, heat flux density has strong dependence on diamagnetism in the central-cell which is time integrated in the plasma duration. While, particle-flux density is proportional to electron line-density in the central-cell. Particle and heat fluxes measured at axially different positions agree well with the calculation results for considering the influence of particle reflection phenomena. c

Vacuum Field Ellipticity Dependence on Radius in Quadrupolar Mirror Machines

The vacuum field flux tube ellipticity dependence on radius for quadrupolar mirror machines has been investigated. A third order expression in the paraxial approximation has been derived for the vacuum field ellipticity. The dependence of ellipticity on midplane radius has been examined in the SFLM Hybrid and the outermost plasma flux tube is 3.5 cm wider than predicted by the first order paraxial approximation, which is within boundaries set by the first wall. The third order approximation has a high accuracy for the ellipticity for long-thin mirrors such as the SFLM Hybrid, and even the first order approximation that is independent of radius is sufficient in many applications. The ellipticity dependence on midplane radius for mirrors with more strongly localized quadrupolar fields than the SFLM Hybrid is also shown to be minor.

Surface Characteristics with respect to Magnetic Abrasive Finishing in Carbon Fiber Reinforced Plastics

Carbon fiber reinforced plastics (CFRP), which have been developed for their high mechanical properties, are insufficient to secure machinery. This paper investigates the use of magnetic abrasive finishing methods and the characteristics of surface roughness for mirror machining of CFRP. The cylindrical surface of CFRP was ground using a diamond paste with sizes of 0.1, 0.5, 1 and 6μm. Consequently, an effective surface roughness of 0.03μm(Ra) could be obtained via a paste size of 0.5μm. The surface roughness was not improved due to epoxy abrasion between the carbon fiber and the epoxy. (Received January 10, 2011)

Research on Precision Mirror Machining Technology for W-Mo Alloy

A new way of precision machining was studied through the experiments of Electrolytic In-Process Dressing (ELID) precision grinding and ultra precision lapping and polishing for W-Mo metal alloy. First a 22nm(Ra) surface was obtained through the ELID grinding, last a 11nm(Ra) surface was obtained after the process of lapping and polishing with 0.1～0.3 N/cm2pressure, 60～100 r/min rotational speed and other optimized parameters. Meanwhile, the formation mechanism of ultra precision mirror surface of the alloy was also analyzed. The experiments prove surface quality of the work piece was guaranteed by ELID grinding, and which was also greatly affected by some parameters in lapping and polishing such as pressure, rotational speed.

Experimental Observation of Zonal Flow and Its Scalings in Axisymmetric Magnetic Field

The anomalous radial transport generated by drift wave turbulence is a fundamental open physics question in magnetic confinement systems, both in modern tokamaks and current and next generation mirror machines. The role of self-generated zonal flows (ZF) in transport regulation via its shear is a potent concept and a physics issue. ZF are believed to be spontaneously excited by drift wave turbulence via Reynolds stress from small-scale fluctuations to large-scale flow.A basic physics experimental study of zonal flows associated with ITG (ion temperature gradient) drift modes has been performed in the Columbia Linear Machine (CLM). The difficult problem of detection of ZF has been solved via a novel diagnostic using the paradigm of FM (frequency modulation) in radio transmission. We find a power spectrum peak at ITG (‘carrier’) frequency of ~120 kHz and FM sidebands at frequency of ~2 kHz. We have definitively identified ZF with azimuthal and axial symmetry (kθ = 0, k// ≈ 0) and radially inhomogeneous (kr [not equal] 0) flow structures in cylindrical plasmas in uniform axisymmetric magnetic field. However, quantitatively, the stabilizing effect of ZF shear appears to be small and no significant isotopic effects are observed. The unique complementary roles of ion acoustic damping and ZF shearing in the saturation of ITG have been experimentally demonstrated using stabilizing and destabilizing feedback techniques. Theoretically ZF is supposed to be saturated via νii. As this is very small both in tokamaks and CLM, we investigate the scaling ZF with in νin which can be significant in CLM.

超音波振動切削に関する研究（第１報）―背分力方向振動切削機構の基礎的解析と実験的考察―

The final objective of this investigation is an invention of theory and technics for multiple precision cylindrical machining with combining ultrasonic vibration cutting on thrust force direction, and superposition superfinishing process. This finishing process is a high efficient mirror machining providing (20-50Hz) low frequent vibration and ultrasonic vibration above 20 kHz to fine abrasive stone. In this report, ultrasonic vibration cutting mechanism on thrust force direction is analyzed and it is shown that high precision machining can be achieved by using the effect of insensitive vibration cutting mechanism. And cutting phenomenon are observed by some experiments. In the results, it is confirmed that chips on low feed rate and high cutting speed conditions have been cut to pieces as same as analyzing, and that work displacement on vibration turning can be reduced according to growing up tool amplitude. This report indicates that ultrasonic vibration cutting on thrust force direction can be possibly used for multiple precision cylindrical machining.

Fusion-Fission Hybrid Reactor Studies for the Straight Field Line Mirror

A comparatively small mirror fusion hybrid device may be developed for industrial transmutation and energy production from spent nuclear waste. This opportunity ensues from the large fission to fusion energy multiplication ratio, Qr = Pfis/Pfus ≤ 150, in a subcritical fusion device surrounded by a fission mantle with the neutron multiplicity keff ≈ 0.97. The geometry of mirror machines is almost perfectly suited for a hybrid reactor application, and the requirements for plasma confinement can be dramatically relaxed in correspondence with a high value of Qr. Steady state power production in a mirror hybrid seems possible if the electron temperature reaches 500 eV. A moderately low fusion Q factor, the ratio of fusion power to the power necessary to sustain the plasma, could be sufficient, i.e. Q ≈ 0.15. Theoretical predictions for the straight field line mirror (SFLM) concept are presented, including results from radio frequency heating, neutron Monte Carlo and magnetic coil computations. Means to achieve an electron temperature of 500 eV are briefly discussed. The basic study considers a 25 m long confinement region with 40 cm plasma radius with 10 MW fusion power and a power production of 1.5 GW thermal.

Field and Coil Design for a Quadrupolar Mirror Hybrid Reactor

A vacuum magnetic field from a superconducting coil set for a single cell minimum B fusion-fission mirror machine reactor is computed. The magnetic field is first optimized for MHD flute stability, ellipticity and field smoothness in a long-thin approximation. Recirculation regions and magnetic expanders are added to the mirror machine without an optimizing procedure. The optimized field is thereafter reproduced by a set of circular and quadrupolar coils. The coils are modelled using filamentary line current distributions. Basic scaling assumptions are implemented for the coil design, with a maximum allowed current density of 1.5 kA/cm2. The coils are optimized using a local optimization method and the resulting field is checked for MHD flute stability and maximum ellipticity.

Modern magnetic mirrors and their fusion prospects

This paper reviews the most important findings from recent experiments on modern magnetic mirrors, apart from tandem mirrors and rotating plasma devices. These modern magnetic mirrors are represented by a multiple mirror device GOL-3 and a gas dynamic trap, experiments on which are carried out in Novosibirsk. Both devices are characterized by axial symmetry and improved axial confinement of plasma compared with conventional mirror machines. Recent findings from experiments enable us to more practically consider applications of the gas dynamic trap as a high-flux 14 MeV neutron source for fusion materials testing, and possibly as a driver for fusion–fission hybrids. They also indicate that effective axial plasma confinement in a multiple mirror can be obtained with a smaller plasma density compared with theory and β < 1. This is beneficial from the point of view of the technical realization of a multiple mirror reactor.

Feasibility Studies of Alpha-Particle Channeling in Mirror Machines

The linear magnetic trap is an attractive concept both for fusion reactors and for other plasma applications because of its relative engineering simplicity and high-beta operation. Applying the α-channeling technique to linear traps such as mirror machines can benefit this concept by efficiently redirecting α-particle energy to fuel ion heating or by otherwise sustaining plasma confinement, thus increasing the effective fusion reactivity. To identify waves suitable for α channeling, a rough optimization of the energy extraction rate with respect to the wave parameters is performed. After the optimal regime is identified, a systematic search for modes with similar parameters in mirror plasmas is performed, assuming quasi-longitudinal or quasi-transverse wave propagation. Several modes suitable for α-particle energy extraction are identified for both reactor designs and for proof-of-principle experiments.

Studies of a Straight Field Line Mirror with Emphasis on Fusion-Fission Hybrids

The straight field line mirror (SFLM) field with magnetic expanders beyond the confinement region is proposed as a compact device for transmutation of nuclear waste and power production. A design with reactor safety and a large fission-to-fusion energy multiplication is analyzed. Power production is predicted with a fusion Q = 0.15 and an electron temperature of ~500 eV. A fusion power of 10 MW may be amplified to 1.5 GW of fission power in a compact hybrid mirror machine. In the SFLM proposal, quadrupolar coils provide stabilization of the interchange mode, radio-frequency heating is aimed to produce a hot sloshing ion plasma, and magnetic coils are computed with an emphasis on minimizing holes in the fission blanket through which fusion neutrons could escape. Neutron calculations for the fission mantle show that nearly all fusion neutrons penetrate into the fission mantle. A scenario to increase the electron temperature with a strong ambipolar potential suggests that an electron temperature exceeding 1 keV could be reached with a modest density depletion by two orders in the expander. Such a density depletion is consistent with stabilization of the drift cyclotron loss cone mode.

重畳超仕上げに関する研究 : 第3報,重畳振動砥石の損耗現象(機械要素,潤滑,設計,生産加工,生産システムなど)

The objective of this investigation is to clarify a new and high efficient mirror machining process with using ultrasonic and low frequent vibrations, that has been named "superposition superfinishing (SSF process)". In the first report, it was confirmed that a stock removal rate of superposition vibration cutting as cutting process on this SSF process can be higher than that of low frequent superfinishing, and that a high efficient mirror machining can be practically achieved. In the second report, by analysis and consideration on superposition cutting track, it was found that cutting length of this superposition vibration cutting strongly influence on a stock removal amount. In this report, as one reason of obtaining a high stock removal rate, we have adopt equivalent grade characteristics of ultrasonically vibrated abrasive stone. It has be shown that the grade of abrasive stone decreases according as amplitude incleases. On the result of some experiments, it is found that this phenomenon can be controlled by an amplitude value of ultrasonic vibration, and that a rate of abrasive stone ware per one meter of superposition superfinishing is lower than that of low frequent superfinishing in the case of large maximum cross angle.

Waves for alpha channeling in mirror machines

Alpha channeling can, in principle, be implemented in mirror machines via exciting weakly damped modes in the ion cyclotron frequency range with perpendicular wavelengths smaller than the alpha-particle gyroradius. Assuming quasilongitudinal or quasitransverse wave propagation, we search systematically for suitable modes in mirror plasmas. Considering two device designs, a proof-of-principle facility and a fusion rector prototype, we in fact identify candidate modes suitable for alpha channeling.

On Possibilities for Transmutation of Nuclear Waste and Energy Production with a “Straight Field Line Mirror” Neutron Source

A pure fusion mirror device suffers from the predicted low values of the Q factor (energy gain factor). A much higher energy production may be achieved in a fusion-fission reactor, where the fusion plasma neutron source is surrounded by a fission mantle. The fusion neutrons are capable of initiating energy producing fission reactions in the surrounding mantle. A mirror machine can probably be designed to provide sufficient space for a 1.1 m wide fission mantle inside the current coils, and the power production from the fission reactions can in such a case exceed the fusion power by more than two orders of magnitude (Pfis/Pfus ≈ 150), suggesting a realistic reactor regime for a mirror based fusion-fission device. An energy producing device may operate with an electron temperature around 1 keV. Transmutation of long-lived radio active isotopes (minor actinides) from spent nuclear fuel from fission reactors can reduce geological storage from 100 000 years to only 300 years. Since the energy of D-T fusion neutrons are above the threshold for the most important transmutation reactions desired for treatment of nuclear waste, there may be an interest for a mirror transmutation device even if no net energy is produced. Recent theoretical simulations have considered the possibility to use the Gas Dynamic Trap (GDT) at Novosibirsk as a subcritical burner for transmutation by fusion neutrons. In the present work, possibilities for mirror based fusion-fission machines are discussed. Means to achieve sufficient end confinement for a straight field line mirror fusion-fission system with a thermal barrier are briefly analyzed. End leakage can alternatively be avoided by connecting the ends of a magnetic mirror with a stellerator tube, while the fusion neutrons are produced in the mirror part where a high energy sloshing ion component is confined. A zero dimensional model for such a mirror-stellarator system has been developed. The computed results indicate some possible parameter regimes for industrial transmutation and power production.

重畳超仕上げに関する研究(第1報)(機械要素,潤滑,工作,生産管理など)

The objective or this investigation is to clarify a new and high efficient mirror machining, that can be named superposition superfinishing (SSF process) using a fine abrasive stone. In this machining, at first process (SVC process: superposition vibration cutting) high efficient cutting is achieved with simultaneously providing (20〜50Hz) low frequent vibration and ultrasonic vibration above 20kHz on fine abrasive stone, and at next process (SF process: superfinishing) mirror surface can be obtained in a short time with only low frequent vibration. In this report, it is shown that basic machining characteristics and possibility of high efficient mirror machining on this new process are practically confirmed by several experiments.

αChanneling in a Rotating Plasma

The wave-particle $\ensuremath{\alpha}$-channeling effect is generalized to include rotating plasma. Specifically, radio frequency waves can resonate with $\ensuremath{\alpha}$ particles in a mirror machine with $E\ifmmode\times\else\texttimes\fi{}B$ rotation to diffuse the $\ensuremath{\alpha}$ particles along constrained paths in phase space. Of major interest is that the $\ensuremath{\alpha}$-particle energy, in addition to amplifying the rf waves, can directly enhance the rotation energy which in turn provides additional plasma confinement in centrifugal fusion reactors. An ancillary benefit is the rapid removal of alpha particles, which increases the fusion reactivity.

Analysis and modeling of edge fluctuations and transport mechanism in the Maryland Centrifugal Experiment

The Maryland Centrifugal Experiment [R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005)] is a mirror machine designed to have a plasma axially confined by supersonic rotation and dominantly interchange stable by the radial shear in the azimuthal velocity. Nevertheless, residual fluctuations still persist. To investigate the presence of such fluctuations, an azimuthal array of 16 magnetic pickup coils at the edge region of the plasma has been employed. A comprehensive analysis of the magnetic fluctuations reveals that, under the imposed shear flow, only m=0 and m=2 modes are dominant; yet, the observed frequency spectrum is broadband. Using higher order spectral analysis, clear evidence of nonlinear mode coupling is detected. It is also observed that the amplification of magnetic fluctuations leads to enhanced transport consistent with the drop of the plasma density and voltage. As a result, the magnetic fluctuations start to decrease in amplitude as the central plasma pressure drops. In return, the anomalous radial particle and momentum transport are reduced; thus, the plasma confinement improves. As the plasma pressure starts to build up, the plasma voltage increases, destabilizing the m=2 interchange mode. The cycle of enhanced transport and intermittent fluctuations repeats itself. A two-dimensional magnetohydrodynamics code in slab geometry is employed to investigate the dynamics of the primary interchange instability and to assess the level of transport. For very low sheared rotation, a broad spatial spectrum of unstable modes is obtained. As the sheared rotation is increased, the high mode numbers become stabilized and low mode numbers dominate the spectrum. Both the experimental data obtained from the azimuthal array probes and the simulations in case of parabolic shear flow show clear evidence of nonlinear mode coupling, explaining the broadband frequency spectrum for low mode numbers. A detailed comparison of spatiotemporal dynamics of simulations with the experimental data is presented.

Enhancement of Polytechnic University of Puerto Rico's plasma machine cooling system

The Mirror and Cusp Plasma Machine at the Polytechnic University of Puerto Rico is a unique research and development machine to create plasma. Due to the high current, high magnetic field confinement and large chamber capacity, this machine is the only plasma machine in the Caribbean to reach high plasma temperatures and densities. Certainly these parameters are achieved by a high dc power supply that produces high currents in order to create a fine magnetic field; these currents range from 300 to 800 A, and the heat dissipation created by this process limits the use of the machine. Originally the machine had a water cooling line to circulate water at room temperature, but this line was not cool enough to efficiently remove heat from the system for large periods of time. Also, the high vacuum diffusion pumps used are water-cooled. The present study was developed to design a more efficient cooling system for the Plasma Laboratory using a water-cooled chiller; the main goals are to operate at lower temperatures but at stable currents I> 600 A, and to recycle the water. Now the machine can operate for longer periods of time and on a daily basis, resulting in more efficient experiments and investigations.

Simulation of α-channeling in mirror machines

Applying α-channeling techniques to mirror machines can significantly increase their effective reactivity, thus making open configurations more advantageous for practical fusion. A large fraction of α-particle energy can be extracted using rf waves. Effects employed to cool α particles can also in principle be used to heat the fusion ions; the possibility to design a configuration of rf waves that could be used to perform both tasks is demonstrated.