Spherical Device Research Articles

Potential well structures in spherical inertial electrostatic confinement devices

Inertial electrostatic confinement (IEC) devices are of interest as neutron generators for many applications. Experiments by Hirsch inspired further efforts to decipher the potential distribution within IEC devices. In this paper, previous analyzes of potential distributions in IEC devices are reviewed and extended. Three types of IEC systems are classified and analyzed according to the arrangement of electrodes and the species within the system. These systems are the unipolar cathode-anode (UCA) system, the bipolar cathode-anode (BCA) system, and the bipolar anode-cathode (BAC) system. Results of extensive parametric studies are reported through an efficient method for solving the Poisson's equation. The method is benchmarked against prior computations by Hirsch and Swanson. For BCA and BAC systems, it is concluded that the double well depth (DWD) increases as the relative focusing of the secondary particle to the primary particle increases, agreeing with prior work by Momota and Miley. Although collisions are neglected in this model, and unverified energy distributions are employed, the method generally agrees with experimental observations by Gu that the DWD will increase as the perveance of the system is increased. Thus, the computations performed here serve as a valuable benchmark.

Antenna pattern comparison between an outdoor cylindrical near-field test facility and an indoor spherical near-field antenna test facility

A new spherical near-field probe-positioning device has been designed and constructed, consisting of a large 5.0 meter fixed arc. This arc has been installed in a near-field test facility, located at Alenia Marconi Systems, on the Isle of Wight, UK. As part of the near-field qualification, testing was performed on a ground-based radar antenna. The resultant patterns were compared against measurements collected on the same antenna on a large outdoor cylindrical near-field test facility, also located on the Isle of Wight [F. Steiner et al., Jan. 1994]. These measurements included multiple-frequency measurements and multiple pattern comparisons. This paper summarizes the results obtained as part of the measurement program, and includes discussions on the error budgets for the two ranges, along with a discussion of the mutual error budget between the two ranges.

The internal magnetic field structures and current density profiles in the Helicity Injected Spherical Torus plasma driven by coaxial helicity injection

In the Helicity Injected Spherical Torus device [Nagata et al., Proceedings of the 17th International Atomic Energy Agency Fusion Energy Conference, Yokohama, 1998 (International Atomic Energy Agency, Vienna, 1998) CD-ROM, EXP4/10], internal magnetic field and current density structures of spherical torus (ST) plasmas sustained by coaxial helicity injection (CHI) have been revealed via intensive internal magnetic measurements. The internal magnetic configuration of the ST plasma formed by CHI is in good agreement with the results of numerical equilibrium fitting calculations. The generation of closed poloidal flux of ST has been verified by varying the external toroidal field strength in the same device. Interestingly, the transition of the current profile from hollow to peaked has been observed during the sustainment phase, which could be explained by inductive effects of currents on open field lines winding the central conductor.

Resistive wall modes and error field amplification

Resistive wall modes and the rapid damping of plasma rotation by the amplification of magnetic field errors are related physical phenomena that affect the performance of the advanced tokamak and spherical torus plasma confinement devices. Elements of our understanding of these phenomena and the code that is used to design the major experimental facilities are based on the electrical circuit representation of the response of the plasma to perturbations. Although the circuit representation of the plasma may seem heuristic, this representation can be rigorously obtained using Maxwell’s equations and linearity for plasmas that evolve on a disparate time scale from that of external currents. These and related results are derived. In addition methods are given for finding the plasma information that the circuit representation requires using post-processors for codes that calculate perturbed plasma equilibria.

Next-step-targeted experiments on the Mega-Amp Spherical Tokamak

Since its first physics campaign, the principal parameters on MAST (Mega-Amp Spherical Tokamak) [A. Sykes et al., Nuclear Fusion 41, 1423 (2001)] have been brought up towards their design values. Considerable advances have been made in a range of physics areas of direct relevance to the International Thermonuclear Experimental Reactor (ITER) [ITER Physics Basis, Nuclear Fusion 39, 2175 (1999)]. In this paper, results on H-mode access, global confinement and pedestal studies are presented and compared with conventional aspect ratio tokamak scalings. Physics and engineering requirements relevant to next step spherical tokamak devices are discussed, in particular the plasma formation, current ramp-up and sustainment, and plasma exhaust. Results of first experiments directly targeting these issues are presented: Plasma current up to 0.5 MA has been produced without use of the central solenoid flux, and current ramp-up and sustainment without use of the central solenoid flux has been demonstrated. Experiments on neutral beam heating and current drive (CD) demonstrate up to 50% bootstrap current fraction and good CD efficiency, and divertor power loading has been found to be tolerable and have a favorable outboard asymmetry.

System Aspects of Dual-Layer Phase-Change Recording with High Numerical Aperture Optics and Blue Laser

In this publication, key technologies for blue laser recording and dual layer phase-change disks with a total capacity of 50 Gbytes are explained and system margins are evaluated. Key challenges are the phase-change recording layer design, a robust read and write system with low intrinsic noise and the optical pickup design including, e.g., high numerical aperture (NA) objective lens, spherical aberration compensation devices and beam shaping optics. Blue-laser dual-layer technology will be emerging as the next-generation optical data storage standard beyond compact discs (CDs) and digital versatile discs (DVDs).

Numerical Simulation Method for a Liquid-Crystal Aberration Compensation Device

We have developed a numerical simulation method on the basis of the ray tracing technique to evaluate a liquid-crystal (LC) spherical aberration compensation device for next-generation high-capacity optical memory devices. We numerically evaluated the performance and the tolerance of the LC device and showed its adequate capability to compensate the spherical aberration due to the nonuniformity of the disc cover layer thickness. Using the simulation method, we numerically examined the possibility of using LC devices to compensate the large spherical aberration due to the layer jump of the dual-layer optical disc and determined that the gradation-type LC device can be used for dual-layer optical recording.

Simulations of symmetric halo currents

It is proposed that halo current magnitude during vertical disruptions can be significantly affected by a number of strongly interacting processes, involving the generation of neutrals, their ionization and recombination, open-surface parallel losses, Ohmic heating due to the large back-emfs, the generation of impurities, and mixed-circuit effects. To investigate the balance of such processes we present and interpret results from one-dimensional multi-fluid transport simulations. The simulations show that, as expected, core plasma current is mainly lost due to the reduction of the core cross-sectional area rather than the thermal quench, and that large values of the safety factor tend to produce low poloidal halo current. In addition, it is seen that increasing the resistance of the external halo path suppresses halo current, as do impurities, naturally released from the divertor during the termination event. High values of the safety factor insulate the halo plasma from parallel transport effects, increasing electrical conductance and toroidal halo current. For mega ampere spherical tokamak device parameters, values of the poloidal halo current are of the order of 15% of the original plasma current.

Steady magnetic fields degrade plasma created by rotating magnetic fields in the Flinders ST*

The Flinders ST is a spherical tokamak project which has concentrated on the production by induction, of a low resistivity target plasma suitable for rotating magnetic field (RMF) current drive tests. Antennas for the production of the RMF fields were installed on the spherical tokamak device, so that it could operate as a device with pure RMF current drive (Rotamak), or purely as an inductively driven ST. Experiments with combinations of inductive and RMF current drive have been made. Experimental results obtained in this small Flinders ST device indicate areas of concern for the RMF current drive technique. First, the experiments show that the strong steady magnetic fields required for the spherical tokamak configuration degrade, and probably extinguish the RMF created plasma, when these transverse steady fields exceed some critical value, which is small compared to that required for ST operation. Second, a measure of the resistance of the pure RMF current drive device, using the ST solenoid and OH coils to create a small loop voltage purely as a diagnostic, shows that the RMF steady current is driven in a very resistive path.

SHaDe, a new 3-DOF haptic device

This paper presents a new type of haptic device using spherical geometry. The basic idea of haptic devices is to provide users with feedback information on the motion and/or force that he or she generates. Haptic devices have several potential applications such as, for example, fine compliant assembly, VR environment simulation, and high-precision teleoperation, especially in hazardous or hostile areas. The particular architecture of the spherical haptic device developed here will be presented and its advantages will be highlighted. Then, its basic kinematic properties, which shall be used for control and geometric optimization purposes, will be reviewed. The design optimization itself will then highlight some important features of the mechanism. The control scheme and real-time force feedback issues will then be presented. Finally, the results obtained with the prototype will be discussed.

Bender transducer design and operation.

An empirical study covering a wide range of bender transducer sizes and operating frequencies is reported. A spherical device model is shown to give good account of bender performance, including interaction effects. A set of empirical rules for scaling equivalent circuit parameters according to the device geometry is identified. An effective spherical radius, approximately half the diaphragm radius, is identified for the typical bender. The effects of pressure and drive voltage on performance are described for particular devices. Sensitivity factors for the equivalent circuit parameters to the operating conditions are determined. These are related to sensitivity factors for the coupling coefficient (Kc) and electromechanical transformer turns ratio (N). Both these parameters are shown to have similar sensitivity responses, decreasing with pressure (planar stress), and increasing with voltage (electric field). The results of high drive tests carried out at Seneca Lake are reported. Values of conventional figures of merit (FOM(V) and FOM(M)), close to the highest claimed for any underwater transducer, are tabled. When allowance for the operating efficiency is included in the figure of merit definitions, the bender appears to be superior to other device types.

Diagnostic requirements for high power auxiliary heating on NSTX (abstract)

National Spherical Torus Experiment (NSTX) has recently completed its first phase of operation. The primary objective for the baseline diagnostic1 set during this phase has been to aid in plasma control and machine operation, which successfully led to attainment of 1 MA of plasma current and some encouraging initial results with coaxial helicity injection (CHI). This first set of diagnostics relied on techniques previously established on tokamaks and related plasma devices. In the next phase of operation, strong auxiliary heating will become available in the form of rf heating through high harmonic fast waves (HHFWs) and neutral beam injection (NBI) for a combined total power of 11 MW. With intense auxiliary heating, accurate and reliable measurements of the plasma parameters for both machine operation and characterization of the plasma performance over a wide range of conditions will require an extended set of diagnostics. Profile diagnostics will be particularly important in this phase, and in some cases this capability can be achieved by upgrading existing diagnostics. However, many new diagnostic approaches must be implemented which take into account the constraints of a spherical torus device. An overview of the full complement of diagnostics planned for NSTX will be presented, and issues related to implementing each diagnostic will be discussed.

Energy gain calculations in Penning fusion systems using a bounce-averaged Fokker–Planck model

In spherical Penning fusion devices, a spherical cloud of electrons, confined in a Penning-like trap, creates the ion-confining electrostatic well. Fusion energy gains for these systems have been calculated in optimistic conditions (i.e., spherically uniform electrostatic well, no collisional ion-electron interactions, single ion species) using a bounce-averaged Fokker–Planck (BAFP) model. Results show that steady-state distributions in which the Maxwellian ion population is dominant correspond to lowest ion recirculation powers (and hence highest fusion energy gains). It is also shown that realistic parabolic-like wells result in better energy gains than square wells, particularly at large well depths (>100 kV). Operating regimes with fusion power to ion input power ratios (Q-value) >100 have been identified. The effect of electron losses on the Q-value has been addressed heuristically using a semianalytic model, indicating that large Q-values are still possible provided that electron particle losses are kept small and well depths are large.

A new poloidal-bundle divertor for a spherical tokamak

In a spherical tokamak, power handling at the divertor region becomes more difficult than that of conventional tokamaks because of the narrow space at the divertor region. Here a new poloidal-bundle divertor is proposed for a spherical tokamak reactor. The bundle divertor is located at the bottom and/or top region of the torus, in addition to several poloidal guide coils. These coils are installed inside of the toroidal coil, because the copper conductor is adaptable in spherical tokamak devices. The magnetic field line outside the separatrix is guided to the lower region of the torus by the poloidal guide coils, and extracted outside of the torus through the adjacent toroidal coils by the bundle divertor coils. The bundle divertor configuration does not affect the shape of the core plasma as much, and the magnetic field ripple, due to these bundle coils, is negligible.

Experimental study of potential structure in a spherical IEC fusion device

The spherical inertial-electrostatic confinement (SIEC) concept is designed to focus and accelerate ions and electrons radially inward towards the center of a negatively biased, highly transparent spherical grid. The converging ions create a high-density plasma core where a high fusion rate occurs. In addition, under proper conditions, the ion and electron flows create a space-charge induced well (a negative potential well nested inside a positive potential well). This structure traps high-energy ions within the virtual anode created by the double potential, providing a high fusion density in the trap volume. The present experiment was designed to verify double potential well formation and trapping by a measurement of the radial birth profile of energetic (3-MeV) protons produced by D-D fusion reactions in a deuterium discharge. This experiment was designed to operate at high perveance (0.4 to 1.4 mA/kV/sup 3/2/), where formation of a double well is predicted theoretically. Additional steps to aid well formation included: use of the unique Star mode of operation to obtain ion beam focusing down to -1.6 H the ballistic limit and the incorporation of a second electrically floating grid (in addition to the focusing/accelerating cathode grid) to reduce the ion radial energy spread to 0.34 mA/kV/sup 3/2/. As the perveance increased, the depth of the double well also increased. At the maximum perveance studied, 1.38 mA/kV/sup 3/2/ (corresponding to 80 mA and 15 kV), the negative potential well depth, corresponding to the measured proton-rate density, was estimated to be 22%-27% of the applied cathode voltage. This represents the first conclusive demonstration of double well formation in an SIEC, since prior measurements by other researchers typically yielded marginal or negative results.

Treatment of congenital anophthalmos with self-inflating polymer expanders: a new method

Congenital anophthalmos is a rare malformation in which the optic vesicle fails to develop. This leads to a small bony orbit, a constricted mucosal socket, short eyelids, reduced palpebral fissure and malar hypoplasia. The treatment includes both aesthetic and functional aspects. Therefore, a two-step procedure is described using a new self-inflating hydrogel expander. A lens-shaped expander with a diameter of 8 mm expands the lids and the mucosal socket to allow insertion of an eye prosthesis. As a second step, orbital expansion is performed with a spherical device. The expanders absorb lacrimal fluid from the mucosal socket or tissue fluid and start swelling when implanted in the orbital tissue. The insertion of an expander into the orbit as well as into the conjunctival pocket including its fixation by a single suture took only a few minutes and was an easy procedure. The expansion of the small conjunctival sockets was successfully completed in all cases within a period of 2-4 weeks. The weight (= volume in ml) of devices increased from 0.15-1.5 g (lens-shaped expander; weight in grams = volume in ml) respectively, 0.3-3.5 g (spherical device). The expanders inserted in orbital tissue increased from 0.4-4.4 g. This is equivalent to a 10 to 11 fold increase in their water-free volumes. Orbital expansion with spherical devices in combination with the inserted eye prosthesis enlarges the lid and palpebral fissures also. In contrast to conventional silicon balloon expanders, the procedure using self-inflating hydrogel expanders is simple and highly efficient.

Compressible Narrow Groove Analysis—Part 2: Computation of Pressure Field in a Spherical Device Rotating in Either Direction

Compressible Narrow Groove Analysis, as derived in a companion paper (Pan, 1998), is a model implementation of Thin Film High-Resolution Modeling for gas films. This paper describes the numerical procedure to compute the pressure field in a centered spherical device, which has general design features originally intended for a high performance gas bearing gyroscope (Keating and Pan, 1968). The number of groove patterns is varied to bring out the significance of the local compressibility number. Increased local compressibility, associated with reduced number of groove patterns, causes successive degradation of the pressurization capacity until it is entirely suppressed at 32 groove patterns. Further study is made with reversed rotation to create a high vacuum state in the gas film concurrent with a large compressibility number. The evacuation operation (with reversed rotation) is relatively insensitive to the number of groove patterns, but is highly dependent on the accommodation coefficient. Experience in preparing these examples lends evidence to the robustness of Thin Film High-Resolution Modeling. Trouble free iterative computations are routinely performed for the local Knudsen number in excess of 109 and the effective local compressibility number larger than 100.

Compressible Narrow Groove Analysis—Part 1: Derivation

The technique of High-Resolution Modeling of Thin Films is combined with a two-scale analysis to formulate the Compressible Narrow Groove Analysis. The data bank of Fukui and Kaneko (1990) is emulated in the form of an empirical formula to treat the state of arbitrary rarefaction of the gas film. Due to its transcendental character, the compressible fine-scale solution is treated on the fly as a part of the global-scale computation. Derivations for the Compressible Narrow Groove Analysis are presented here. In a companion paper (Pan, 1998), the Compressible Narrow Groove Analysis is used to compute the pressure field of a spherical device operating in both the pressurizing and evacuating modes.

Pulsed Operation of Spherical Inertial-Electrostatic Confinement Device

Inertial electrostatic confinement (IEC) fusion confines high energy ions in potential wells, where their increased energy and density yields a high fusion rate. Studies of the IEC at the University of Illinois (UI) initially concentrated on steady-state operation where neutron yields of approx.10{sup 6} D-D n/s are routinely obtained. However, the development of a pulsed configuration has been undertaken to provide higher neutron yields. Preliminary experiments have demonstrated I{sup 2} scaling during pulsed operation when the perveance threshold of 2.2 mA/kV{sup 3/2} is exceeded. Based on these results, it appears that the present IEC could be operated with 3-A, 100-kV repetitive pulses with a 10% duty factor to produce neutron yields of approx.10{sup 10} neutrons/second. 6 refs., 7 figs.

A particle-in-cell simulation of the breakdown and period doubling thresholds for a RF discharge Ar plasma

RF glow-discharge plasmas provide mild energetic ion bombardment of exposed surfaces. The characteristics of a 10 MHz RF glow-discharge Ar plasma are studied by particle-in-cell simulation. The model simulates a spherical plasma device using a one-dimensional plasma model. The code is used to determine the breakdown voltage, , for a given pd, where p is the gas pressure and d is the plasma discharge length. These values are then compared with Paschen's law for the DC case. Also, the breakdown voltage as a function of driver frequency is investigated. Due to the nonlinear capacitance of the plasma, the current and voltage waveforms are found to contain higher harmonic components and sub-harmonic components. Above a certain power level of the driver, sub-harmonics which are indicative of a period-doubling (PD) bifurcation are found to become dominant. The PD thresholds for varying pd, the magnitude of the current and the voltage of the Fourier-analysed components as a function of applied RF voltages are calculated. The PD threshold is always higher than the breakdown threshold. Finally, the pressure-dependence of the sheath width is discussed.