Publications (202)188.48 Total impact
 Physical Review Special Topicsaccelerators and Beams  PHYS REV SPEC TOPAC. 01/2009; 12(12).
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ABSTRACT: We have developed a semianalytic expression for the total energy loss to a vacuum transmissionline electrode operated at high lineal current densities. (We define the lineal current density jℓ≡B/μ0 to be the current per unit electrode width, where B is the magnetic field at the electrode surface and μ0 is the permeability of free space.) The expression accounts for energy loss due to Ohmic heating, magnetic diffusion, j×B work, and the increase in the transmission line’s vacuum inductance due to motion of the vacuumelectrode boundary. The sum of these four terms constitutes the Poynting fluence at the original location of the boundary. The expression assumes that (i) the current distribution in the electrode can be approximated as onedimensional and planar; (ii) the current I(t)=0 for t<0, and I(t)∝t for t≥0; (iii) jℓ≤10 MA/cm; and (iv) the currentpulse width is between 50 and 300 ns. Under these conditions we find that, to first order, the total energy lost per unit electrodesurface area is given by Wt(t)=αtβBγ(t)+ζtκBλ(t), where B(t) is the nominal magnetic field at the surface. The quantities α, β, γ, ζ, κ, and λ are material constants that are determined by normalizing the expression for Wt(t) to the results of 1D magnetohydrodynamic MACH2 simulations. For stainlesssteel electrodes operated at current densities between 0.5 and 10 MA/cm, we find that α=3.36×105, β=1/2, γ=2, ζ=4.47×104, κ=5/4, and λ=4 (in SI units). An effective timedependent resistance, appropriate for circuit simulations of pulsedpower accelerators, is derived from Wt(t). Resistancemodel predictions are compared to energyloss measurements made with stainlesssteel electrodes operated at peak lineal current densities as high as 12 MA/cm (and peak currents as high as 23 MA). The predictions are consistent with the measurements, to within experimental uncertainties. We also find that a previously used electrodeenergyloss model overpredicts the measurements by as much as an order of magnitude.Physical Review Special Topics  Accelerators and Beams 12/2008; 11(12). · 1.52 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The use of magnetic fields to isentropically compress materials for equationofstate studies has been first demonstrated on the Z machine at Sandia National Laboratories. Sharing similarities with the GEPI pulser, a compact pulser has been designed and built, focusing on isentropic compression experiments. In order to achieve high compacity and fast turn around, the design is built around a solid dielectric transmission line to couple current from eight lowinductance capacitors that are switched with ultralowinductance multichannel gas switches operating in dry air at atmospheric pressure. A peaking stage made of 72 capacitors enhanced by a lowinductance multichannel sharpening switch brings the fundamental rise time of the pulser down to 350 ns (10%90%). A set of inductances in parallel with the sharpening switch as well as using various gases into this switch allow us to modify the current wave shape. The pulser delivers a peak current of 4 MA at a charge voltage of 80 kV on a short circuit. The rise time can be lengthened to around 650 ns for a current of 4.2 MA. The use of postholes convoluting in a solid dielectric insulation design makes that pulser unique, as well as its compact size, ease of use, and ease of access to the load.IEEE Transactions on Plasma Science 11/2008; · 0.95 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have developed a system of differentialoutput monitors that diagnose current and voltage in the vacuum section of a 20MA 3MV pulsedpower accelerator. The system includes 62 gauges: 3 current and 6 voltage monitors that are fielded on each of the accelerator's 4 vacuuminsulator stacks, 6 current monitors on each of the accelerator's 4 outer magnetically insulated transmission lines (MITLs), and 2 current monitors on the accelerator's inner MITL. The innerMITL monitors are located 6 cm from the axis of the load. Each of the stack and outerMITL current monitors comprises two separate Bdot sensors, each of which consists of four 3mmdiameter wire loops wound in series. The two sensors are separately located within adjacent cavities machined out of a single piece of copper. The high electrical conductivity of copper minimizes penetration of magnetic flux into the cavity walls, which minimizes changes in the sensitivity of the sensors on the 100ns time scale of the accelerator's power pulse. A model of flux penetration has been developed and is used to correct (to first order) the Bdot signals for the penetration that does occur. The two sensors are designed to produce signals with opposite polarities; hence, each current monitor may be regarded as a single detector with differential outputs. Commonmodenoise rejection is achieved by combining these signals in a 50Omega balun. The signal cables that connect the Bdot monitors to the balun are chosen to provide reasonable bandwidth and acceptable levels of Compton drive in the bremsstrahlung field of the accelerator. A single 50Omega cable transmits the output signal of each balun to a doublewall screen room, where the signals are attenuated, digitized (0.5ns/sample), numerically compensated for cable losses, and numerically integrated. By contrast, each innerMITL current monitor contains only a single Bdot sensor. These monitors are fielded in oppositepolarity pairs. The two signals from a pair are not combined in a balun; they are instead numerically processed for commonmodenoise rejection after digitization. All the current monitors are calibrated on a 76cmdiameter axisymmetric radial transmission line that is driven by a 10kA current pulse. The reference current is measured by a currentviewing resistor (CVR). The stack voltage monitors are also differentialoutput gauges, consisting of one 1.8cmdiameter Ddot sensor and one null sensor. Hence, each voltage monitor is also a differential detector with two output signals, processed as described above. The voltage monitors are calibrated in situ at 1.5 MV on dedicated accelerator shots with a shortcircuit load. Faraday's law of induction is used to generate the reference voltage: currents are obtained from calibrated outerMITL Bdot monitors, and inductances from the system geometry. In this way, both current and voltage measurements are traceable to a single CVR. Dependable and consistent measurements are thus obtained with this system of calibrated diagnostics. On accelerator shots that deliver 22 MA to a lowimpedance zpinch load, the peak lineal current densities at the stack, outerMITL, and innerMITL monitor locations are 0.5, 1, and 58MA/m, respectively. On such shots the peak currents measured at these three locations agree to within 1%.Physical Review Special Topicsaccelerators and Beams  PHYS REV SPEC TOPAC. 01/2008; 11(10).  [Show abstract] [Hide abstract]
ABSTRACT: Sharing similarities with the GEPI pulser which is dedicated to Isentropic Compression Experiments (ICE), VELOCE, an even more compact electrical pulser, has been designed and built in duplicate for SNL and WSU. This type of machine complements gun and laser facilities in the study of material response. In order to achieve a broad loading capability and fast turn around, the design is built around a solid dielectric transmission line to couple current from low inductance capacitors and electrically triggered switches. Peaking capacitors enhanced by a low inductance, multichannel sharpening switch reduce the quarter period of the pulser to about 470 ns (0100%). Gas mixtures in the switch cavity and inductances in parallel allow modification of the shape of the induced pressure wave. At 80 kV of charge voltage, the peak current can reach 3.5 MA. Design of the pulser, range of pressures and velocities, as well as potential applications are presented. A consistent numerical tool developed for pulsers design based on a circuit code coupled to a 1D MHD code is also introduced.12/2007;  [Show abstract] [Hide abstract]
ABSTRACT: The use of magnetic fields to isentropically compress materials for equationofstate studies has been first demonstrated on the Z machine at SNL [1]. Sharing similarities with the GEPI pulser [2], a compact pulser has been designed and built, focusing on Isentropic Compression Experiments. In order to achieve high compacity and fast turn around, the design is built around a solid dielectric transmission line to couple current from eight lowinductance capacitors that are switched with ultralowinductance multichannel gas switches operating in dry air at atmospheric pressure. A peaking stage made of 72 capacitors enhanced by a low inductance, multichannel sharpening switch brings the fundamental rise time of the pulser down to 350 ns (1090%). A set of inductances in parallel with the sharpening switch as well as using various gases into this switch allow us to modify the current wave shape. The pulser delivers a peak current of 4 MA at a charge voltage of 80 kV on a short circuit. The rise time can be lengthened to around 650 ns for a current of 4.2 MA. The use of postholes convolutes in a solid dielectric insulation design makes that pulser unique as well as its compact size, ease of use and ease of access to the load.Magagauss Magnetic Field Generation and Related Topics, 2006 IEEE International Conference on; 12/2006 
Article: Analytic model of a magnetically insulated transmission line with collisional flow electrons
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ABSTRACT: We have developed a relativisticfluid model of the flowelectron plasma in a steadystate onedimensional magnetically insulated transmission line (MITL). The model assumes that the electrons are collisional and, as a result, drift toward the anode. The model predicts that in the limit of fully developed collisional flow, the relation between the voltage Va, anode current Ia, cathode current Ik, and geometric impedance Z0 of a 1D planar MITL can be expressed as Va=IaZ0h(chi), where h(chi)≡[(chi+1)/4(chi1)]1/2lnf ⌊chi+(chi21)1/2⌋/2chi(chi1) and chi≡Ia/Ik. The relation is valid when Va≳1MV. In the minimally insulated limit, the anode current Ia,minf =1.78Va/Z0, the electronflow current If,minf =1.25Va/Z0, and the flow impedance Zf,minf =0.588Z0. {The electronflow current If≡IaIk. Following Mendel and Rosenthal [Phys. Plasmas 2, 1332 (1995)PHPAEN1070664X10.1063/1.871345], we define the flow impedance Zf as Va/(Ia2Ik2)1/2.} In the wellinsulated limit (i.e., when Ia≫Ia,minf ), the electronflow current If=9Va2/8IaZ02 and the flow impedance Zf=2Z0/3. Similar results are obtained for a 1D collisional MITL with coaxial cylindrical electrodes, when the inner conductor is at a negative potential with respect to the outer, and Z0≲40Omega. We compare the predictions of the collisional model to those of several MITL models that assume the flow electrons are collisionless. We find that at given values of Va and Z0, collisions can significantly increase both Ia,minf and If,minf above the values predicted by the collisionless models, and decrease Zf,minf . When Ia≫Ia,minf , we find that, at given values of Va, Z0, and Ia, collisions can significantly increase If and decrease Zf. Since the steadystate collisional model is valid only when the drift of electrons toward the anode has had sufficient time to establish fully developed collisional flow, and collisionless models assume there is no net electron drift toward the anode, we expect these two types of models to provide theoretical bounds on Ia, If, and Zf.Physical Review Special Topics  Accelerators and Beams 09/2006; 9. · 1.52 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have developed wirearray z pinch scaling relations for plasmaphysics and inertialconfinementfusion (ICF) experiments. The relations can be applied to the design of z pinch accelerators for highfusionyield (approximately 0.4 GJ/shot) and inertialfusionenergy (approximately 3 GJ/shot) research. We find that (delta(a)/delta(RT)) proportional (m/l)1/4 (Rgamma)(1/2), where delta(a) is the implodingsheath thickness of a wireablationdominated pinch, delta(RT) is the sheath thickness of a RayleighTaylordominated pinch, m is the total wirearray mass, l is the axial length of the array, R is the initial array radius, and gamma is a dimensionless functional of the shape of the current pulse that drives the pinch implosion. When the product Rgamma is held constant the sheath thickness is, at sufficiently large values of m/l, determined primarily by wire ablation. For an ablationdominated pinch, we estimate that the peak radiated xray power P(r) proportional (I/tau(i))(3/2)Rlphigamma, where I is the peak pinch current, tau(i) is the pinch implosion time, and phi is a dimensionless functional of the currentpulse shape. This scaling relation is consistent with experiment when 13 MA < or = I < or = 20 MA, 93 ns < or = tau(i) < or = 169 ns, 10 mm < or = R < or = 20 mm, 10 mm < or = l < or = 20 mm, and 2.0 mg/cm < or = m/l < or = 7.3 mg/cm. Assuming an ablationdominated pinch and that Rlphigamma is held constant, we find that the xraypower efficiency eta(x) congruent to P(r)/P(a) of a coupled pinchaccelerator system is proportional to (tau(i)P(r)(7/9 ))(1), where P(a) is the peak accelerator power. The pinch current and accelerator power required to achieve a given value of P(r) are proportional to tau(i), and the requisite accelerator energy E(a) is proportional to tau2(i). These results suggest that the performance of an ablationdominated pinch, and the efficiency of a coupled pinchaccelerator system, can be improved substantially by decreasing the implosion time tau(i). For an accelerator coupled to a doublepinchdriven hohlraum that drives the implosion of an ICF fuel capsule, we find that the accelerator power and energy required to achieve highyield fusion scale as tau(i)0.36 and tau(i)1.36, respectively. Thus the accelerator requirements decrease as the implosion time is decreased. However, the xraypower and thermonuclearyield efficiencies of such a coupled system increase with tau(i). We also find that increasing the anodecathode gap of the pinch from 2 to 4 mm increases the requisite values of P(a) and E(a) by as much as a factor of 2.Physical Review E 09/2005; 72(2 Pt 2):026404. · 2.33 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have conducted a series of experiments designed to measure the flashover strength of various azimuthally symmetric 45° vacuuminsulator configurations. The principal objective of the experiments was to identify a configuration with a flashover strength greater than that of the standard design, which consists of a 45° polymethylmethacrylate (PMMA) insulator between flat electrodes. The thickness d and circumference C of the insulators tested were held constant at 4.318 and 95.74 cm, respectively. The peak voltage applied to the insulators ranged from 0.8 to 2.2 MV. The rise time of the voltage pulse was 40 60 ns; the effective pulse width [as defined in Phys. Rev. ST Accel. Beams 7, 070401 (2004), PRABFM, 10984402, 10.1103/PhysRevSTAB.7.070401] was on the order of 10 ns. Experiments conducted with flat aluminum electrodes demonstrate that the flashover strength of a crosslinked polystyrene (Rexolite) insulator is (18±7)% higher than that of PMMA. Experiments conducted with a Rexolite insulator and an anode plug, i.e., an extension of the anode into the insulator, demonstrate that a plug can increase the flashover strength by an additional (44±11)%. The results are consistent with the Anderson model of anodeinitiated flashover, and confirm previous measurements. It appears that a Rexolite insulator with an anode plug can, in principle, increase the peak electromagnetic power that can be transmitted across a vacuum interface by a factor of [(1.18)(1.44)]2=2.9 over that which can be achieved with the standard design.Review of Modern Physics 05/2005; · 42.86 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Advances in fast, pulsedpower technologies have resulted in the development of very high current drivers that have current rise times ~100 ns. The largest such pulsed power driver today is the new Z accelerator located at Sandia National Laboratories in Albuquerque, New Mexico. Z can deliver more than 20 MA with a timetopeak of 105 ns to low inductance (~1 nH) loads. Such large drivers are capable of directly generating magnetic fields approaching 3 kT in small, 1 cm3 volumes. In addition to direct field generation, Z can be used to compress an applied, axial seed field with a plasma. Flux compression schemes are not new and are, in fact, the basis of all explosive fluxcompression generators, but we propose the use of plasma armatures rather than solid, conducting armatures. We present experimental results from the Z accelerator in which magnetic fields of ~2 kT are generated and measured with several diagnostics. Issues such as energy loss in solid conductors and dynamic response of currentcarrying conductors to very large magnetic fields are reviewed in context with Z experiments. We describe planned fluxcompression experiments that are expected to create the highestmagnitude uniformfield volumes yet attained in the laboratory.11/2004;  [Show abstract] [Hide abstract]
ABSTRACT: We have measured the xray power and energy radiated by a tungstenwirearray z pinch as a function of the peak pinch current and the width of the anodecathode gap at the base of the pinch. The measurements were performed at 13 and 19MA currents and 1, 2, 3, and 4mm gaps. The wire material, number of wires, wirearray diameter, wirearray length, wirearrayelectrode design, normalizedpinchcurrent time history, implosion time, and diagnostic package were held constant for the experiments. To keep the implosion time constant, the mass of the array was increased as I2 (i.e., the diameter of each wire was increased as I), where I is the peak pinch current. At 19 MA, the mass of the 300wire 20mmdiam 10mmlength array was 5.9 mg. For the configuration studied, we find that to eliminate the effects of gap closure on the radiated energy, the width of the gap must be increased approximately as I. For shots unaffected by gap closure, we find that the peak radiated xray power P(r) proportional to I1.24+/0.18, the total radiated xray energy E(r) proportional to I1.73+/0.18, the xraypower rise time tau(r) proportional to I0.39+/0.34, and the xraypower pulse width tau(w) proportional to demonstrate that the internal energy and radiative opacity of the pinch are not responsible for the observed subquadratic power scaling. Heuristic wireablation arguments suggest that quadratic power scaling will be achieved if the implosion time tau(i) is scaled as I(1/3). The measured 1sigma shottoshot fluctuations in P(r), E(r), tau(r), tau(w), and tau(i) are approximately 12%, 9%, 26%, 9%, and 2%, respectively, assuming that the fluctuations are independent of I. These variations are for onehalf of the pinch. If the half observed radiates in a manner that is statistically independent of the other half, the variations are a factor of 2(1/2) less for the entire pinch. We calculate the effect that shottoshot fluctuations of a single pinch would have on the shotsuccess probability of the doublepinch inertialconfinementfusion driver proposed by Hammer et al. [Phys. Plasmas 6, 2129 (1999)]. We find that on a given shot, the probability that two independent pinches would radiate the same peak power to within a factor of 1+/alpha (where 0< or =alpha<1) is equal to erf(alpha/2sigma), where sigma is the 1sigma fractional variation of the peak power radiated by a single pinch. Assuming alpha must be < or =7% to achieve adequate oddLegendremode radiation symmetry for thermonuclearfusion experiments, sigma must be <3% for the shotsuccess probability to be > or =90%. The observed (12/2(1/2))%=8.5% fluctuation in P(r) would provide adequate symmetry on 44% of the shots. We propose that threedimensional radiativemagnetohydrodynamic simulations be performed to quantify the sensitivity of the xray emission to various initial conditions, and to determine whether an imploding z pinch is a spatiotemporal chaotic system.Physical Review E 04/2004; 69(4 Pt 2):046403. · 2.33 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Absorption spectroscopy measurements of the timedependent heating of thin foils exposed to intense zpinch radiation sources are presented. These measurements and their analysis provide valuable benchmarks for, and insights into, the radiative heating of matter by xray sources. Zpinch radiation sources with peak powers of up to 160 TW radiatively heated thin plastictamped aluminum foils to temperatures approximately 60 eV. The foils were located in open slots at the boundary of zpinch hohlraums surrounding the pinch. Timeresolved Kalpha satellite absorption spectroscopy was used to measure the evolution of the Al ionization distribution, using a geometry in which the pinch served as the backlighter. The timedependent pinch radius and xray power were monitored using framing camera, xray diode array, and bolometer measurements. A threedimensional view factor code, within which onedimensional (1D) radiationhydrodynamics calculations were performed for each surface element in the view factor grid, was used to compute the incident and reemitted radiation flux distribution throughout the hohlraum and across the foil surface. Simulated absorption spectra were then generated by postprocessing radiationhydrodynamics results for the foil heating using a 1D collisionalradiative code. Our simulated results were found to be in good general agreement with experimental xray spectra, indicating that the spectral measurements are consistent with independent measurements of the pinch power. We also discuss the sensitivity of our results to the spectrum of the radiation field incident on the foil, and the role of nonlocal thermodynamic equilibrium atomic kinetics in affecting the spectra.Physical Review E 11/2002; 66(4 Pt 2):046416. · 2.33 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The Doppler broadening of ion line profiles emitted by zpinch plasma provides information about the thermalization of the implosion kinetic energy and the radiation efficiency of the pinch. Measurements of these line profiles are often complicated by source broadening in the instrument and opacity broadening of the emitted radiation. A high resolution concave crystal spectrometer in the Johann geometry was used to record the time averaged spectra of optically thin trace elements in the load. An imaging slit provided radially resolved but axially averaged spectra. The measurements indicate that lower ion temperatures (3–5 keV) are observed for Al wire loads on both the Saturn and Double EAGLE accelerators in the short current pulse mode (60–100 ns) than in the long pulse mode (125–225 ns) where values of 6.3–9.5 keV are observed. These values are smaller than those observed on Saturn by others. Furthermore, the wavelength at the line center of axially resolved ion line profiles on the DM2 accelerator at Titan was observed to vary about some average value which implies an axially varying fluid motion of the plasma column transverse to the pinch axis. © 2002 American Institute of Physics.Journal of Applied Physics 09/2002; 92(7):34583462. · 2.19 Impact Factor 
Article: Z pinch driven inertial confinement fusion target physics research at Sandia National Laboratories
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ABSTRACT: Three hohlraum concepts are being pursued at Sandia National Laboratories (SNL) to investigate the possibility of using pulsed power driven magnetic implosions (Z pinches) to drive targets capable of fusion yields in the range 2001000 MJ. This research is being conducted on SNL's Z facility, which is capable of driving peak currents of 20 MA in various Z pinch load configurations that produce implosion velocities as high as 7.5 × 107cm/s, X ray energies of 12 MJ and X ray powers of 100250 TW. The first concept, denoted dynamic hohlraum, has achieved a temperature of 180 ± 14 eV in a configuration suitable for driving capsules. In addition, this concept has also achieved a temperature of 230 ± 18 eV in an arrangement suitable for driving an external hohlraum. The second concept, denoted static walled hohlraum, has achieved temperatures of ~80100 eV. Experimental investigation of the third concept, denoted Z pinch driven hohlraum, has recently begun. The article discusses each of these hohlraum concepts and provides an overview of the experiments that have been conducted on these systems to date.Nuclear Fusion 05/2002; 39(9Y):1283. · 3.24 Impact Factor 
Conference Paper: Progress in doubleshell gas puff zpinches
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ABSTRACT: An experimental study of high current (3–15 MA), high fidelity (multiple atomic number) and long implosion time (100–200 ns) gas puff loads using the 1–2–3–4 cm doubleshell gas puff is in progress at Titan/PSD. Results of experiments conducted on DoubleEAGLE, Saturn, Decade Quad and the Z accelerators will be analyzed and presented. The principal observations are: (1) The overall pinch quality and radiative characteristics of all the argon double shell zpinches are quite satisfactory. The Ar Kshell yields varies from the expected I4 scaling in the inefficient regime for 3 to 7 MA to I2 scaling in the efficient regime from 7 to 15 MA. (2) On all experiments from 3–15 MA, selective seeding of the shells demonstrates that the hottest mass of the pinch originates from the inner shell. This suggests that mixing between the two plasma shells during their collision and final implosion is limited. (3) On the 15 MA Sandia Z accelerator, with a load mass of 0.8 mg/cm, the Kshell xray output reached 275 kJ in a 15 TW peak power, 12 ns pulse. The analyzed ion and electron densities reach 5 × 1019 and 1.0 × 1021 /cc and the highest electron temperature observed is up to 2.2 keV with a 2.0 keV continuumHighPower Particle Beams (BEAMS), 2002 14th International Conference on; 01/2002 
Article: Particleincell simulations of electron flow in the posthole convolute of the Z accelerator
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ABSTRACT: The threedimensional, particleincell code QUICKSILVER [J. P. Quintenz et al., Lasers Part. Beams 12, 283 (1994)] is now being used to simulate the inner region of the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] at Sandia National Laboratories. The simulations model electron flow and anode losses in the double posthole convolute, which couples four radial, magnetically insulated transmission lines (MITLs) in parallel to a single MITL that drives a Zpinch load. To efficiently handle the large range in the magnetic field, 0<B<200 T, the particle pusher is modified to subcycle the electron advance relative to the field solver. Results from a series of simulations using a constantimpedance load are presented. The locations of electron losses to the anode in the convolute are in qualitative agreement with damage to the Z hardware. The electron energy deposited in these anode regions rapidly heats the surface to temperatures above 400 °C—the threshold at which anode plasma formation is expected. © 2001 American Institute of Physics.Physics of Plasmas 09/2001; 8(10):45344544. · 2.25 Impact Factor 
Article: Z pinches—A historical view
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ABSTRACT: Z pinches have a long and varied history. Beginning in the 18th century, z pinches have been used to heat plasmas very efficiently. Early in the nuclear fusion program, it was realized that modest currents are required to confine plasma that could produce energy gain. The instability of the confined plasma was convincingly demonstrated in experiments in the 1950s that were performed around the world. These uniformly negative results led to z pinches being dropped as a fusion concept. Recent progress in fast z pinches has reinvigorated the field. We review the field and highlight the recent advances that point the way to a bright future for z pinches.Laser and Particle Beams 09/2001; 19(04):509  525. · 2.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The magnetoRayleigh–Taylor (MRT) instability limits the performance of dynamic z pinches. This instability develops at the plasmavacuum/field interface, growing in amplitude throughout the implosion, thereby reducing the peak plasma velocity and spatial uniformity at stagnation. MRT instabilities are believed to play a dominant role in the case of high wire number arrays, gas puffs and foils. In this article, the MRT instability is discussed in terms of initial seeding, linear and nonlinear growth, experimental evidence, radiation magnetohydrodynamic simulations, and mitigating schemes. A number of experimental results are presented, where the mitigating schemes have been realized. In general, the problem is inherently three dimensional, but twodimensional simulations together with theory and experiment enhance our physical understanding and provide insight into future load design.Laser and Particle Beams 09/2001; 19(04):527  540. · 2.02 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A unified set of hightemperaturehohlraum models has been developed. For a simple hohlraum, P(S)=[A(S)+(1alpha(W))A(W)+A(H)]sigmaT(4)(R)+(4Vsigma/c)(dT(4)(R)/dt), where P(S) is the total power radiated by the source, A(S) is the source area, A(W) is the area of the cavity wall excluding the source and holes in the wall, A(H) is the area of the holes, sigma is the StefanBoltzmann constant, T(R) is the radiation brightness temperature, V is the hohlraum volume, and c is the speed of light. The wall albedo alpha(W) identical with(T(W)/T(R))(4) where T(W) is the brightness temperature of area A(W). The net power radiated by the source P(N)=P(S)A(S)sigmaT(4)(R), which suggests that for laserdriven hohlraums the conversion efficiency eta(CE) be defined as P(N)/P(Laser). The characteristic time required to change T(4)(R) in response to a change in P(N) is 4V/c[(1alpha(W))A(W)+A(H)]. Using this model, T(R), alpha(W), and eta(CE) can be expressed in terms of quantities directly measurable in a hohlraum experiment. For a steadystate hohlraum that encloses a convex capsule, P(N)=[(1alpha(W))A(W)+A(H)+[(1alpha(C))A(C)(A(S)+alpha(W)A(W))/A(T)]]sigmaT(4)(RC), where alpha(C) is the capsule albedo, A(C) is the capsule area, A(T) identical with(A(S)+A(W)+A(H)), and T(RC) is the brightness temperature of the radiation that drives the capsule. According to this relation, the capsulecoupling efficiency of the baseline National Ignition Facility hohlraum is 1523 % higher than predicted by previous analytic expressions. A model of a hohlraum that encloses a z pinch is also presented.Physical Review E 09/2001; 64(2 Pt 2):026410. · 2.33 Impact Factor 
Article: Experimental configuration for isentropic compression of solids using pulsed magnetic loading
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ABSTRACT: A capability to produce quasiisentropic compression of solids using pulsed magnetic loading on the Z accelerator has recently been developed and demonstrated [C. A. Hall, Phys. Plasmas 7, 2069 (2000)]. This technique allows planar, continuous compression of materials to stresses approaching 1.5 Mbar. In initial stages of development, the experimental configuration used a magnetically loaded material cup or disk as the sample of interest pressed into a conductor. This installation caused distortions that limited the ability to attach interferometer windows or other materials to the rear of the sample. In addition, magnetic pressure was not completely uniform over sample dimensions of interest. A new modular configuration is described that improves the uniformity of loading over the sample surface, allows materials to be easily attached to the magnetically loaded sample, and improves the quality of data obtained. Electromagnetic simulations of the magnetic field uniformity for this new configuration will also be presented. Comparisons between data on copper to ∼300 kbar using the old and new experimental configurations will also be made. Results indicate that to within experimental error, the configurations produce similar results in the pressurevolume plane. © 2001 American Institute of Physics.Review of Scientific Instruments 08/2001; 72(9):35873595. · 1.58 Impact Factor
Publication Stats
2k  Citations  
188.48  Total Impact Points  
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Institutions

1985–2006

Sandia National Laboratories
 Advanced Materials Laboratory
Albuquerque, New Mexico, United States


2000

Lawrence Livermore National Laboratory
 Physics Division
Livermore, California, United States


1976–1981

University of California, Davis
Davis, California, United States
