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Effects of the intensity pickets on laser imprinting were investigated using laser-driven, planar plastic and foam targets on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Intensity pickets are used in adiabat-shaping techniques designed to improve stability of inertial confinement fusion targets. The measurements were performed in planar foam targets using the decaying shock method of adiabat shaping and in planar plastic targets using the relaxation (RX) method. In foam targets, the picket reduced the target areal-density modulations by ∼6 times at shorter spatial wavelengths of 30 and 60μm, while at a longer wavelength of 120μm there was no reduction. The “imprint efficiency,” the equivalent surface amplitude produced by imprinting, was reduced by the intensity picket by a factor of ∼3 at a spatial wavelength of 60μm, while it was increased by a factor of ∼2 at a 120-μm spatial wavelength. In plastic targets, used to test the RX method, the imprint efficiency was reduced b...

The optical properties of x-ray preheated planar-window materials relevant for shock-wave timing experiments were studied on the OMEGA Laser System. The x-ray radiation was generated by 100 ps, 1×1015W∕cm2 laser pulses incident on planar plastic targets, instantaneously affecting samples located ∼0.7mm away. An abrupt onset of strong absorption of an optical probe beam (λ=532nm) and a temporally varying refractive index were measured in polystyrene and diamond windows. The behavior of diamond windows exposed to x rays is consistent with a simple model based on the generation of free charge carriers. Polystyrene windows showed indications of optical transitions due to molecular states that are created by the ionizing radiation.

The temperature-density profiles of imploded cryogenic D2 capsules are inferred for a low-adiabat (α∼6) drive using a Monte Carlo technique described earlier [V. A. Smalyuk et al., Phys. Plasmas 12, 052706 (2005)]. The analysis has been improved by incorporating differential x-ray imaging using narrowband channels at ∼4.5 and ∼6.5keV. The inferred compression is significantly higher with the low-adiabat drive than with reported previously high-adiabat (α∼25) drive.

The National Ignition Facility (NIF) will provide up to 1.8MJ of laser energy for imploding inertial confinement fusion (ICF) targets. Ignited NIF targets are expected to produce up to 1019 DT neutrons. This will provide unprecedented opportunities and challenges for the use of nuclear diagnostics in ICF experiments. In 2005, the suite of nuclear-ignition diagnostics for the NIF was defined and they are under development through collaborative efforts at several institutions. This suite includes PROTEX and copper activation for primary yield measurements, a magnetic recoil spectrometer and carbon activation for fuel areal density, neutron time-of-flight detectors for yield and ion temperature, a gamma bang time detector, and neutron imaging systems for primary and downscattered neutrons. An overview of the conceptual design, the developmental status, and recent results of prototype tests on the OMEGA laser will be presented.

Radial profiles of nuclear burn in directly driven, inertial-confinement-fusion implosions have been systematically studied for the first time using a proton emission imaging system sensitive to energetic 14.7MeV protons from the fusion of deuterium (D) and 3-helium (He3) at the OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. Experimental parameters that were varied include capsule size, shell composition and thickness, gas fill pressure, and laser energy. Clear relationships have been identified between changes in a number of these parameters and changes in the size of the burn region, which we characterize here by the median “burn radius” Rburn containing half of the total DHe3 reactions. Different laser and capsule parameters resulted in burn radii varying from 20to80μm. For example, reducing the DHe3 fill pressure from 18to3.6atm in capsules with 20μm thick CH shells resulted in Rburn changing from 31to25μm; this reduction is attributed to increased fuel-shell mix for the mor...

An understanding of the nonlinear evolution of Rayleigh-Taylor (RT) instability is essential in inertial confinement fusion and astrophysics. The nonlinear RT growth of three–dimensional (3-D) broadband nonuniformities was measured near saturation levels using x-ray radiography in planar foils accelerated by laser light. The initial 3-D target modulations were seeded by laser nonuniformities and subsequently amplified by the RT instability. The measured modulation Fourier spectra and nonlinear growth velocities are in excellent agreement with those predicted by Haan's model [S. Haan, Phys. Rev. A 39, 5812 (1989)]. These spectra and growth velocities are insensitive to initial conditions. In a real-space analysis, the bubble merger was quantified by a self-similar evolution of bubble size distributions, in agreement with the Alon–Oron–Shvarts theoretical predictions [D. Oron et al. Phys. Plasmas 8, 2883 (2001)].

Polar direct drive (PDD) [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)] will allow direct-drive ignition experiments on the National Ignition Facility (NIF) [J. Paisner et al., Laser Focus World 30, 75 (1994)] as it is configured for x-ray drive. Optimal drive uniformity is obtained via a combination of beam repointing, pulse shapes, spot shapes, and∕or target design. This article describes progress in the development of standard and “Saturn” [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 0952002 (2005)] PDD target designs. Initial evaluation of experiments on the OMEGA Laser System [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)] and simulations were carried out with the two-dimensional hydrodynamics code SAGE [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005)]. This article adds to this body of work by including fusion particle production and transport as well as radiation transport within the two-dimensional DRACO [P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)] hydr...

More than 100 spherical deuterium ice layers have been formed to make cryogenic targets for direct-drive ICF implosion experiments on OMEGA. These ice layers have an inner surface roughness that ranges from 1.3to6μm root-mean-square (rms), with the best layers having a value less than 2μm rms. These surface roughness values are averaged two-dimensional roughness measurements that cover the entire surface and includes all of the Fourier cosine modes. The ice thickness variation within the layer is predominately in the low spectral modes (mode 5 and lower) and is caused by the support used to hold the target. Changing the design of this support to minimize the thermal effect is constrained by the necessity of having a dynamically stable target for the implosion. We have demonstrated that it is possible to form crystalline ice layers that are facet-free and transparent by slowing the solidification rate of the liquid. Faster freezing rates form layers comprised of polycrystalline ice with a greater roughness (1to2μm greater). Cooling an ice layer 0.5K below the triple point temperature does not affect the roughness of the layer. Cooling the layer a further 1K to achieve the desired internal gas pressure sometimes induces additional ice roughness; this roughness is manifest over low- to mid-spectral modes. Removing the thermal shrouds from around the target causes the ice to melt and the internal gas pressure to increase. Using the behavior of a cryogenic deuterium target as a reference, calculations of the response of the more interesting National Ignition Facility-scale deuterium and tritium targets show that exposing the target for 0.8s to ambient radiation will cause ∼10% of the ice to melt and partially slump whereas the gas pressure will increase by 15%.

A proton emission imaging system has been developed and used extensively to measure the nuclear burn regions in the cores of inertial confinement fusion implosions. Three imaging cameras, mounted to the 60-beam OMEGA laser facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], use the penetrating 14.7MeV protons produced from DHe3 fusion reactions to produce emission images of the nuclear burn spatial distribution. The technique relies on penumbral imaging, with different reconstruction algorithms for extracting the burn distributions of symmetric and asymmetric implosions. The hardware and design considerations required for the imaging cameras are described. Experimental data, analysis, and error analysis are presented for a representative symmetric implosion of a fuel capsule with a 17-μm-thick plastic shell and 18atm DHe3 gas fill. The radial burn profile was found to have characteristic radius Rburn, which we define as the radius containing half the DHe3 reactions, of 32±2μm (burn radii measure...

Excessive increase in the shell entropy and degradation from spherical symmetry in inertial confinement fusion implosions limit shell compression and could impede ignition. The entropy is controlled by accurately timing shock waves launched into the shell at an early stage of an implosion. The seeding of the Rayleigh-Taylor instability, the main source of the asymmetry growth, is also set at early times during the shock transit across the shell. In this paper we model the shock timing and early perturbation growth of directly driven targets measured on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. By analyzing the distortion evolution, it is shown that one of the main parameters characterizing the growth is the size of the conduction zone Dc, defined as a distance between the ablation front and the laser deposition region. Modes with kDc>1 are stable and experience oscillatory behavior [V. N. Goncharov, Phys. Rev. Lett. 82, 2091 (1999)]. The model shows that the main stabiliz...

Extended x-ray absorption fine structure (EXAFS) measurements have been used to demonstrate the phase transformation from body-centered-cubic (bcc) to hexagonal-closely packed (hcp) iron due to nanosecond, laser-generated shocks. The EXAFS spectra are also used to determine the volume compression and temperature in the shocked iron, which are consistent with hydrodynamic simulations and with the compression inferred from velocity interferometric (VISAR) measurements. This is a direct, atomic-level and in situ proof of shock-induced transformation in iron. Thus, the EXAFS data can be used both for studying the phase transformation as well as for characterizing the crystal conditions (density, temperature) during the transformation.

The Hugoniot of quartz has been measured using laser-driven shock waves with pressures from 2 to 15 Mbars. Within this pressure range silica transforms from a liquid near melt into a dense plasma. Results are in good agreement with previous studies in part of this range performed using explosive- and nuclear-driven shocks indicating the absence of time-dependent effects for time scales between several hundred picoseconds and several hundred microseconds. These data combined with earlier data at lower pressures clearly show the increasing compressibility of silica as it transitions from solid to liquid to dense plasma regimes.

In a direct-drive, inertial confinement fusion implosion, a spherical target is irradiated by a large number of overlapped laser beams. Imprinting of laser modulations depends on the relative arrival time of laser beams and their angles of incidence. This dependence was measured in planar plastic targets using six overlapping beams on the OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton, R. L. Keck, J. P. Knauer, J. H. Kelly, T. J. Kessler, S. A. Kumpan, S. J. Loucks, S. A. Letzring, F. J. Marshall, R. L. McCrory, S. F. B. Morse, W. Seka, J. M. Soures, and C. P. Verdon, Opt. Commun. 133, 495 (1997)]. One of the beams (the imprint beam) had a special phase plate that produced two-dimensional modulations on the target, easily distinguishable from the features imprinted by the other five drive beams. The timing of the imprint beam was varied with respect to the drive beams to study imprinting sensitivity to beam mistiming. Shifting the imprint beam to arrive before the other beams significantly increased the imprint efficiency. The results are in very good agreement with the model predictions.

The 60-beam Omega laser system at the University of Rochester's Laboratory for Laser Energetics (LLE) has been a workhorse on the frontier of laser fusion and high-energy-density physics for more than a decade. LLE scientists are currently extending the performance of this unique, direct-drive laser system by adding high-energy petawatt capabilities.

Hydrodynamic simulations have shown that a picket pulse preceding the main target drive pulse in a direct-drive inertial confinement fusion implosion can reduce both the ablation-interface Rayleigh–Taylor (RT) seed and growth rate by increasing the adiabat (ratio of the plasma pressure to the Fermi-degenerate pressure) while maintaining the low adiabat in the inner-fuel layer for optimal target compression and a minimal drive energy for ignition. Experiments with planar and spherical targets have been carried out on the OMEGA [T. R. Boehly, D. L. Brown, R. S. Craxton, R. L. Keck, J. P. Knauer, J. H. Kelly, T. J. Kessler, S. A. Kumpan, S. J. Loucks, S. A. Letzring, F. J. Marshall, R. L. McCrory, S. F. B. Morse, W. Seka, J. M. Soures, and C. P. Verdon, Opt. Commun. 133, 495 (1997)] laser system, showing that the RT growth of nonuniformities was reduced by picket-pulse laser illumination. Adiabat shaping in spherical targets has been investigated with two types of picket pulses—a “decaying shock wave” and a ...

Imprinting of laser-beam modulations at various angles of incidence is measured for the first time in planar CH foils. The imprinted target modulations were seeded by special probe beams at a spatial wavelength of 60μm and subsequently amplified by five drive beams. The measured imprint efficiency decreases by a factor of 3 as the angle of incidence of the probe beam is increased from 20° to 60°, as predicted by theoretical modeling. The imprinting is very sensitive to the relative arrival time of the probe and drive beams.

Measurements from three classes of direct-drive implosions at the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] were combined with Monte Carlo simulations to investigate models for determining hot-fuel areal density (ρRhot) in compressed, D2-filled capsules, and to assess the impact of mix and other factors on the determination of ρRhot. The results of the Monte Carlo simulations were compared to predictions of simple, commonly used models that use ratios of either secondary D3He proton yields or secondary DT neutron yields to primary DD neutron yields to provide estimates ρRhot,2p or ρRhot,2n, respectively, for ρRhot. For the first class of implosion, where ρRhot is low (⩽3 mg/cm2), ρRhot,2p and ρRhot,2n often agree with each other and are often good estimates of the actual ρRhot. For the second class of implosion, where ρRhot is of order 10 mg/cm2, ρRhot,2p often underestimates the actual value due to secondary proton yield saturation; in addition, fuel-shell mix causes ρRhot,2p...

Multidimensional hydrodynamic properties of high-adiabat direct-drive plastic-shell implosions on the OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] are investigated using the multidimensional hydrodynamic code, DRACO [D. Keller et al., Bull. Am. Phys. Soc. 44, 37 (1999)]. Multimode simulations including the effects of nonuniform illumination and target roughness indicate that shell stability during the acceleration phase plays a critical role in determining target performance. For thick shells that remain integral during the acceleration phase, target yields are significantly reduced by the combination of the long-wavelength (l

In direct-drive laser fusion, the tradeoff between stability and overall efficiency requires precise control of the implosion isentrope. Most target designs use the temporal shape of the drive pulse to create shocks that slightly preheat the capsule shell and establish the isentrope for the rest of the implosion. Also, the use of foam overcoatings has been proposed as a means to reduce laser imprinting. These foams can alter the structure and intensity of the initial shock. To ensure that our hydrocodes adequately model these effects it is important that shock heating of targets be measured and understood. We report on measurements of shock heating in planar targets irradiated with the OMEGA laser system. Planar 20-μm-thick CH targets were irradiated with six ultraviolet (UV) beams at intensities of ∼2×1014 W/cm2 with temporally square and ramped pulses. Some targets also have low-density foam (30 mg/cc) on the irradiated surface. A thin (0.5 μm) Al layer, imbedded in the target, is probed with x rays fr...

The use of measured spectra of secondary fusion protons for studying physical characteristics of D2-filled inertial confinement fusion capsules is described theoretically and demonstrated with data from implosions in the OMEGA 60-beam laser facility. Spectra were acquired with a magnet-based charged-particle spectrometer and with a range-filter-based spectrometer utilizing filters and CR39 nuclear track detectors. Measurement of mean proton energy makes possible the study of a capsule’s total areal density (ρR), since that is what affects the energy loss suffered by protons as they pass through fuel and shell while leaving the capsule. Details of specific shots will be presented. It is also shown that similar techniques should prove useful for diagnosis of future experiments with cryogenic D2-filled capsules.

With charged-particle spectroscopy implemented on OMEGA, we have been able to routinely measure the particle spectra (both nuclear lines and continua) from a variety of capsule implosions. Important parameters such as fusion yields, fuel and shell areal densities, and ion temperatures can be readily deduced. We will report on details of this work with emphasis on the implosion physics.

The generation and numerical and optical characterization of a single-beam, ponderomotive-optical trap are discussed. A novel segmented wave-plate technique is described, and experimental results of the trap formation are presented. Several methods to tune the traps are discussed, including techniques to realize a bright trap for dark-seeking electrons. The effects of non-Gaussian incident beams and beams with phase aberrations on trap formation are also described.

We have extended the technique of plasma temperature measurement by ratio of isoelectronic lines, recently developed with nanosecond laser plasmas, to the diagnosis of picosecond and subpicosecond plasmas. We have found a major benefit—particularly for short-pulse laser plasmas—that even where level populations are far from steady-state values, the ratio of isoelectronic lines may be nearly steady state, which considerably simplifies interpretation. We describe theoretical and experimental investigations for plasmas created from solid targets by 100-fs–10-ps high-intensity laser pulses and report the experimental application of the technique to λ = 1.05-μm laser pulses at 1016 W cm−2.

Observations of multiphoton scattering of electrons born in a high-intensity laser focus are extended to intensities at which the relativistic mass shift associated with the quiver motion must be taken into account.

We have developed and demonstrated a simple, single-shot method to measure the duration of a weak ultrafast pulse. The density of free electrons in a semiconductor is changed by a pump beam, causing a time-dependent variation of the refractive index, which in turn determines the spatial distribution of the reflected probe-beam intensity. The derivative of the spatial distribution in the transition region is a cross correlation of the pump and probe pulses. This technique is suitable for measuring weak pulses such as low-level synchrotron light in the mid-infrared-to-far-infrared range.

A new technique based on frequency-domain interferometry has been used to measure the group-velocity walk-off between ordinary and extraordinary waves with picosecond duration in birefringent crystals. The dependence of walk-off on incident angle with respect to the birefringent crystal has also been measured.

The optimum conditions for optical beam steering by spatial phase modulation in nonlinear Kerr media are obtained by use of the conservation laws of the nonlinear Schrodinger equation together with the moment method. The operating conditions under which the deflection angle is largest and the deflected beam carries the most energy in the form of a spatial soliton are determined. The analytical theory is applied to both planar waveguides and bulk Kerr media. The analytical predictions are compared with numerical simulations for the case of sinusoidal spatial phase modulation. Good agreement has been found between the analytical results and computer simulations.

The collisions of spatial vector solitons have been investigated numerically. Both the polarization and propagation directions of the solitons are changed after collision, which could be useful in the applications of all-optical switching. It is also found that both the polarization mixing and the beam deflection become exponentially smaller as theta increases. The advantages of using vector solitons in optical switching are discussed.

Soliton collisions in birefringent optical fibers have been investigated numerically. It is found that soliton collisions can be divided into three kinds of behavior depending on the value of the linear birefringence. Both soliton mixing and soliton center-frequency shifts have been studied in detail. A new kind of soliton state has been found. The Cerenkov emission generated by soliton collision has also been found numerically.

Ionization suppression can be defined as the decrease of photoinduced atomic ionization rates with an increased applied field intensity. The temporal and spatial intensity distribution of a focused laser pulse introduces difficulty in observing ionization suppression. An analysis of the effect of a temporally Gaussian laser pulse on ionization suppression from Rydberg levels is given. It is found that ionization suppression by the Gaussian pulse is still apparent, even though the low-intensity tails of the pulse produce moderate ionization. Further, the spatial effects are examined for a scheme whereby a narrow atomic beam is passed through the focus of a laser at which ionization is detected. Ionization suppression in the more intense region of the laser focus is predicted to be apparent above the background ionization from the relatively large region of low intensity surrounding the laser focus.

The pedestal (prepulse and postpulse) associated with a chirped-pulse-amplification (CPA) laser is studied. Four components have been identified that contribute to the pedestal. Pulses are spectrally shaped by gain narrowing in a frequency-matched, regenerative amplifier, while self-phase modulation is avoided. The intensity contrast is further improved through the use of a saturable absorber, resulting in Gaussian pulses of ~0.9-ps duration with an intensity contrast exceeding 105:1. Both experimental and numerical descriptions of these processes are presented. This investigation makes possible the study of high-intensity ultrashort laser–plasma interactions with a fiber–grating CPA system.

Laser ionization of noble gases was studied with a 1.053-μm, 1-psec Nd:glass laser. A systematic scan of intensities from mid-1013 W/cm2 to mid-1016 W/cm2 was performed, resulting in the production of charge states as high as Xe12+. Ionization occurs exclusively in the tunneling regime. We compare experimental ion production rates with those predicted by several different theories. Agreement between experimental ion-production curves and theoretical predictions is good for two theoretical models: (1) an elaboration of the Keldysh tunneling model, developed by Ammosov et al. [ Sov. Phys. JETP64, 1191 ( 1986)] and (2) a much more primitive model, based on Coulomb-barrier suppression, in which tunneling and other quantum-mechanical effects are ignored completely. The success of the more primitive model suggests that a new term, barrier-suppression ionization, rather than tunneling or multiphoton ionization, may be the most appropriate at this wavelength and in this range of intensities.

Picosecond chirped-pulse technology is used to generate two spectrally separate, time-synchronized pulses for χ(3) measurements by nearly degenerate four-wave mixing. Near 1053 nm, nonresonant relative measurements of χ1111(3) are carried out on three model substances, nitrobenzene, α-chloronaphthalene, and 4′-decyloxynaphthyl-1′-(4-decyloxybenzoate). Their χ(3) values are normalized to CS2.

The local carbon particle diffusion coefficient was measured in the Proto S-1/C spheromak with use of a test-particle injection scheme. When the plasma was not in a force-free Taylor state, and when there were pressure gradients in the plasma, the particle diffusion was 5 times that predicted by Bohm and was consistent with collisional drift-wave diffusion. The diffusion appears to be driven by correlations of the fluctuating electric field and density. During the decay of the discharge when the plasma was in the Taylor state, the diffusion coefficient of the carbon was classical.

The current penetration phase of PDX discharges is examined. The Fast-Ion Diagnostic Experiment has been used to measure the temporal evolution of the central q (r/a < 0.4) and to show the effect of magnetic perturbations on fast ions. During plasma current penetration, a series of magnetic perturbations was observed in the plasma. When the plasma current was rising rapidly, the perturbations were accompanied by increases in β + li/2 and decreases in the loop voltage, suggesting a rapid penetration of the plasma current. When the plasma current was rising slowly, a series of 'minor' disruptions occurred. These were accompanied by decreases in β + li/2 and in the loop voltage, and increases in the plasma current. During this phase, current penetration may be enhanced by the change in the resistivity profile which accompanies the disruption.

Compression of low-temperature spheromak plasmas has been studied with the aid of a zero-dimensional two-fluid computer code. The major direct effect of compression is to raise the current and plasma densities, keeping their ratios constant. Above critical pre-compression values of the electron temperature, Te, and the product of electron density and particle confinement time, neτp, the rising plasma density shifts the impurity ions to higher ionization states, with correspondingly lower radiative losses. High post-compression temperatures may thus be achieved. In oxygen radiation-dominated plasmas with pre-compression values above the critical ones, and with constant τp, the electron temperature can be increased by up to a factor of seven for a compression of a factor of two. This can be compared with a factor-of-four temperature rise expected for adiabatic compression. If the energy balance is dominated by particle confinement losses rather than radiation losses, the effect of compression is to raise the temperature as Te ~ C6/5, for constant τp.

Local impurity particle diffusion coefficients have been measured in a low temperature plasma by the injection of test particles at the center of the plasma. The injection is accomplished by a high‐voltage discharge between two small graphite electrodes on a probe. The probe can be located anywhere in the plasma. The diffusion is observed spectroscopically. An analysis of the spatial and temporal evolution of the C ii radiation from the carbon discharge can determine the parallel and perpendicular diffusion of the impurity ions. Results with the diagnostic have been obtained in the Proto S‐1/C spheromak. The measured value of the diffusion coefficient in the afterglow plasma is in good agreement with classical predictions.

The light ion accelerator (1.5 MV, 4 Ω) at Cornell University is being used to study the characteristics of an applied Bz , or ‘‘barrel,’’ diode. We report here the results of a series of experiments utilizing a plasma opening switch. With a magnetically insulated ion diode load, the peak diode voltage increased from 1.5 to 1.8 MV and the ion power increased from 50 to 80 GW when a plasma opening switch was used. Our results are compared with existing theories of plasma opening switch operation.

A special purpose charge‐exchange analyzer was constructed to measure the spatial distribution of hot‐plasma ions, as a function of energy and time, in the poloidal divertor experiment (PDX). The fast neutrals produced by charge exchange within the tokamak are reionized as they pass through a helium stripping cell in the detector. The energies of these ions are determined by the trajectories they follow between cylindrical deflection plates which are set at known electrostatic potentials. We describe the technique used to calibrate the response of this system as it depends on the energies and the masses of the particles which are being detected.

The horizontally scanning, multi-angle charge-exchange analyser on the Poloidal Divertor Experiment (PDX) was used to study the beam ion slowing-down process with high-power perpendicular injection. Measurements were made over a wide range in toroidal field (8 kG

Radial distributions of fully ionized oxygen and carbon in the PDX tokamak plasma are reported. These ions were detected via radiation emitted in charge-exchange recombination reactions between the impurities and hydrogen atoms from a non-perturbing neutron beam. The C/sup 6 +/ and O/sup 8 +/ ions are observed out to radii beyond the limiter, which is in contrast to expectations based on coronal equilibrium but consistent with a simple diffusive transport model. Central values of Z/sub eff/ obtained with these measurements agree with values obtained from plasma resistivity and visible bremsstrahlung measurements.

A comparison is made of the tilting instability and resulting nonlinear reconnection between a three-dimensional magnetohydrodynamic (3D MUD) simulation and the proto S-1/C spheromak experiment. A good agreement is obtained. The observed fast reconnection process is found to be explained by the driven reconnection theory.

The angular dependence of the flux of fast neutrals was measured in conjunction with Ion Cyclotron Range of Frequency experiments on the Princeton Large Torus. A regime was found where the energetic tail of the charge-exchange neutral flux was strongly anisotropic. Unexpectedly, however, the flux from co-going ions was peaked at an angle intermediate between perpendicular and parallel. This could be the consequence of a preferential filling of that class of banana orbits whose tips are located within the ion cyclotron resonance layer.

The horizontally scanning, multiangle charge-exchange analyzers on the Princeton Large Torus (PLT) and the Poloidal Divertor Experiment (PDX) were used to study the effects of MHD activity on the background ion distribution function and on the beam ion slowing-down process during high-power neutral injection. Sawtooth oscillations were observed in the fast ion flux on PLT and PDX, and measurements with neutral beams providing local neutral density enhancement indicate that ions are transported radially when these events occur. With near-perpendicular injection in PDX, at the lower toroidal fields necessary to maximize beta, rapid, repetitive bursts of greatly enhanced charge-exchange flux were observed. These are associated with the ''fishbone'' MHD instability, and a substantial depletion of the perpendicular slowing-down spectrum below the injection energy was seen. A simple phenomenological model for this loss mechanism was developed, and its use in simulation codes has been successful in providing good agreement with the data. The behavior and characteristics of this model are well matched by the direct theoretical calculations.

The local carbon diffusion coefficient has been measured in the S-l spheromak by detecting the radial spread of injected carbon impurity. The radial impurity density profile is determined by the balance of ionization and diffusion. Using the measured local electron temperature and density, the ionization rate is determined from which the particle diffusion coefficient is inferred. The results found in this work are consistent with Bohm diffusion, and the absolute magnitude of the local particle transport coefficient D⊥ is found to be (4-6) × DBohm. The measured particle transport rates are correlated with the plasma pressure gradients, suggesting a pressure driven transport mechanism.

The local carbon diffusion coefficient was measured in the S - 1 spheromak by detecting the radial spread of injected carbon impurity. The radial impurity density profile is determined by the balance of ionization and diffusion. Using measured local electron temperature T/sub e/ and density n/sub e/, the ionization rate is determined from which the particle diffusion coefficient is inferred. The results found in this work are consistent with Bohm diffusion. The absolute magnitude of D/sub /perpendicular// was determined to be (4/approximately/6) /times/ D/sub Bohm/. 25 refs., 13 figs., 2 tabs.

We have measured the absorption of 1-ps laser pulses interacting with matter at intensities from ${10}^{10}$ to ${10}^{16}$ W/${\mathrm{cm}}^{2}$. The variations of absorption with incidence angle and polarization have been used to infer submicron plasma-density-gradient scale lengths. The results show a transition between a regime of laser interaction with sharply bounded dense cold matter ($I\ensuremath{\le}5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ W/${\mathrm{cm}}^{2}$), where absorption is by the usual skin depth effect, to a regime of interaction with a plasma of very steep density gradient ($\frac{L}{\ensuremath{\lambda}}\ensuremath{\le}0.2$) ($5\ifmmode\times\else\texttimes\fi{}{10}^{12}\mathrm{W}/{\mathrm{cm}}^{2}\ensuremath{\le}I\ensuremath{\le}{10}^{15}\mathrm{W}/{\mathrm{cm}}^{2}$).

Compression of low temperature spheromak plasmas has been studied with the aid of a O-D two-fluid computer code. It is found that in a plasma which is radiation dominated, the electron temperature can be increased by up to a factor of seven for a compression of a factor of two, provided the temperature is above some critical value (approx.25eV) and the electron density particle confinement time product n/sub e/tau/sub p/ greater than or equal to 1 x 10/sup 9/s/cm/sup 3/. If the energy balance is dominated by particle confinement losses rather than radiation losses, the effect of compression is to raise the temperature as T/sub e/ approx.C/sup 6/5/, for constant tau/sub p/.