Edward C. Morse

Ultrafast x-ray Thomson scattering of shock-compressed matter.

Spectrally resolved scattering of ultrafast K-α x-rays has provided experimental validation of the modeling of the compression and heating of shocked matter. The elastic scattering component has characterized the evolution and coalescence of two shocks launched by a nanosecond laser pulse into lithium hydride with an unprecedented temporal resolution of 10 picoseconds. At shock coalescence, we observed rapid heating to temperatures of 25,000 kelvin when the scattering spectra show the collective plasmon oscillations that indicate the transition to the dense metallic plasma state. The plasmon frequency determines the material compression, which is found to be a factor of 3, thereby reaching conditions in the laboratory relevant for studying the physics of planetary formation.

Proton spectra from ultraintense laser-plasma interaction with thin foils: Experiments, theory, and simulation

A beam of high energy ions and protons is observed from targets irradiated with intensities up to 5×1019 W/cm2. Maximum proton energy is shown to strongly correlate with laser-irradiance on target. Energy spectra from a magnetic spectrometer show a plateau region near the maximum energy cutoff and modulations in the spectrum at approximately 65% of the cutoff energy. Presented two-dimensional particle-in-cell simulations suggest that modulations in the proton spectrum are caused by the presence of multiple heavy-ion species in the expanding plasma.

Optical parametric chirped-pulse amplifier as an alternative to Ti:sapphire regenerative amplifiers

We demonstrated a high-pulse energy, femtosecond-pulse source based on optical parametric chirped-pulse amplification. We successfully amplified 1-microm broadband oscillator pulses to 31 mJ and recompressed them to 310-fs pulse duration, at a 10-Hz repetition rate. The gain in our system is 6 x 10(7), achieved by the single passing of only 40 mm of gain material pumped by a commercial Q-switched Nd:YAG laser. This relatively simple system replaces a more complex Ti:sapphire regenerative-amplifier-based chirped-pulse amplification system. Numerous features in design and performance of optical parametric chirped-pulse amplifiers make them a preferred alternative to regenerative amplifiers based on Ti:sapphire in the front end of high-peak-power lasers.

The use of a preconditioned bi-conjugate gradient method for hybrid plasma stability analysis

Abstract The numerical stability analysis of compact toroidal plasmas using implicit time differencing requires the solution of a set of coupled, 2-dimensional, elliptic partial differential equations for the field quantities at every timestep. When the equations are spatially finite-differenced and written in matrix form, the resulting matrix is large, sparse, complex, non-Hermitian, and indefinite. The use of the preconditioned bi-conjugate gradient method for solving these equations is discussed. The effect of block-diagonal preconditioning and of incomplete block-LU preconditioning on the convergence of the method is investigated. For typical matrices arising in our studies, the eigenvalue spectra of the original and preconditioned matrices are calculated as an illustration of the effectiveness of the preconditioning. We show that the preconditioned bi-conjugate gradient method converges more rapidly than the conjugate gradient method applied to the normal equations, and that it is an effective iterative method for the class of non-Hermitian, indefinite problems of interest.

Plasma diagnostics for the sustained spheromak physics experiment

In this article we present an overview of the plasma diagnostics operating or planned for the sustained spheromak physics experiment device now operating at Lawrence Livermore National Laboratory. A set of 46 wall-mounted magnetic probes provide the essential data necessary for magnetic reconstruction of the Taylor relaxed state. Rogowski coils measure currents induced in the flux conserver. A CO2 laser interferometer is used to measure electron line density. Spectroscopic measurements include an absolutely-calibrated spectrometer recording extended domain spectrometer for obtaining time-integrated visible ultraviolet spectra and two time-resolved vacuum monochrometers for studying the time evolution of two separate emission lines. Another time-integrated spectrometer records spectra in the visible range. Filtered silicon photodiode bolometers provide total power measurements, and a 16 channel photodiode spatial array gives radial emission profiles. Two-dimensional imaging of the plasma and helicity injec...

Linear stability analysis of compact toroidal plasmas using particle simulation

A technique is presented for the linear three‐dimensional stability analysis of plasmas in which ion kinetic effects are important. This technique is appropriate for the analysis of compact toroidal plasmas such as spheromaks, field‐reversed mirrors and theta pinches, field‐reversed configurations, and ion rings and layers. The plasma is modeled by the hybrid quasineutral model, in which the ions are represented by particles. An initial value approach is used to find the most rapid instabilities, in conjuction with the numerical integration of the equations. The model is verified against known analytic and numerical results for the linear stability of ion layers and theta pinches. The tilt instability in the spheromak is investigated, and comparison is made to magnetohydrodynamic (MHD) stability results for an equilibrium with a low ion beta.

Downscattered neutron imaging

Images with 14 MeV neutrons of inertial confinement fusion (ICF) D,T fusion show the regions of most intense fusion burn, while images based on lower-energy “downscattered” neutrons can reveal regions of nonburning D,T fuel. The downscattered images can help to understand ICF implosion dynamics. Recording downscattered images is difficult because the images are relatively weak, and because they may be obscured by residual “afterglow” of more intense 14 MeV images. The effect of afterglow can be estimated by adding a sequence of images for neutron energies from 14 MeV down to the downscatteed energy of interest. The images will be subject to decay factors which depend on the time response of the neutron scintillator. Preliminary analyses suggest that afterglow will not prevent the recording of useful downscattered images.

Image reconstruction algorithms for inertial confinement fusion neutron imaging

A neutron imaging system is required to diagnose ignition implosions at the National Ignition Facility. Such a system is required to be able to resolve features in the imploded target core as small as 5μm. The system will use a pinhole-camera-type geometry with a nonideal coded aperture and will employ image restoration techniques. The choice of image reconstruction method will be important in recovering the best possible source images from the recorded data. Monte Carlo transport simulations with MCNP5 make it possible to estimate the performance of the neutron imaging system based on calculated energy-dependent image edits of a failed inertial confinement fusion implosion. Simulations of the recorded neutron images include specific aperture designs, a pixelated energy- and time-dependent scintillator array, and an intensified gated charge coupled device camera for recording the images. An initial series of simulations used a source that was binned into 1MeV increments from 6to18MeV, an imaging aperture ...

Eigenfunctions of the curl in cylindrical geometry

Eigenfunctions of the equation ∇×B=λB are found for finite cylindrical geometry with normal boundary condition B∙n=0 and nonaxisymmetric modes ∼eimθ,m≠0. The vector field B can be represented by a scalar generating function of the Chandrasekhar-Kendall type with radial Bessel functions for the nondegenerate cases. A general set of solutions can also be generated by transformation of variables. A series solution in terms of radial Bessel functions is found which has excellent convergence properties (an∼1∕n4) and a robust method of locating eigenvalues is described.

An ion Doppler spectrometer instrument for ion temperature and flow measurements on SSPX.

A high-resolution ion Doppler spectrometer (IDS) has been installed on the sustained spheromak plasma experiment to measure ion temperatures and plasma flow. The system is composed of a 1 m focal length Czerny-Turner spectrometer with a diffraction grating line density of 2400 lines/mm, which allows for first order spectra between 300 and 600 nm. A 16-channel photomultiplier tube detection assembly combined with output coupling optics provides a spectral resolution of 0.0126 nm/channel. We calculate in some detail the mapping of curved slit images onto the linear detector array elements. This is important in determining the wavelength resolution and setting the optimum vertical extent of the slit. Also, because of the small wavelength window of the IDS, a miniature fiber-optic survey spectrometer sensitive to a wavelength range 200-1100 nm and having a resolution of 0.2 nm is used to obtain a time-integrated spectrum for each shot to verify specific impurity line radiation. Several measurements validate the systems operation. Doppler broadening of C III 464.72 nm line in the plasma shows time-resolved ion temperatures up to 250 eV for hydrogen discharges, which is consistent with neutral particle energy analyzer measurements. Flow measurements show a sub-Alfvenic plasma flow ranging from 5 to 45 kms for helium discharges.