Creating an X-ray Nebula in the Laboratory: Spectral Diagnostics of Neon Photoionization Experiments on the Z-Machine

D. S. Conners, D. H. Cohen (Swarthmore College & Prism Computational Sciences, J. J. MacFarlane (Prism Computational Sciences), D. A. Liedahl (Livermore National Laboratory), J. E. Bailey (Sandia National Laboratory)

Gas Cell ExperimentsExperimental Design

VisRad Viewfactor Simulations of the X-ray Source

Motivation

Spectral DiagnosticsBacklit Absorption

Spectral DiagnosticsEmission

Introduction: We have begun a campaign of ride-along shots on the Z-machine at Sandia in order to study the response of low-density gases to irradiance by strong x-rays. Our interest is in the excitation/ionization kinematics and the physical conditions of the x-ray photoionized plasma. We would like to minimize gradients, shocks, and collisional effects and isolate the atomic physics. The motivation behind these experiments is to study basic physics, explore fusion and X-ray laser applications, and to benchmark codes used to study astrophysical plasmas.

The experimental package consists of a cm-scale neon-filled cell with mylar windows, mounted several cm from the Z-machine’s current return can, inside of which is the pinch itself.

Ultimately, we plan on making simultaneous, time-resolved absorption and emission spectroscopic measurements of the gas cell, which can be filled with a variety of gases, including mixtures. The absorption spectroscopy uses the pinch itself as a backlighter.

Experiments already completed have used 30 Torr neon (nion ~ 1018 cm-3) observed in absorption with a time-integrated spectrometer.

With the advent of high-resolution ( ~ 1000) x-ray spectroscopy in the last several years, a wealth of spectroscopic data for photoionized sources has been generated, and the level of detail and accuracy demanded of models has increased dramatically.

By producing and measuring a well-characterized x-ray photoionized plasma in the laboratory, we hope to benchmark the codes used to model x-ray spectra of active galactic nuclei, x-ray binaries, and other astrophysical sources for which photoionization dominates collisional ionization.

The Chandra telescope (left) and XMM telescope, both launched in 1999, are the first

high-resolution spectroscopic missions for the astronomical community. Both employ

gratings. X-ray missions planned for the future will use microcalorimeters for high

spectral resolution.

The conditions in the plasma in proximity to the compact object are traditionally described by the ionization parameter (flux/density), which is a measure of the dominance of radiation processes over collisional processes.

Contours of constant ionization parameter (logarithmic, in cgs units) are shown for the HMXRB Vela X-1 (above, right; Figure from Sako et al. Ap.J., 525, 921 (1999)). Our previous experiments have achieved an

ionization parameter of ~7, but significantly higher values are possible.

Viewfactor simulations of the pinch and target are used to calculate the incident spectrum on the gas cell, and also to investigate the spatial uniformity of the irradiance on the cell.

Accretion onto compact objects leads to photoionization-dominated X-ray spectra

The circumstellar gas near the source of hard x-rays is highly ionized--overionized for its temperature compared to coronal equilibrium--and produces an x-ray spectrum dominated by radiative recombination continua and recombination cascades.

Calculation of iron emission spectrum for a photoionized plasma (left) and a collisional plasma (right). Though the ionization distribution is the same in both models, the spectra are quite different (and the photoionized plasma is

much colder). (Figure taken from Liedahl et al. Ap.J., 350, L37 (1990).)

Two snapshots (an early time, during the low-temperature foot of the pulse on the left and a late time from the high-temperature peak) from a VisRad simulation of

the imploding pinch and gas cell. The four frames, in each snapshot are a complete view of the pinch, current return can, and target (represented by the flat square)

(top left), a zoomed-in view of the pinch through the current return can (the opaque slats of which are represented by ‘scaffolding’ in the front so as not to obstruct the view in this representation) (top right), a view from inside the can, looking through

the slats at the target (lower left), and the instantaneous spectrum incident on the front of the gas cell (lower right).

Viewfactor simulation output is used as input to our hydrodynamic simulations.

The time-dependent pinch drive temperature (left). On the right, we show, for one simulation time, the incident spectrum (red) and blackbody

equivalent (black) for one point on the front of the cell. Note that the incident spectrum is quite non-Planckian.

Temperature (left) and density (right) at four different times in the simulation of our experiments. The radiation is incident from the left. The initial neon gas density is 1018 cm-3. Note the shock

waves launched from the mylar walls at late times. Note also the radiation wave (as evidenced by the temperature gradient) traversing the gas at t=100 ns.

Conclusions from hydro: To optimize uniformity of the physical conditions, a lower gas density is desirable. To minimize compression of the gas by the mylar walls, thicker walls are desirable. By doping the walls or coating them with thin foils, we may be able to optimally filter the x-rays flowing into the gas cell as well as putting off the implosion of the gas by the wall plasma until very late times.

A series of K absorption spectra, representing three different sets of physical conditions in the gas cell (warm to hot from top to

bottom). Spectral resolution is 5 eV (FWHM).

The time-integrated absorption spectrum from our first round of shots (top, above) are well-matched by our

calculations (lower, above).

A section of our raw data from the same shot (above). Note the very high principle quantum number

lines of He-like neon.

A spectral simulation of neon, based on a representative hydro calculation. The intrinsic spectrum (left) shows numerous lines of B-like through H-like neon, as well as a

recombination edge (near 1200 eV) and recombination and free-free continua. At 2 eV (center) and 5 eV (right) spectral resolution, many of the features are blended. The 2 eV

resolution spectrum is probably good enough to quantitatively test atomic/spectral models, whereas a lot of information is lost at a resolution of 5 eV.

Sandia’s Z-Machine is the worlds most powerful source of X-rays

An pulsed-power imploding tungsten wire array: creating a cm-scale, >150 eV plasma, generating 200 TW of X-rays in a ~10 ns pulse (2 MJ of X-rays).

The Z-Machine’s Marx capacitor banks before a ‘shot’.

Experimental Schematic

Experimental DesignHydrodynamic Simulations of the Gas Cell

Data

Conclusions•Absorption spectra can be measured in this configuration;

•Ionization parameters of ~100 should be achievable at nion~1017 cm-3;

•Measurement of recombination spectra and accurate wavelengths of inner-shell transitions should be possible in the next round of experiments.

gas

cell