Titanium and germanium lined hohlraums and halfraums as multi-keV x-ray radiators

Abstract

As multi-keV x-ray radiators, hohlraums and halfraums with inner walls coated with metallic materials (called liner) have been tested for the first time with laser as the energy drive. For titanium, conversion efficiencies (CEs) are up to ∼14% for emission into 4π, integrating between 4.6 and 6.5 keV when a large diameter hohlraum is used. Germanium CE is ∼0.8% into 4π between 9 and 13 keV. The highest CEs have been obtained with a 1 ns squared pulse and phase plates giving laser absorption near 99%. These high CEs are due to long-lasting, good plasma conditions for multi-keV x-ray production maintained by plasma confinement inside the plastic cylinder and plasma collision leading to a burst of x rays at a time that depends on target size. As photon emitters at 4.7 keV, titanium-lined hohlraums are the most efficient solid targets and data are close to CEs for gas targets, which are considered as the upper limit for x-ray yields since their low density allows good laser absorption and low kinetics losses. As 10.3 keV x-ray emitters, exploded germanium foils give best results one order of magnitude more efficient than thick targets; doped aerogels and lined hohlraums give similar yields, about three times lower than those from exploded foils.

A computational study of x-ray emission from high-Z x-ray sources on the National Ignition Facility laser

“…Thus, the laser beam supersonically and volumetrically heats the low-density material on a time scale shorter than the time scale for the rarefaction wave to decompress and cool the plasma, providing much higher x-ray conversion efficiency (XRCE) in the non-LTE plasma than is obtained by simply irradiating solid targets [4,5,6]. Under-dense high-Z radiators have been confined in the past mainly to high-Z noble gases (Ar, Kr, and Xe) [7,8,9], but some efficient Ti x-ray sources (K-shell emission ~ 4 keV) have also been created with nano-fiber targets [10]; with pre-pulsed Ti, Cu, and Ge foils (K-shell emission ~ 4 keV, ~8 keV, and ~10 keV respectively) [11,12]; and by irradiating the inside surface of Ti-and Ge-lined cans [13,14].…”

Multi-keV x-ray source development experiments on the National Ignition Facility

“…For certain applications, such as the study of x-ray driven electromagnetic phenomena, or radiography of high-density objects, high-energy, deeply penetrating x-ray photons are required. In addition to the K-shell Kr and Xe sources, x-ray sources at other energies need to be developed; for example, pre-exploded metal foils 22,23 , foil-lined cylinders 24 , metal nano-fiber low-density targets 21 , and metal-doped aerogel materials 20 are all candidates for highly efficient xray sources. In conclusion, an enormous expanse of x-ray source development activities exist, now that an unexplored regime of laser energy and power is available at the NIF.…”

“…The target design for the current experiments builds on experience at the Nova 29 and Omega 30 laser facilities using modeling and simulation tools that have been validated against experiments [12][13][14][15][16][17][18][19][20][21][22][23][24] . The desired x-ray source characteristics for the present experiments were: a) significant multi-keV x-ray output with energies >3 keV, b) x-ray pulse width of order 5 ns, and c) x-ray rise-time of order 1 ns or longer.…”

“…During the last decade, many research groups have been developing bright, multi-keV x-ray sources [12][13][14][15][16][17][18][19][20][21][22][23][24] . The goal has been to create millimeter-scale, homogeneous, keV-scale-temperature plasmas that are efficient laser-to-x-ray converters.…”

Bright x-ray sources from laser irradiation of foams with high concentration of Ti

“…% yielded twice the x-ray flux. Thus, such foams could achieve the ∼ 10% CE of gas 5 or metal-lined-cavity targets 29 . The experiment at Gekko XII also provided measurements of the heating dynamics in a simple geometry.…”