T. D. Shepard

Gas‐filled targets for large scale‐length plasma interaction experiments on Nova

Stimulated Brillouin backscatter from large scale‐length gas‐filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te≳3 keV), and gradient scale lengths (Ln∼2 mm, Lv≳6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas‐filled balloons with polyimide walls (gasbags) and gas‐filled hohlraums. Detailed characterization using x‐ray imaging and x‐ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time‐resolved SBS backscatter from these targets is <3% for conditions similar to ignition target designs.

Characterization of titanium laser‐produced plasmas

The development of a plasma environment that is appropriate for the study of laser‐plasma processes in laser‐fusion plasma is reported. A material (titanium) with medium atomic number (Z) was used to provide x‐ray measurements of radial and axial plasma symmetry as well as electron temperature. The electron density evolution was measured using stimulated scattering processes and odd half‐harmonic generation from probe lasers of different wavelengths. The plasmas were created by two‐sided irradiation of thin foils with 24 kJ of 351 nm laser light. When the peak electron density had decayed to about 4×1020 cm−3, the density profile was estimated to have a full width at half‐maximum of 2 mm and the electron temperature was measured to be about 3 keV using K‐shell spectroscopy. Two‐dimensional computer simulations were found to reproduce some features of both electron density and temperature evolution.

Bragg‐diffraction x‐ray spectrographs for the determination of Te in 2–3‐mm‐sized laser‐produced plasmas on NOVA

We have developed spectrographs to measure the electron temperature in gas‐filled targets and low‐density foams, and find it to nominally fall in the 2–4‐keV range. The instrument we designed, built, and fielded can simultaneously record the Ti Heα and Cr Heα line emission. After compensating for the instrumental response, we can estimate the electron temperature from this line ratio to within ±15%.

Dynamics of gas‐filled hohlraums

In order to prevent high‐Z plasma from filling in the hohlraum in indirect drive experiments, a low‐Z material, or tamper is introduced into the hohlraum. This material, when fully ionized is typically less than one‐tenth of the critical density for the laser light used to illuminate the hohlraum. This tamper absorbs little of the laser light, thus allowing most of the laser energy to be absorbed in the high‐Z material. However, the pressure associated with this tamper is sufficient to keep the hohlraum wall material from moving a significant distance into the interior of the hohlraum. In this paper we discuss measurements of the motion of the interface between the tamper and the high‐Z hohlraum material. We also present measurements of the effect the tamper has on the hohlraum temperature.

Spectroscopic temperature measurements of non‐equilibrium plasmas

The characterization of laser‐produced plasmas has required the application of spectroscopic techniques to non‐standard conditions where kinetics models have not been extensively tested. The plasmas are produced by the Nova laser for the study of inertial confinement fusion, can be mm in size, and evolve on sub‐nanosecond time scales. These targets typically achieve electron temperatures from 2–4 keV and electron densities of 1020–1022 cm−3. We have measured the electron temperature of two types of targets: bags of gas and hohlraums, Au cylinders with laser entrance holes in the flat ends. By comparing data from different targets, we examine the time‐dependence of spectroscopic plasma diagnostics.

Te measurements in open‐ and closed‐geometry long‐scale‐length laser plasmas via isoelectronic x‐ray spectral line ratios

We have successfully employed isoelectronic line ratios to measure the electron temperature in gas‐filled hohlraums and ‘‘gas bags’’ shot with the Nova laser. Isoelectronic line ratios are well suited to this measurement because they are relatively insensitive to radiation field effects (in hohlraums), opacity, transients, and variations in electron density compared to conventional line ratios. Targets were designed to produce plasma parameters Te∼3 keV and Ne∼1021 cm−3 over a scale length of ∼2 mm. Collisional‐radiative, transient K‐shell atomic kinetics calculations including line transfer were performed by post‐processing the Lasnex results. By comparing these calculations with experimental data, we infer electron temperatures of at least 3 keV for both types of targets.