S. Compton

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

We report results from a five shot campaign carried out with Ar–Xe gas-filled targets at the National Ignition Facility (NIF). The targets were shot with ≈350 kJ of 3ω laser energy delivered with a 5 ns trapezoidal laser pulse. We report measured x-ray output from the target in different spectral bands both below and above 1.5 keV photon energies: We find yields of ≈20.5 kJ/sr with peak x-ray power approaching 4 TW/sr over all energies, as measured for the unique viewing angle of our detector, and ≈3.6 kJ/sr with peak x-ray power of 1 TW/sr for x-rays with energies >3 keV. This is a laser-to-x-ray conversion efficiency of 13±1.3% for isotropic x-rays with energies >3 keV. Laser energy reflected by the target plasma for both inner and outer-cone beams is measured and found to be small, between 1% and 4% of the drive energy. The energy emitted in hard x-rays (with energies >25 keV) is measured and found to be ≈1 J/sr. Two-dimensional imaging of the target plasma during the laser pulse confirms a fast, volum...

Experimental measurement of Au M-band flux in indirectly driven double-shell implosions

Indirectly driven double-shell implosions are being investigated as a possible noncryogenic path to ignition on the National Ignition Facility [J. A. Paisner, J. D. Boyes, S. A. Kumpan, W. H. Lowdermilk, and M. S. Sorem, Laser Focus World 30, 75 (1994)]. In recent double-shell experiments, the inner shell trajectory was shown to exhibit a strong sensitivity to the temporal history of the M-band (2–5keV) radiation emitted from the Au hohlraum wall. A large time-dependent discrepancy was observed between measurement and simulation of the x-ray flux in this range. In order to better characterize the radiation environment seen in these implosions, an experimental campaign was conducted on the Omega laser. A number of diagnostics were used to measure both the temporal and spectral nature of the M-band flux. Results were obtained from an absolutely calibrated 12-channel filtered x-ray diode array (Dante) as well as two streaked crystal spectrometers and an absolutely calibrated time-integrated spectrometer (Hen...

Laser coupling to reduced-scale hohlraum targets at the Early Light Program of the National Ignition Facility

A platform for analysis of material properties under extreme conditions, where a sample is bathed in radiation with a high temperature, is under development. Depositing maximum laser energy into a small, high-Z enclosure produces this hot environment. Such targets were recently included in an experimental campaign using the first four of the 192 beams of the National Ignition Facility [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)], under construction at the University of California Lawrence Livermore National Laboratory. These targets demonstrate good laser coupling, reaching a radiation temperature of 340 eV. In addition, there is a unique wavelength dependence of the Raman backscattered light that is consistent with Brillouin backscatter of Raman forward scatter [A. B. Langdon and D. E. Hinkel, Phys. Rev. Lett. 89, 015003 (2002)]. Finally, novel diagnostic capabilities indicate that 20% of the direct backscatter from these reduced-scale targets is in the polarization or...

Time-resolved soft x-ray imaging diagnostic for use at the NIF and OMEGA lasers

The soft x-ray imager (SXRI) built for the first experiments at the National Ignition Facility (NIF) has four soft x-ray channels and one hard x-ray channel. The SXRI is a snout that mounts to a four strip gated imager. This produces four soft x-ray images per strip, which can be separated in time by ∼60ps. Each soft x-ray channel consists of a mirror plus a filter. The diagnostic was used to study x-ray burnthrough of hot Hohlraum targets at the NIF and OMEGA lasers. The SXRI snout design and issues involved in selecting the desired soft x-ray channels are discussed.

Transmitted Laser Beam Diagnostic at the Omega Laser Facility

We have developed and commissioned a transmitted beam diagnostic (TBD) for the 2ω high intensity interaction beam at the Omega laser facility. The TBD consists of a bare-surface reflector mounted near the target, which collects and reflects 4% of the transmitted light to a detector assembly outside the vacuum chamber. The detector includes a time integrating near-field camera that measures beam spray, deflection, and the absolute transmitted power. We present a detailed description of the instrument and the calibration method and include first measurements on laser heated gas bag targets to demonstrate the performance of the diagnostic.

Shielding a Streak Camera from Hard X-rays

The targets used in the hot halfraum campaign at OMEGA create many hot electrons, which result in a large flux of hard x rays. The hard x rays produce a high background in the streak camera. The background was significantly reduced by wrapping the streak camera with a high-Z material; in this case, 1/8 in. of Pb. The large hard x-ray flux also adds noise to images from framing cameras which use charge-coupled devices.

Fluorescence spectroscopy as a diagnostic of the radiation environment in high energy density experiments (invited)

A fluorescence spectroscopy technique has been developed to measure conditions in high energy density (HED) experiments. The experimental technique and modeling of the spectra are described and results of fluorescence measurements are presented. Fluorescence spectra were measured from an aluminium microdot over a small hole in the wall of an experimental package or a hohlraum. The aluminium was photopumped from a broadband radiation source, without perturbing the temperature. To date, fluorescence spectroscopy has been used to diagnose the radiative heating of plasmas in the temperature range 20–80 eV. Fluorescence spectroscopy has several advantages over x-ray absorption and self-emission spectroscopy in the diagnosis of HED experiments and these are discussed in the article. Extension of the technique to higher temperature plasma is discussed.

Multi-kev X-ray yields from HIGH-Z gas targets fielded at the national ignition facility

The National Ignition Facility (NIF) is a 192-beam laser system now operating at the LLNL. We report on the measured X-ray flux from gas-filled targets shot with 112-132 laser beams at the NIF. The targets were driven with up to 75 TW of laser power (350 kJ of 3ω laser energy delivered in a 5 ns modified-flat-top pulse). The laser delivered power and energy within 2% of the requested values on all shots. The targets were thin walled (25 μm), 4 mm long, 4 mm inner-diameter epoxy pipes designed to transmit X rays in the 1 - 10 keV spectral band. The pipes were filled with 1.2 atm of an Ar: Xe mixture. The emitted X-ray flux was monitored with multiple channels of the NIF facilitys two X-ray-diode based DANTE instruments in the sub-keV range, as well as around 3.2 keV (Ar K-shell emission) and in the 4.0-6.5 keV band (Xe L-shell emission). Temporal waveforms of the emitted X rays, and the total emitted fluence are presented. Two-dimensional X-ray imaging (for energies > 8 keV) of the targets was performed with a gated X-ray detector that used a four-strip microchannel plate. The two dimensional images confirm supersonic, volumetric heating of the gas targets. Laser light scattered via laser-plasma instabilities from the target plasma was monitored with the facilitys full aperture backscatter system (FABS), and hard X rays produced by hot electrons from the target plasma were measured with FFLEX diagnostic. We measure 15-20% laser-to-X-ray conversion efficiency for X rays with energies greater than 3.0 keV This is significantly greater than any previous measurements in this spectral band.

Coupling of High Power Lasers to Small, Hot Targets at the National Ignition Facility

Summary form only given. An experimental campaign to study radiation drive in small-scale halfraums has been carried out using the first four beams of the National Ignition Facility (NIF) at the Lawerence Livermore National Laboratory (Livermore, CA). The targets fill with plasma so quickly that, late in time, most of the laser energy is deposited at the laser entrance hole. Experiments have shown the effect of laser beam conditioning, laser power, and target size on hohlraum performance. The experimental results on X-radiation drive, laser backscatter, hard X-rays, hard X-ray imaging, and X-ray burnthrough are discussed