Daniel Headley

Development of a variable focal length concave mirror for on-shot thermal lens correction in rod amplifiers

An optical surface of variable concave parabolic shape and a clear aperture of 30 mm was created using two rings to deform a flat 50.8 mm diameter mirror. The deformable mirror assembly was modeled using finite element analysis software as well as analytical solutions. Measured parabolic surface deformation showed good agreement with those models. Mirror performance was quantitatively studied using an interferometer and focal lengths from hundreds of meters down to the meter scale have been achieved. In this publication, the deformable mirror has been applied to compensate on shot thermal lensing in 16 mm diameter and 25 mm diameter Nd:Phosphate glass rod amplifiers by using only a single actuator. The possibility to rapidly change focal lengths across two to three orders of magnitude has applications for remote sensing, such as laser induced breakdown spectroscopy, LIDAR, and control of laser filament formation.

Studies on thin films as short pulse laser debris shields

Optical properties of various thin films such as Nitrocellulose, Mylar, and Polyimide were investigated with respect to their application as laser debris shields. Studies on optical and spectral transmission quality, absorption, stress induced birefringence, and damage threshold have been performed. Scalability to large apertures was also considered. Studies were performed of how focusing geometry, target alignment, and mechanical components can help mitigate target debris traveling back to the focusing optic.

Status of the 2 MA driver for creating 2 MG magnetic fields for cluster fusion experiments

Now that the 400 kA version of a pulsed-power driver for magnetic coils for cluster fusion experiments has been completed, we are currently assembling a 2 MA version for generating magnetic fields up to 2 MG. Both versions are intended to drive single-turn, single-use 1-cm diameter magnetic field coils that enclose laser-produced, high-density deuterium cluster plasmas in vacuum. The driver is being built at the Sandia National Laboratories and laser-plasma experiments are being done at the University of Texas in Austin. The coils provide axial magnetic fields to slow radial loss of electrons from the plasma. Peak field with the 400 kA system is about 40 T, and will be up to 200 T for the 2 MA upgrade, with a current rise time of 1.7 µs for both systems. For these experiments the driver output must pass through a vacuum feed-through to couple to a short transmission line in vacuum that is terminated with a single-turn coil. This is done with a cylindrical insulator and conical transmission lines. A description of the device, and results of initial tests of the high-current version of the driver is given.

LDRD final report on confinement of cluster fusion plasmas with magnetic fields.

Two versions of a current driver for single-turn, single-use 1-cm diameter magnetic field coils have been built and tested at the Sandia National Laboratories for use with cluster fusion experiments at the University of Texas in Austin. These coils are used to provide axial magnetic fields to slow radial loss of electrons from laser-produced deuterium plasmas. Typical peak field strength achievable for the two-capacitor system is 50 T, and 200 T for the ten-capacitor system. Current rise time for both systems is about 1.7 {mu}s, with peak current of 500 kA and 2 MA, respectively. Because the coil must be brought to the laser, the driver needs to be portable and drive currents in vacuum. The drivers are complete but laser-plasma experiments are still in progress. Therefore, in this report, we focus on system design, initial tests, and performance characteristics of the two-capacitor and ten-capacitors systems. The questions of whether a 200 T magnetic field can retard the breakup of a cluster-fusion plasma, and whether this field can enhance neutron production have not yet been answered. However, tools have been developed that will enable producing the magnetic fields needed to answer these questions. These are a two-capacitor, 400-kA system that was delivered to the University of Texas in 2010, and a 2-MA ten-capacitor system delivered this year. The first system allowed initial testing, and the second system will be able to produce the 200 T magnetic fields needed for cluster fusion experiments with a petawatt laser. The prototype 400-kA magnetic field driver system was designed and built to test the design concept for the system, and to verify that a portable driver system could be built that delivers current to a magnetic field coil in vacuum. This system was built copying a design from a fixed-facility, high-field machine at LANL, but made to be portable and to use a Z-machine-like vacuum insulator and vacuum transmission line. This system was sent to the University of Texas in Austin where magnetic fields up to 50 T have been produced in vacuum. Peak charge voltage and current for this system have been 100 kV and 490 kA. It was used this last year to verify injection of deuterium and surrogate clusters into these small, single-turn coils without shorting the coil. Initial test confirmed the need to insulate the inner surface of the coil, which requires that the clusters must be injected through small holes in an insulator. Tests with a low power laser confirmed that it is possible to inject clusters into the magnetic field coils through these holes without destroying the clusters. The university team also learned the necessity of maintaining good vacuum to avoid insulator, transmission line, and coil shorting. A 200-T, 2 MA system was also constructed using the experience from the first design to make the pulsed-power system more robust. This machine is a copy of the prototype design, but with ten 100-kV capacitors versus the two used in the prototype. It has additional inductance in the switch/capacitor unit to avoid breakdown seen in the prototype design. It also has slightly more inductance at the cable connection to the vacuum chamber. With this design we have been able to demonstrate 1 MA current into a 1 cm diameter coil with the vacuum chamber at air pressure. Circuit code simulations, including the additional inductance with the new design, agree well with the measured current at a charge voltage of 40 kV with a short circuit load, and at 50 kV with a coil. The code also predicts that with a charge voltage of 97 kV we will be able to get 2 MA into a 1 cm diameter coil, which will be sufficient for 200 T fields. Smaller diameter or multiple-turn coils will be able to achieve even higher fields, or be able to achieve 200-T fields with lower charge voltage. Work is now proceeding at the university under separate funding to verify operation at the 2-MA level, and to address issues of debris mitigation, measurement of the magnetic field, and operation in vacuum. We anticipate operation at full current with single-turn, magnetic field coils this fall, with 200 T experiments on the Texas Petawatt laser in the spring of 2012.

Recent Results from Laser Solid Interaction Experiments using 100 TW Sandia Laser

Summary form only given. We present results from experiments performed on 100 TW facility at Sandia National Laboratory. The main goals of the experiment were to quantify the pointing stability, Kalpha spot size and pre-pulse effects of the Z-PW laser. The targets were 25 mum thick Cu foils. The specific diagnostics used include a Cu Kalpha spherical crystal imager which provided information on the shot to shot variations in Kalpha spot intensity and position, an optical probe used for transverse shadowgraphy of pre-pulse plasma formation and radiochromic film RCF stacks to characterize the rear surface proton energy. The Copper Kalpha images show an average spot diameter of ~71plusmn11 mum and a pointing error of les50 mum in both the vertical and horizontal directions. The stacks of RCF showed an average peak proton energy of 7.7plusmn2.4 MeV.

Commissioning experiments on the 100 TW Sandia laser

We present results from laser-target interaction experiments performed within the 100 TW facility at Sandia National Laboratory. A series of K-alpha images of 25μm thick copper foils reveal an average K-alpha spot diameter of ∼70±10μm and a pointing error of less than 50μm in both the vertical and horizontal directions. 2 omega optical probe pulse shadowgraphy images show front surface plasma growth and transverse focal structure from self-emission. Stacks of radiochromic film (RCF) positioned along the rear normal to the Cu foil indicate an average peak proton energy of 8±2 MeV.