Showera Haque

Measurement of pulsed-power-driven magnetic fields via proton deflectometry

Measuring magnetic field and current distribution in Z-pinch plasma systems is crucial to the validation of Z-pinch theory. In this letter, the demonstration of proton deflectometry to pulsed-power-driven loads at the mega-amp scale is presented, which is capable of making more detailed field maps in high-density regions of plasmas. In this method, a laser-driven, broad-spectrum, MeV-energy proton beam is directed through a pulsed-power-driven plasma system, and the resulting deflections are measured to examine configuration of magnetic fields and to infer the currents that support them. The technique was first demonstrated on simple short-circuit loads, and the results are in excellent agreement with numerical simulations providing reliable estimates of the field and current configurations. It was then applied to a more complex—radial foil—plasma load. The measurements show unexpected proton deflections that exhibit the complexity of the plasma load and that with further analysis will reveal details about the current and magnetic field topology in this complex configuration.

Spatially resolved single crystal x-ray spectropolarimetry of wire array z-pinch plasmas

A recently developed single-crystal x-ray spectropolarimeter has been used to record paired sets of polarization-dependent and axially resolved x-ray spectra emitted by wire array z-pinches. In this measurement, two internal planes inside a suitable crystal diffract the x-rays into two perpendicular directions that are normal to each other, thereby separating incident x-rays into their linearly polarized components. This paper gives considerations for fielding the instrument on extended sources. Results from extended sources are difficult to interpret because generally the incident x-rays are not separated properly by the crystal. This difficulty is mitigated by using a series of collimating slits to select incident x-rays that propagate in a plane of symmetry between the polarization-splitting planes. The resulting instrument and some of the spatially resolved polarized x-ray spectra recorded for a 1-MA aluminum wire array z-pinch at the Nevada Terawatt Facility at the University of Nevada, Reno will be presented.

Development of a spectroscopic technique for simultaneous magnetic field, electron density, and temperature measurements in ICF-relevant plasmas

Spectroscopic techniques in the visible range are often used in plasma experiments to measure B-field induced Zeeman splitting, electron densities via Stark broadening, and temperatures from Doppler broadening. However, when electron densities and temperatures are sufficiently high, the broadening of the Stark and Doppler components can dominate the emission spectra and obscure the Zeeman component. In this research, we are developing a time-resolved multi-axial technique for measuring the Zeeman, Stark, and Doppler broadened line emission of dense magnetized plasmas for Z-pinch and Dense Plasma Focus (DPF) accelerators. The line emission is used to calculate the electron densities, temperatures, and B-fields. In parallel, we are developing a line-shape modeling code that incorporates the broadening effects due to Stark, Doppler, and Zeeman effects for dense magnetized plasma. This manuscript presents the details of the experimental setup and line shape code, along with the results obtained from an Al iii doublet at the University of Nevada, Reno at Nevada Terawatt Facility. Future tests are planned to further evaluate the technique and modeling on other material wire array, gas puff, and DPF platforms.

Development of a spectroscopic technique for simultaneous magnetic field, electron density, and temperature measurements in ICF-relevant plasmas(Conference Presentation)

Visible spectroscopic techniques are often used in plasma experiments to measure B-field induced Zeeman splitting, electron densities via Stark broadening and temperatures from Doppler broadening. However, when electron densities and temperatures are sufficiently high, the broadening of the Stark and Doppler components can dominate the emission spectra and obscure the Zeeman component. In this research, we are developing a time-resolved multi-axial technique for measuring the Zeeman, Stark, and Doppler broadened line emission of dense magnetized plasmas for Z-pinch and Dense Plasma Focus (DPF) accelerators. The line emission is used to calculate the electron densities, temperatures, and B-fields. In parallel, we are developing a line-shape modeling code that incorporates the broadening effects due to Stark, Doppler, and Zeeman effects for dense magnetized plasma. Experiments conducted at the University of Nevada (Reno) at the Nevada Terawatt Facility (NTF) using the 1 MA Z-pinch (Zebra). The research explored the response of Al III doublet, 4p 2P3/2 to 4s 2S1/2 and 4p 2P1/2 to 4s 2S1/2 transitions. Optical light emitted from the pinch is fiber coupled to high-resolution spectrometers. The dual spectrometers are coupled to two high-speed visible streak cameras to capture time-resolved emission spectra from the experiment. The data reflects emission spectra from 100 ns before the current peak to 100 ns after the current peak, where the current peak is approximately the time at which the pinch occurs. The Al III doublet is used to measure Zeeman, Stark, and Doppler broadened emission. The line emission is then used to calculate the temperature, electron density, and B-fields. The measured quantities are used as initial parameters for the line shape code to simulate emission spectra and compare to experimental results. Future tests are planned to evaluate technique and modeling on other material wire array, gas puff, and DPF platforms. This work was done by National Security Technologies, LLC, under Contract No. DE-AC52-06NA25946 with the U.S. Department of Energy and supported by the Site-Directed Research and Development Program. DOE/NV/25946--2749.

Magnetic Field Measurement in Magnetized Laser Plasmas Using Zeeman Broadening Diagnostics

The Zeeman effect has been used to measure the magnetic field in high energy density plasmas. The measurements are difficult when the field orientation is fluctuating in the plasma volume or when the line broadening due to the high plasma density and temperature surpasses the Zeeman splitting. Based on an idea proposed by Tessarin et al. (2011), we implemented a solution to this problem to the field measurement in magnetized laser plasmas. Time resolved spectra were obtained at the Nevada Terawatt Facility for plasmas created by 20 J, 1 ns Leopard laser pulses in the azimuthal magnetic field produced by the 0.6 MA Zebra pulsed power generator. The time history of the components of the Al III 4s 2S1/2 - 4p 2P1/2,3/2 doublet were recorded via optical streak camera (approximately 400 ns frame) at various locations along the magnetic field radius. In these measurements the Zeeman splitting was not resolved, but the magnetic field strength can be measured from the difference between the widths of the line profiles.

The influence of insulating coatings upon single wire explosions

Summary form only given. Experiments have been initiated on the 1-MA Z-pinch generator at the Nevada Terawatt Facility to investigate the influence of insulating coatings on the ablation and energy deposition in current-carrying metallic wires. The Z-pinch loads used included single 50 μm Cu wires with and without polymer coatings, as well as pairs of such wires in a two-wire array configuration. The dynamics of these Z-pinches was diagnosed with laser shadowgraphy and X-pinch driven X-ray backlighting. It was observed that the ablation and the instability growth of the coated wires was delayed compared with that of the uncoated ones. Besides possible direct applications to enhanced energy deposition in Z-pinches, the coated exploding wires represent a platform for studying mixing in conditions that may be relevant to astrophysical systems and to directly or indirectly laser-driven fusion capsules.

Electron beams and x-ray emission of conical wire array Z-pinches

Summary form only given. It is well known that intense beams of energetic electrons are accelerated in z-pinches. Using magnetic deflection and Faraday cup detection we measured electron energies and the current of beams produced in conical wire array z-pinches and x-pinches. The variations in pointing and divergence of the beams reduced the accuracy of these measurements. However, the beam activity can be correlated with the x-ray emission of the z-pinches. We will present results of these measurements.

Strip velocity measurements for gated x-ray imagers using short pulse lasers

Strip velocity measurements of gated X-ray imagers are presented using an ultra-short pulse laser. Obtaining time- resolved X-ray images of inertial confinement fusion shots presents a difficult challenge. One diagnostic developed to address this challenge is the gated X-ray imagers. The gated X-ray detectors (GXDs) developed by Lawrence Livermore National Laboratory and Los Alamos National Laboratory use a microchannel plate (MCP) coated with a gold strip line, which serves as a photocathode. GXDs are used with an array of pinholes, which image onto various parts of the GXD image plane. As the pulse sweeps over the strip lines, it creates a time history of the event with consecutive images. In order to accurately interpret the timing of the images obtained using the GXDs, it is necessary to measure the propagation of the pulse over the strip line. The strip velocity was measured using a short pulse laser with a pulse duration of approximately 1-2 ps. The 200nm light from the laser is used to illuminate the GXD MCP. The laser pulse is split and a retroreflective mirror is used to delay one of the legs. By adjusting the distance to the mirror, one leg is temporally delayed compared to the reference leg. The retroreflective setup is calibrated using a streak camera with a 1 ns full sweep. Resolution of 0.5 mm is accomplished to achieve a temporal resolution of ~5 ps on the GXD strip line.

X-ray yield from pinch target implosions

The wire array z-pinch is an efficient x-ray source which has been proposed for use in indirect drive inertial confinement fusion schemes. Extensive research has focused on methods to enhance and manipulate the x-ray yield in a z-pinch. In recent experiments performed at the Nevada Terawatt Facility, it was observed that a center wire added as a target for conical array implosions resulted in an increase in x-ray yield when the diameter of the center wire was smaller than a threshold value which depended on the wire material. Investigation of this behavior was performed on Zebra, a 2 TW z-pinch generator which delivers a 1 MA current pulse to a load, with a 90 ns rise time. Aluminum cylindrical and conical wire arrays with similar implosion times were used to investigate the role of the center wire in the implosion. For each configuration the array wires diameter remained unchanged for all experiments, while Al, Ti, Cu, SS and W targets were used with diameters ranging from 10 µm – 1 mm. Comparing the soft x-ray yield (20 eV – 5 keV) without a center wire, the cylindrical arrays produced more x-rays than the conical wire arrays, which was expected since a portion of the kinetic energy of the conical implosion goes to producing a plasma jet. With the addition of a center wire, the conical wire array showed a positive correlation between the soft x-ray yield and the diameter of the target. This increase was significant enough to surpass the cylindrical wire array in soft x-ray yield. In conical wire array implosion the narrow end of the cone has an increased J×B force causing the narrow region to implode faster than the rest of the array, similar to an x-pinch. During the initial implosion, time-gated pinhole images recorded a bright x-ray burst at the narrow region of the cone. In addition there was also observed a bright, narrow, hard x-ray source along the length of the pinch. This talk will present the differences in soft (20 eV – 5 keV) and hard (1 keV – 5 keV) x-ray yield form different arrays and targets and will discuss the possible sources for the x-ray yield increase.

Investigation of plasma flow redirection by an externally applied magnetic field

Magnetic field - plasma interactions play an important role in plasma opening switches, the closure of magnetically insulated transmission lines, as well as many other laboratory and astrophysical systems.