P. C. Schrafel

Initial experiments using radial foils on the Cornell Beam Research Accelerator pulsed power generator

A novel technique involving radial foil explosions can produce high energy density plasmas. A current flows radially inward in a 5 μm thin aluminum foil from a circular anode, which contacts the foil on its outer rim, to the cathode, which connects to the foil at its geometrical center. When using small “pin” cathodes (∼1 mm in diameter) on a medium size pulsed-current generator such as the Cornell Beam Research Accelerator, the central magnetic field approaches 400 T, yielding magnetic pressures larger than 0.5 Mbar. While the dynamics is similar to radial wire arrays, radial foil discharges have very distinct characteristics. First a plasma jet forms, with densities near 5×1018 cm−3. J×B forces lift the foil upward with velocities of ∼200 km/s. A plasma bubble with electron densities superior to 5×1019 cm−3 then develops, surrounding a central plasma column, carrying most of the cathode current. X-ray bursts coming from the center of this column were recorded at 1 keV photon energy. As the magnetic bubb...

Study of gas-puff Z-pinches on COBRA

Gas-puff Z-pinch experiments were conducted on the 1 MA, 200 ns pulse duration Cornell Beam Research Accelerator (COBRA) pulsed power generator in order to achieve an understanding of the dynamics and instability development in the imploding and stagnating plasma. The triple-nozzle gas-puff valve, pre-ionizer, and load hardware are described. Specific diagnostics for the gas-puff experiments, including a Planar Laser Induced Fluorescence system for measuring the radial neutral density profiles along with a Laser Shearing Interferometer and Laser Wavefront Analyzer for electron density measurements, are also described. The results of a series of experiments using two annular argon (Ar) and/or neon (Ne) gas shells (puff-on-puff) with or without an on- (or near-) axis wire are presented. For all of these experiments, plenum pressures were adjusted to hold the radial mass density profile as similar as possible. Initial implosion stability studies were performed using various combinations of the heavier (Ar) a...

Initial magnetic field compression studies using gas-puff Z-pinches and thin liners on COBRA

This magnetic compression of cylindrical liners filled with DT gas has promise as an efficient way to achieve fusion burn using pulsed-power machines. However, to avoid rapid cooling of the fuel by transfer of heat to the liner an axial magnetic field is required. This field has to be compressed during the implosion since the thermal insulation is more demanding as the compressed DT plasma becomes hotter and its volume smaller. This compression of the magnetic field is driven both by the imploding liner and plasma. To highlight how this magnetic field compression by the plasma and liner evolves we have separately studied Z-pinch implosions generated by gas puff and liner loads. The masses of the gas puff and liner loads were adjusted to match COBRAs current rise times. Our results have shown that Ne gas-puff implosions are well described by a snowplow model where electrical currents are predominately localized to the outer surface of the imploding plasma and the magnetic field is external to the imploding plasma. Liner implosions are dominated by the plasma ablation process on the inside surface of the liner and the electrical currents and magnetic fields are advected into the inner plasma volume; the sharp radial gradient associated with the snowplow process is not present.

The impact of Hall physics on magnetized high energy density plasma jetsa)

Hall physics is often neglected in high energy density plasma jets due to the relatively high electron density of such jets (ne ∼ 1019 cm−3). However, the vacuum region surrounding the jet has much lower densities and is dominated by Hall electric field. This electric field redirects plasma flows towards or away from the axis, depending on the radial current direction. A resulting change in the jet density has been observed experimentally. Furthermore, if an axial field is applied on the jet, the Hall effect is enhanced and ignoring it leads to serious discrepancies between experimental results and numerical simulations. By combining high currents (∼1 MA) and magnetic field helicity (15° angle) in a pulsed power generator such as COBRA, plasma jets can be magnetized with a 10 T axial field. The resulting field enhances the impact of the Hall effect by altering the density profile of current-free plasma jets and the stability of current-carrying plasma jets (e.g., Z-pinches).

Gas puff Z-pinch implosions with external Bz field on COBRA

We present preliminary experimental results on mitigating Magneto-Rayleigh-Taylor (MRT) instabilities by applying an external Bz field. The experiments were conducted on the 1-MA, 200-ns COBRA generator at Cornell University. In the experiments, a triple-nozzle was used to produce z-pinch loads from concentric outer and inner annular gas puffs and a center gas puff column. A single coil was used to produce a Bz field in the pinch region. We have used two 4-frame 2-ns gated EUV cameras to obtain images of the imploding plasmas, in which the MRT instabilities were observed. The MRT instabilities can grow when the plasma accelerates toward the axis. With a triple gas puff (outer, inner and center puff), reduced acceleration or de-acceleration of the imploding plasma occurred when the outer puff plasma imploded onto the inner annular puff plasma resulting a relatively stable implosion. In the absent of the inner annular gas puff, the imploding outer annular plasma continued to accelerate toward the axis. Large turbulent flares at the edge of the implosion or pinch plasma were observed. The implosion was not stable. To stabilize the implosion without the inner gas puff, a Bz field was applied. This external Bz field was compressed by the outer imploding plasma shell. A relatively stable implosion was observed. Increasing the Bz field to 2-kG resulted in a relatively fatter pinch plasma.

The Impact of Cathode Diameter on Radial Foil Explosions

Radial foil configurations prove to be a very simple experimental setup to study high-energy-density plasmas. A thin metallic foil lies flat over a stretcher which is connected to the anode of a pulsed-power generator. The cathode contacts the foil at its geometrical center using a hollow stainless steel pin. Force densities should increase dramatically as the pin diameter diminishes, and we expect plasma properties to change accordingly. Based only on pin diameter considerations, radial foil explosions at 1 MA could produce magnetic pressures ranging from 160 kbar (for 2-mm pins) to 2.5 Mbar (for 0.5-mm pins). However, magnetohydrodynamic instabilities limit plasma performances. For large-diameter pins (2 mm), the force density is low, but the plasma is stable until the discharge current reaches 1.1 MA. For a smaller diameter (0.5 mm), instabilities appear when the discharge is 600 kA before the current peaks. While an increase in the local plasma electron density and temperature is noticeable as the cathode size diminishes, instabilities do limit overall plasma properties and require stabilization to obtain higher pressures.

Radiative precursors driven by converging blast waves in noble gases

A detailed study of the radiative precursor that develops ahead of converging blast waves in gas-filled cylindrical liner z-pinch experiments is presented. The experiment is capable of magnetically driving 20 km/s blast waves through gases of densities of the order 1E-5 g/cc. Data was collected for Ne, Ar and Xe gas-fills. The geometry of the setup allows a determination of the plasma parameters both in the precursor and across the shock, along a nominally uniform line of sight that is perpendicular to the propagation of the shock waves. Radiation from the shock was able to excite NeI, ArII and XeII/XeIII precursor spectral features. It is shown that the combination of interferometry and optical spectroscopy data is inconsistent with upstream plasmas being in LTE. Specifically, radial electron density gradients do not correspond to any apparent temperature change in the emission spectra.

Plasma jets subject to adjustable current polarities and external magnetic fields

In the present research, collimated plasma jets form from ablation of a radial foil (Al 20 μm thin disk) using a pulsed power generator (COBRA) with 1 MA peak current and 100 ns rise time. Plasma dynamics of the jet are diagnosed with and without an applied uniform axial magnetic field (1 T) and under a change of current polarities, which correspond to current moving either radially outward or inward from the foils central axis. Experimental results are compared with numerical simulations (PERSEUS). The influence of the Hall effect on the jet development is observed under opposite current polarities. Additionally, the magnetic field compression within the jet is examined. Further studies will compare the laboratory-generated plasma jets and astrophysical jets with embedded magnetic fields.

Extended MHD plasma jets with external magnetic fields

Summary form only given. In the present research, collimated plasma jets form from Joule heating and ablation of a radial foil (Al 20 μm thin disk) using a pulsed power generator (COBRA) with 1 MA peak current and 100 ns rise time. Plasma dynamics of the jet are diagnosed with and without an applied uniform external magnetic field (~1 T axial Bz) and under a change of current polarities, which correspond to current moving either radially outward or inward from the foils central axis. Experimental results are compared with predictions made by numerical simulations (PERSEUS)1. The influence of the Hall effect on the jet development is observed under opposite current polarities. Before jet formation, initial plasma on top of the foil surface develops discrete narrow current path channels (tendrils) that display characteristics of electrothermal-filamentation plasma instability. A disruption of the tendrils and subsequent jet is noticed after a modest increase in the applied field strength from 1 to 1.5 T, which is not predicted by the 2D (r-z) simulations.

Triple nozzle gas-puff z-pinch implosions on COBRA

Summary form only given. We present investigations using a triple-nozzle, 6cm outer diameter gas puff for fast z-pinch implosions. Experiments are conducted on the 1MA, 200ns COBRA generator at Cornell University. Centimeter thickness cylindrical shells of neon/argon gas at typical densities of no ~ 1018cm-3 are imploded onto a 4cm outer diameter inner gas shell and both subsequently converge at the pinch axis. The configuration is examined with and without a central gas jet and the influence on instability growth rates is investigated. The structure of the imploding plasma sheath and Rayleigh-Taylor (RT) instabilities are imaged and quantified under different mass loading and radiative efficacy in each of the three puffs.The diagnostic suite used to characterize implosion dynamics enables measurement of density distributions before and after the arrival of current in the gas shells. Neutral gas density distributions are characterized prior to each experiment using a Planar Laser Induced Fluorescence (PLIF) system. Plasma density profiles are captured during each implosion by a three-frame Laser Shearing Interferometer (LSI) and a twoframe Laser Wavefront Analyzer (LWA). Plasma dynamics are recorded using two 4-frame, 2ns gated EUV pinhole cameras and thermodynamic properties by a multi-spherical crystal x-ray spectrometer capturing spatially resolved spectra. These measurements enable us to obtain implosion veSummary form only given. We present investigations using a triple-nozzle, 6cm outer diameter gas puff for fast z-pinch implosions. Experiments are conducted on the 1MA, 200ns COBRA generator at Cornell University. Centimeter thickness cylindrical shells of neon/argon gas at typical densities of no ~ 1018cm-3 are imploded onto a 4cm outer diameter inner gas shell and both subsequently converge at the pinch axis. The configuration is examined with and without a central gas jet and the influence on instability growth rates is investigated. The structure of the imploding plasma sheath and Rayleigh-Taylor (RT) instabilities are imaged and quantified under different mass loading and radiative efficacy in each of the three puffs.The diagnostic suite used to characterize implosion dynamics enables measurement of density distributions before and after the arrival of current in the gas shells. Neutral gas density distributions are characterized prior to each experiment using a Planar Laser Induced Fluorescence (PLIF) system. Plasma density profiles are captured during each implosion by a three-frame Laser Shearing Interferometer (LSI) and a twoframe Laser Wavefront Analyzer (LWA). Plasma dynamics are recorded using two 4-frame, 2ns gated EUV pinhole cameras and thermodynamic properties by a multi-spherical crystal x-ray spectrometer capturing spatially resolved spectra. These measurements enable us to obtain implosion velocity, mean ion charge states and plasma temperature at various positions and implosion times.locity, mean ion charge states and plasma temperature at various positions and implosion times.