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The RITS-6 inductive voltage adder (IVA) accelerator (3.5–8.5 MeV) at Sandia National Laboratories produces highpower (TW) focused electron beams (< 3mm diameter) for flash x-ray radiography applications. The Self-Magnetic Pinch (SMP) diode utilizes a hollowed metal cathode to produce a pinched focus onto a high-Z metal anode converter. There is not a clear understanding as to the effects various contaminants such as: C, CO, H, H2O, HmCn, O2, and N2, on the anode surface or in the bulk, may have on impedance dynamics, beam stability, beam spot size, and reproducibility.

The electrothermal instability (ETI) arises whenever a current-carrying material has a resistivity that depends on temperature. When resistivity, η, increases with increasing temperature, ETI causes striations to form perpendicular to the direction of current. On pulsed-power-driven, ablating metallic loads, this process can cause sections of the target to ablate earlier than the bulk material, creating a macroscopic surface perturbation on the plasma-vacuum interface. Experiments are underway on the MAIZE 1-MA linear transformer driver at the University of Michigan to study surface perturbations produced by ETI as seeding for the Rayleigh-Taylor (MRT) instability on imploding liner [1] and accelerating foil plasmas [2]. Target foils are fabricated at the Lurie Nanofabrication Facility at UM by depositing ultrathin (200 to 500 nm) coatings of aluminum or titanium on 1.5 μm Chemplex Ultra-Polyester films. Foil thicknesses are chosen to maintain the same mass between shots, and the materials are chosen to provide substantially different values of dη/dt, which impacts the growth rate of the electrothermal instability. Targets are ablated and accelerated by driving a current of 500 to 600 kA on MAIZE, and the accelerated plasmas are imaged using a 12-frame laser imaging system. Images of these plasmas are compared to determine if initial plasma interface perturbations are measurably different on targets of different materials, with the same mass, but different ETI growth rates.

Experiments are underway on the 1-MA, 100-kV MAIZE linear transformer driver (LTD) z-pinch experiment to explore the characterization and stabilization of the magneto-Rayleigh-Taylor (MRT) and electrothermal instabilities (ETI)1. Instability experiments at UM utilize 400 nm thick, planar Al foil loads and Al-backed Mylar foils. Theory has shown that axial magnetic fields and magnetic shear can reduce the growth rate of MRT2. To test effects of axial magnetic fields, pseudo-helical return current electrodes are substituted in place of planar electrodes. Instability growth is measured by sub-ps laser shadowgraphy viewing across the foil plasma, perpendicular to the current.

Experiments are underway to study the effects an axial magnetic field on the magneto Rayleigh-Taylor instability (MRT) in ablating planar foils on the 1-MA LTD at the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) facility at the University of Michigan. In planar foil ablation experiments at UM, MRT is observed when the expanding plasma-vacuum interface decelerates as the magnetic pressure exceeds the plasma pressure during the drive current1. Theoretical investigation at UM has shown that an axial magnetic field along with magnetic shear may reduce the MRT growth rate in general2. To test this experimentally, axial magnetic fields are generated using helical return current posts. The axial field is proportional to the drive current and peaks at 13 T for 600 kA peak current. A 775 nm Ti:sapphire laser is used to shadowgraph the foil in order to study the MRT instability. Results indicate improved confinement in addition to significant anisotropy on the left and right sides of the foil when compared to experiments at UM using planar return current posts with no axial field. Recent work utilizes a new load configuration where return current plates run perpendicular to the foil current, producing an axial field that can be adjusted based on the proximity of the plates to the foil.

Wire z-pinch experiments at the University of Michigan are in progress to test potential spectral lines for use as a local magnetic field diagnostic in dense plasmas driven by high currents. The goal of these experiments is to determine several spectral lines that can be used to reliably characterize the magnetic field and current density profiles of high energy density plasmas. Initial feasibility experiments were performed on a compact pulser, in which 50–60 kA currents were conducted in a single wire with 400 ns risetime. A lower-inductance, higher-current pulser is currently under construction. An optical fiber collected visible light emission from the wire ablation plasma for measurement by a timegated ICCD coupled to a 0.75-m optical spectrograph. Spectra have been collected for several plasmas including W, Mo, Na, and Al. Magnetic fields of 5–6 T have been measured using Zeeman splitting.

Summary form only given. X-pinch experiments are underway on the MAIZE Linear Transformer Driver (LTD) at the University of Michigan. The MAIZE LTD can supply 1 MA, 100 kV pulses with 100 ns risetime into a matched load. The x-pinch consists of a single 35-50 μm Al or Mo wire separated by conical electrodes, between two current return plates. The LTD is charged to +/-70 kV resulting in approximately 0.4-0.5 MA through the wire. Initial tests show multiple x-ray bursts over the length of the current pulse.The x-pinch will ultimately backlight the Magneto RayleighTaylor (MRT) instability on a planar Al foil. The foil load contains a 1 cm wide, 400 nm thick foil placed between two current return plates. Ongoing MRT experiments involve seeding the MRT instability with arrays of 30 micron holes micromachined in the foil by a 150 fs Ti:sapphire laser. Laser shadowgraphy has previously been used to image the seeded foil as well as determine the MRT growth rate.[1,2] Future plans for the x-pinch include placing it in parallel with the foil in order to more accurately image and characterize the MRT instability. Plans for a smaller 100-150 kA compact pinch driver are also in development; (see poster by YagerEliorraga at this conference).

Summary form only given. A 70kV, 100kA compact pulse generator (0.7m × 0.9m × 0.3m) has been constructed and successfully tested with a resistive load using a linear LTD-type capacitor-switch configuration. The generator consists of 6 bricks connected in parallel, where each brick contains two oppositely charged capacitors (+/-70kV, 40nF) and a low inductance L-3 spark-gap switch (93nH). The bricks are connected to the load through a parallel plate transmission line. The generator is designed to drive a hybrid x-pinch to serve as a diagnostic for planar foil ablation experiments on the 1-MA LTD at the Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) facility.[1,2] The hybrid x-pinch diagnostic consists of a 35-50μm Al or Mo wire between two conical tungsten electrodes and will be used as a backlighter in addition to the current 775nm Ti:sapphire laser. The construction of the hybrid x-pinch chamber and transmission line is currently underway. In addition, the generator may be used to create external magnetic fields for magneto Rayleigh-Taylor (MRT) experiments on the 1-MA LTD. Preliminary results of generator characterization will be presented.

Experimental, theoretical and simulation research investigations are underway on the Magneto Rayleigh-Taylor instability driven by the Mega Ampere Linear Transformer Driver at the University of Michigan. Since the Linear Transformer Driver operates at 100- kV output, inductance minimization was crucial in design of the coaxial and radial magnetically insulated transmission line that transmits power to the load. Experiments ablate a 400 nm-thick, 1 cm wide, planar, aluminum foil located between two parallel-plate anodes. The initial position of the foil relative to the anodes controls the foil-plasma acceleration. Laser-micromachined, periodic hole patterns on foils are utilized to seed the wavelength of Magneto Rayleigh-Taylor growth. Sub-ns laser shadowgraphy diagnoses the instability growth at the edges of the ablation plasma. Early instability is believed to originate from the Electro-Thermal instability. Later exponential growth rates have been measured whose trends are consistent with Magneto Rayleigh Taylor theory. As expected, the fastest Magneto Rayleigh-Taylor growth rate corresponds to the largest foil-plasma acceleration. Effects of magnetic shear on Magneto Rayleigh-Taylor growth have been predicted theoretically.

Summary form only given. X-pinch experiments are currently underway on the Linear Transformer Driver (LTD) at the University of Michigan. The MAIZE LTD can supply 1 MA, 100 kV pulses with 100 ns risetime into a matched load. The x-pinch consists of a single wire separated by conical electrodes1, between two current return plates. The LTD was charged to +/−70 kV resulting in approximately 0.5 MA passing through a 50 µ Mo wire. During initial tests a 12.5 µ Ti filter was placed in front of the film to screen out visible emission from the wire. Laser shadowgraphy is also used to diagnose the x-pinch plasma.

Summary form only given. Recyclable transmission lines (RTL) have recently been of interest to the inertial confinement fusion and pulsed power community as a means to increase repetition rate and decrease cost per shot in Z-pinch driven inertial confinement fusion devices [1–3]. The ability to remove surface contaminants from the surface of RTLs is important to their successful operation. These contaminants, which consist of residual atmospheric gases and hydrocarbons, physically and chemically adsorb to the transmission line surfaces. Some contaminants have sufficient binding energies such that they are not desorbed even in vacuums as high as 10−6 Pa at room temperature. When a pulse is initiated, remaining contaminants are rapidly emitted through joule heating and stimulated desorption, causing local pressures to increase as high as 103 Pa [4]. These areas of local high pressure support plasma formation, which leads to breakdown and loss of power delivery capability in the transmission line. In order to satisfy the RTL concept, conditioning of the transmission lines to remove contamination prior to shot must be done quickly and in situ. A new magnetically insulated transmission line (MITL) with repetitive pulse capability is being designed and installed on the 1-MA linear transformer driver at the University of Michigan to evaluate in situ conditioning methods. This test-bed will evaluate the effect of multiple “conditioning pulses” on contaminant inventory and ability to improve MITL power flow. Preliminary findings will be presented.

Summary form only given. Recent research on the 1-MA Michigan Linear Transformer Driver, MAIZE, has focused on the Magneto Rayleigh-Taylor (MRT) instability and validation of analytic theory, developed at UM [1,2]. MRT is a concern to all forms of magnetically imploding experiments, most recently with the imploding liners anticipated in the MagLIF geometry.[3] Eliminating or mitigating MRT is crucial to success of these programs.

Recent work on the 1-MA Michigan Linear Transformer Driver, MAIZE, has focused on the Magneto Rayleigh-Taylor (MRT) instability and validation of analytic theory, developed at UM (see abstract by Lau et al.). MAIZE is a nominal 1-MA, 100 ns, 100 kV driver, capable of driving 0.1 Ω matched loads. We present here the results of a series of shots using different techniques to seed the MRT instability on planar or pseudo-planar foils. The planar geometry is unique in that it eliminates the complication of the m=0 and m=1 instabilities, allowing extricated analysis of the MRT instability.

Linear Transformer Drivers (LTDs) represent the most compact, high-current accelerators. LTDs have numerous advantages over Marx PFL systems: a) fast risetime (100 ns) without additional pulse forming, b) high efficiency (∼70%), c) inductive voltage adder and d) repetitively pulsed operation. LTDs have been utilized in a patented Sandia design for a PW pulsed power driver for fusion.[1] MYKONOS, a 1-MV, 1-MA LTD system is being constructed at Sandia. The 1-MA, 100 kV LTD at UM was designed and developed at the IHCE under Sandia sponsorship. Initial tests were performed at IHCE in an inductive-adder with 5 LTD modules at 1-MA and 0.5 MV. At UM, the LTD was coupled to a Magnetically Insulated Transmission Line (MITL) to drive a load in the MAIZE z-pinch facility. Resistive load tests agreed well with PSPICE simulations. Modifications have been made to the UM LTD to improve its serviceability in an experimental system. Magneto-Rayleigh-Taylor plasma instability experiments have been performed on the UM LTD facility in a low-inductance, planar-foil plasma load, located between two planar anode plates.[2] Laser shadowgraphy was performed by a 100-sps doubled-YAG laser. Instability growth rate have been compared to MRT theory (Lau et al., this conference). PSPICE simulations of LTD component voltages and currents have been performed, accounting for calculated foil motion. UM LTD convolute experiments were performed.[3]

Spectroscopic analysis has been performed on Al foil plasmas ablated by the Linear Transformer Driver (LTD) at the University of Michigan. The MAIZE LTD can supply 1-MA, 100 kV pulses with 100 ns risetime into a matched load. The plasma load used in this experiment consists of a 400 nm Al foil (cathode) placed between two, planar, current return anode posts. The LTD was charged to +−70 kV, resulting in approximately 0.65 MA with a 170 ns risetime passing through the foil. An optical fiber was placed about 1 cm away from the load; plasma light passed through a 0.75-m optical spectrograph and was gated for 10 ns by an intensified CCD detector.

Initial dynamic load experiments were performed on UM's 1-MA linear transformer driver (LTD) facility, MAIZE, to characterize magneto-Rayleigh-Taylor (MRT) instability growth and plasma dynamics on planar-foil plasmas. The MAIZE LTD is capable of delivering a 1-MA, < 100 ns risetime drive pulse into a 0.1 Ω matched load with a ±100 kV charge. For these dynamic load experiments the LTD was charged to ±70 kV to deliver up to 0.7 MA with a 170 ns risetime into the foil load.

Foils may soon become necessary to achieve the required mass for higher current-driven x-ray sources. They may also offer useful options for x-ray pulse shaping and as imploding liners for magnetized target fusion. This paper reports our latest design and experimental progress on the dominant instability, the magneto-Rayleigh-Taylor instability (MRT). Planar Al foils as thin as 400 nm driven by the 1-MA linear transformer driver (LTD), MAIZE, at the University of Michigan, are used as the dynamic loads for this investigation. Sub-nanosecond laser backlighting and spectroscopy diagnostic deployment, transmission line and load hardware, inductance considerations, and experimental progress will be presented along with MRT theory.

Summary form only given. Post-hole convolutes are used to combine the current from several magnetically insulated transmission lines into one anode-cathode gap. The post-hole convolute geometry produces a complex magnetic field, which can contain several magnetic nulls. Electron flow in the post-hole region can cross the anode-cathode gap, which can produce plasma and lead to further current losses.A streaked spectroscopy diagnostic has recently been set up to observe plasma emission at Sandia's Z Machine. The diagnostic is fiber-optically coupled to the experimental hardware, allowing the diagnostic to measure light in the visible spectrum (~ 400 nm to ~ 700 nm). Hardware has been designed to observe the light emitted by the plasma forming in the post hole region. Experimental shots are planned for late winter / early spring. Single post-hole convolute experiments on the 1 MA Michigan Accelerator for Inductive Z-pinch Experiments (MAIZE) are also being conducted. These experiments utilize a static load, which allows 10's of shots per hour. The current flowing into and out of the convolute is monitored in order to determine the current lost in the post hole region. Visible spectroscopy of the plasma formed will be used to infer a temperature. Different components of the convolute are made from different materials allowing the plasma source to be observed spectroscopically Laser imaging will be used to determine the electron density and closure velocity of the plasma. MAGIC PIC 3D simulations of the post-hole convolute fielded on MAIZE predicted plasma formation at a magnetic null in the convolute. Light emission was observed experimentally from the predicted region using an open pinhole camera. Results from experiments as well as simulations will be presented.