Microwave oscillation has been measured for the first time in a 12-cavity axial-magnetic-field recirculating planar magnetron, designed to operate in π mode at 1 GHz. The device operates with a -300-kV pulsed cathode voltage and a 0.2-T axial magnetic field, and oscillates at transverse currents exceeding 1 kA when driven by an electron beam pulselength between 0.5 and 1 μs. Microwave pulses were measured at frequencies between 0.97-1 GHz and achieved several hundred nanoseconds in length. Mode competition was observed between the π and 5 π/6 modes.

Experiments to study the ablation dynamics of coiled wire arrays were performed on the MAGPIE generator (1 MA, 240 ns) at Imperial College, and on the COBRA generator at Cornell University's Laboratory of Plasma Studies (1 MA, 100 ns). The MAGPIE generator was used to drive coiled wires in an inverse array configuration to study the distribution of ablated plasma. Using interferometry to study the plasma distribution during the ablation phase, absolute quantitative measurements of electron line density demonstrated very high density contrasts between coiled ablation streams and inter-stream regions many millimetres from the wire. The measured density contrasts for a coiled array were many times greater than that observed for a conventional array with straight wires, indicating that a much greater axial modulation of the ablated plasma may be responsible for the unique implosion dynamics of coiled arrays. Experiments on the COBRA generator were used to study the complex redirection of plasma around a coiled wire that gives rise to the ablation structure exhibited by coiled arrays. Observations of this complex 3D plasma structure were used to validate the current model of coiled array ablation dynamics [Hall et al., Phys. Rev. Lett. 100, 065003 (2008)], demonstrating irrefutably that plasma flow from the wires behaves as predicted. Coiled wires were observed to ablate and implode in the same manner on both machines, indicating that current rise time should not be an issue for the scaling of coiled arrays to larger machines with fast current rise times.

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.

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 μm Mo wire. During initial tests a 12.5 μm 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. Currently, the LTD drives a 400 nm Al foil with +/- 70 kV at 600 kA. Magnetic pressure causes the foil to accelerate, which results in the magneto-Rayleigh-Taylor (MRT) instability. Laser shadowgraphy has been used to image the foil and determine the growth rate, and the x-pinch is expected to be used as a backlighter with the goal of x-ray probing the foil plasma at higher densities.

Experiments have been performed on a nominal 100 ns rise time, MegaAmpere (MA)-class linear transformer driver to explore the magneto-Rayleigh-Taylor (MRT) instability in planar geometry. Plasma loads consisted of ablated 400 nm-thick, 1 cm-wide aluminum foils located between two parallel-plate return-current electrodes. Plasma acceleration was adjusted by offsetting the position of the foil (cathode) between the anode plates. Diagnostics included double-pulse, sub-ns laser shadowgraphy, and machine current B-dot loops. Experimental growth rates for MRT on both sides of the ablated aluminum plasma slab were comparable for centered-foils. The MRT growth rate was fastest (98 ns e-folding time) for the foil-offset case where there was a larger magnetic field to accelerate the plasma. Other cases showed slower growth rates with e-folding times of about ∼106 ns. An interpretation of the experimental data in terms of an analytic MRT model is attempted.

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.

The Recirculating Planar Magnetron (RPM) [1] is a crossed field device that combines the advantages of high-efficiency recirculating devices with those of planar devices: both large area cathode (high current) and anode (improved thermal management). Two embodiments of the RPM are modeled a under design: 1) Axial magnetic field with radial electric field (experiments underway), and 2) Radial magnetic field and axial electric field.

Summary form only given. This paper concentrates on the effects of cylindrical geometry on the magneto-Rayleigh-Taylor instability (MRT), a major concern in the magnetized liner inertial fusion concept (MagLIF) [1]. Several issues are being studied, such as the Bell-Plesset effect [2], the effects of magnetic shear and feedthrough [3], and the nonzero MRT growth rate that remains (but was hardly noticed) in the k = m = 0 limit in Harris' seminal paper on a cylindrical liner [4], where k and m are respectively the azimuthal and axial wavenumber. In liner implosions, such as those for MagLIF, significant magnetic shear can be present and feedthrough of MRT during compression can introduce unwanted fuel mix. We shall use recent simulation and experimental results [5] to compare with direct integration of the eigenvalue equation to investigate the importance of the cylindrical geometry in liner implosions. These analytic results may provide a parametric design in optimizing such parameters as the liner aspect ratio and magnetic field configuration.

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.

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.

Summary form only given. 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. This work was conducted on 400-nm thick, 1-cm wide aluminum foils placed between two planar or pseudo-planar current return plates. The driver charge was limited to ±70 kV, giving ~700 kA with a risetime of ~150 ns. This reduced charge voltage improved machine and hardware lifetime. Experiments were performed employing various methods to seed the MRT instability on either the foil (cathode) or return current plates (anode). Cathode seeding was performed by imposing a periodic ripple in the foil. Anode seeding was performed by using electrodes with a periodic structure machined into them. The progress of these experiments is presented here. Analysis of MRT was derived from laser shadowgraphic images, obtained using a sub-ns, frequency doubled Nd: YAG laser.

Summary form only given. 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. The density of the edge plasma can be determined through Stark broadening of the H-alpha line. The Fourier transform was taken of the Voigt profile, which was then used to approximate the density of the Al plasma. This method resulted in a density of approximately 1015 cm-3 in the outer regions of the Al plasma at peak current. Additionally, spectra taken during the current rise yielded 1-3-eV plasma temperatures from the slope of the continuum emission. These data will be shown as well as planned future experiments.

The presented research focuses on investigation of Z-pinch plasma formation, implosion, and radiation characteristics as a function of the load configuration. The single planar and multi-planar wire arrays as well as compact cylindrical wire arrays were studied on the 1.3 MA UNR Zebra and I MA Cornell COBRA generators. The largest yields and powers were found for W and Mo double planar and compact wire arrays. A possibility of radiation pulse shaping was demonstrated. Two types of bright spots were observed in plasmas. A comparison of Mo double planar and compact wire array data indicates the possibility that the same heating mechanism operates during the final implosion and stagnation stages.

Summary form only given. We are investigating the development of the axial instability that occurs on each exploding wire in wire-array Z-pinches. The axial instability is a growing modulation of the size of the coronal plasma around individual wires of the array that results in non-uniform ablation of material from the cold wire core. It has long been known that the wavelength of this modulation is constant late in time and, since it is unique to different materials, it has come to be known as the fundamental mode. In these experiments we have been imaging individual wires with laser shadowgraphy and an XUV framing camera primarily in low wire number, large wire diameter aluminum array. We show also some results from various other arrays for comparison. We document the development of this modulation from the beginning of plasma formation and show its dominant wavelength and amplitude growth as a function of time. The magnetic field topology is also probed using small B-dot probes inside the array. The change from a closed to an open field topology is correlated with the instability growth. Growth of the instability is seen to slow and eventually stop as the magnetic field 1-2 mm inside a wire decreases and changes sign, signifying the change in topology from locally to globally dominated field. Magnetic probe measurements of the field advected inside the array during the ablation phase are used to calculate the amount of current that is associated with the precursor plasma before implosion. With aluminum arrays, the basic scaling is that the rate of field advection towards the axis is roughly proportional to the field outside the array. Thus the precursor current is proportional to driving current and inversely proportional to array radius, and insensitive to wire number, at least for wire spacing of ~1 mm or greater. Further results with aluminum and tungsten arrays and closer wire spacings will be presented.

The results of experiments with combined aluminum (Al) and stainless steel (SS) alloy 304, nested wire arrays from the 1 MA COBRA generator at Cornell University are presented. The loads studied consisted of a 6 mm diameter inner array and a 13 mm diameter outer array with a different material in each array: SS or aluminum. Al implodes before SS in all loads studied, even when Al was on the inner array. The new wire ablation dynamic model and spectroscopic modeling are used to interpret these data. The observed implosion dynamics are likely a result of the higher ablation rate of Al. These initial results suggest that combining wire materials with different ablation rates in wire array loads could be developed into a useful technique for x-ray pulse shaping and radiation yield optimization.

The experimental results described in this paper were motivated by earlier, low current, single wire experiments. In these experiments, single 10–25 μm diameter wires were driven by 1–5 kA current pulses with variable dI/dt from 5 to 60 A/ns. The amount of energy deposited in the wires, the expansion rate, and expansion uniformity that occurred before a plasma induced voltage collapse were found to depend on the polarity, dI/dt, and the quality of the contacts between the wires and the electrodes. This paper reports the results of experiments with cylindrical wire arrays driven by Cornell Beam Research Accelerator (COBRA) [J. B. Greenly, J. D. Douglas, D. A. Hammer et al., Rev. Sci. Instrum. 79, 073501 (2008)] current pulses that reached 1 MA. The pulse lengths were varied from 100 to 200 ns. These larger current pulses drove the wires of the array through the initiation phase studied in the single wire experiments and through ablation and Z-pinch implosion to stagnation on the cylindrical axis of the array. Regardless of the current pulse length, the COBRA dI/dt per wire during initiation reached approximately 175 A/ns and resistive voltage breakdown occurred at ∼13 ns. Wire-electrode contacts were modified by soldering the cathode ends of the wires to the brass electrode. With the 100 ns COBRA pulse, voltage monitor data suggested that soldering produced a smaller radius pinch, but bolometer data showed that this did not affect the total energy emitted from the array compared to nonsoldered contacts. With the 200 ns COBRA pulse and soldered contacts, the bolometer data showed an average of 69% increase in time integrated x-ray emission and the photoconducting detector data showed an increase in x-ray power and yield compared with nonsoldered contacts. Under these same conditions the four-frame extreme ultraviolet images showed a more pronounced “Christmas tree” effect at the cathode.