D.A. Chalenski

Effects of magnetic shear on magneto-Rayleigh-Taylor instability

The magnetized liner inertial fusion concept [S. A. Slutz et al., Phys. Plasmas 17, 056303 (2010)] consists of a cylindrical metal liner enclosing a preheated plasma that is embedded in an axial magnetic field. Because of its diffusion into the liner, the pulsed azimuthal magnetic field may exhibit a strong magnetic shear within the liner, offering the interesting possibility of shear stabilization of the magneto-Rayleigh-Taylor (MRT) instability. Here, we use the ideal MHD model to study this effect of magnetic shear in a finite slab. It is found that magnetic shear reduces the MRT growth rate in general. The feedthrough factor is virtually independent of magnetic shear. In the limit of infinite magnetic shear, all MRT modes are stable if buu2009>u20091, where bu is the ratio of the perturbed magnetic tension in the liner’s interior region to the acceleration during implosion.

Recirculating-Planar-Magnetron Simulations and Experiment

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.

Magneto-Rayleigh-Taylor experiments on a MegaAmpere linear transformer driver

Experiments have been performed on a nominal 100u2009ns rise time, MegaAmpere (MA)-class linear transformer driver to explore the magneto-Rayleigh-Taylor (MRT) instability in planar geometry. Plasma loads consisted of ablated 400u2009nm-thick, 1u2009cm-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 (98u2009ns 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 ∼106u2009ns. An interpretation of the experimental data in terms of an analytic MRT model is attempted.

Microwave oscillation in a recirculating planar magnetron

Summary form only given. 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.

Seeded Magneto-Rayleigh-Taylor instability experiments on A 1-MA LTD

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.

Foil MRT and X-pinch experiments on a MA linear transformer driver

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).

Microwave oscillation, mode control and extraction in the recirculating planar magnetron

Summary form only given. The recirculating planar magnetron (RPM) is a crossedfield 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). Preliminary experiments using the RPM-12a, the first L-band prototype, have successfully produced high power microwave pulses 50-300 ns in length at approximately 1 GHz . The device is driven using the Michigan Electron Long Beam Accelerator with Ceramic insulator (MELBA-C) which delivers a pulsed cathode bias of -300 kV for durations of 0.3-1.0 μs. The RPM is capable of generating 2-10 kA and is typically operated with an externally applied axial magnetic field between 0.18-0.2 T. Recent experimental work has focused on enhancing mode separation and cross-oscillator coupling using the mode control cathode (MCC). The MCC is a periodically slotted emission structure that provides RF feedback between oscillators and may be geometrically manipulated to improve mode separation independently from the anode structure. Future studies will continue to focus on phase locking using the MCC as well as a guided extraction mechanism for the planar slow wave structures. Power extraction is to be accomplished using on an adaptation of the “all cavity extractor” (Greenwood et al.) that uses a coaxial transmission line to axially extract RF power in the TEM mode. An experimental D-band RPM prototype using the coaxial all cavity extractor system is under design and construction; operation is anticipated for the summer of 2013.

Development of a compact LTD pulse generator for X-ray backlighting of planar foil ablation experiments

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 on the UM 1-MA linear transformer driver

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.

Design and preliminary results of a recyclable transmission line testing experiment

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.