J. P. Quintenz

Experiments on a current‐toggled plasma‐opening switch

Plasma‐opening switches have been used to improve pulsed‐power wave shapes for over a decade. These switches have used the inertia of the plasma to hold the switch closed. This results in conflicting requirements when long hold‐off time and fast opening are required, and also results in variation in opening current due to variation in initial plasma fill. The current‐toggled plasma‐opening switch attempts to overcome these problems by using external magnetic fields rather than inertia to control the plasma conductor. Data will be presented showing several features of the operation of this switch. These data will be compared to models used to design the switch. The comparisons indicate that the mass can be measured approximately from fast coil data and that the slow coil flux does set the opening level of the current. They also indicate that the opening current is somewhat dependent upon plasma mass, and that the design of the field coils that provide the control fields must be done more carefully to provi...

Time-resolved proton focus of a high-power ion diode

An improved understanding of the factors that control the axial focus of applied‐B ion diodes was obtained from time‐resolved diagnostics of ion‐beam trajectories. This resulted in a new selection of anode shape that produced a proton focus of 1.3‐mm diameter from a 4.5‐cm‐radius diode, which is a factor of 2 improvement over previous results. We have achieved a peak proton power density of 1.5±0.2 TW/cm2 on the 1‐TW Proto I accelerator. The radial convergence of this proton beam, defined as the ratio of the anode diameter to focused beam FWHM, is 70. Time‐resolved information about virtual cathode evolution, the self‐ and applied‐magnetic‐field bending, and the horizontal focus of the beam was also obtained. In addition, the diffusion of the magnetic field into the anode plasma is estimated by measuring the horizontal focal position as a function of time. Finally, we discuss the effects of gas cell scattering on the beam focus.

Electron and ion kinetics and anode plasma formation in two applied Br field ion diodes

Two magnetically insulated ion diodes that utilize a radial applied‐B field are described. Both diodes generate an annular beam that is extracted along the diode axis. The first diode operated at 1.2 MV and 600 kA for 25 ns and generated a 300‐kA ion beam. The second operated at 300 kV, 100 kA and generated 15 kA of ion current. The first diode was used to study diode performance as a function of inner and outer anode‐cathode gaps, the applied‐B field, and transmission line current ratios. The second diode was used to study anode plasma formation. The diodes were operated below Bcrit, resulting in electron leakage to the anode, especially near the outer cathode. A definition of Bcrit applicable to extraction diodes is given and methods of improving ion production efficiency in these diodes are suggested. The strong correlation of ion production with visible light emission suggests, however, that the electron loss played an important role in anode turn‐on. The breakdown of neutral gas desorbed by electron ...

Simulation codes for light-ion diode modeling

The computational tools used in the investigation of light-ion diode physics at Sandia National Laboratories are described. Applied-B ion diodes are used to generate intense beams of ions and focus these beams onto targets as part of Sandias inertial confinement fusion program. Computer codes are used to simulate the energy storage and pulse forming sections of the accelerator and the power flow and coupling into the diode where the ion beam is generated. Other codes are used to calculate the applied magnetic field diffusion in the diode region, the electromagnetic fluctuations in the anode-cathode gap, the subsequent beam divergence, the beam propagation, and response of various beam diagnostics. These codes are described and some typical results are shown.

Development Path for Z-Pinch IFE

Abstract The long-range goal of the Z-Pinch IFE program is to produce an economically-attractive power plant using high-yield z-pinch-driven targets (~3GJ) with low rep-rate per chamber (~0.1 Hz). The present mainline choice for a Z-Pinch IFE power plant uses an LTD (Linear Transformer Driver) repetitive pulsed power driver, a Recyclable Transmission Line (RTL), a dynamic hohlraum z-pinch-driven target, and a thick-liquid wall chamber. The RTL connects the pulsed power driver directly to the z-pinch-driven target, and is made from frozen coolant or a material that is easily separable from the coolant (such as carbon steel). The RTL is destroyed by the fusion explosion, but the RTL materials are recycled, and a new RTL is inserted on each shot. A development path for Z-Pinch IFE has been created that complements and leverages the NNSA DP ICF program. Funding by a U.S. Congressional initiative of

Applied‐B field ion diode studies at 3.5 TW

4M for FY04 through NNSA DP is supporting assessment and initial research on (1) RTLs, (2) repetitive pulsed power drivers, (3) shock mitigation [because of the high yield targets], (4) planning for a proof-of-principle full RTL cycle demonstration [with a 1 MA, 1 MV, 100 ns, 0.1 Hz driver], (5) IFE target studies for multi-GJ yield targets, and (6) z-pinch IFE power plant engineering and technology development. Initial results from all areas of this research are discussed.

Self‐magnetically insulated ion diode

The performance of 7 and 14 cm radius, applied B‐field, focusing ion diodes is investigated on the Proto II accelerator at a diode power of 3.5 TW. The ion production efficiency of the 14‐cm diode is determined to be approximately 80% for the 30‐ns power pulse used. When a 2 Torr air background gas was used in the beam propagation region between the diode and the target, the ion beam was observed to be approximately 70% magnetically neutralized in regions of approximately 20 kG transverse B‐field and ≥95% in regions of no B‐field. The local and global divergence of the 1.4‐MV beam were each determined to be approximately 1°. Local source power brightness of >1.5×1013 W/cm2/rad2 is inferred.

Anode plasma behavior in a magnetically insulated ion diode

Light ion diodes for producing 1–100 TW ion beams are required for inertial confinement fusion. The theory, numerical simulations, and experiments on a self‐magnetically insulated ion diode are presented. The treatment is from the point of view of a self‐magnetically insulated transmission line with an ion loss current and differs from the usual treatment of the pinched electron beam diode. The simulations show that the ratio V/IZ0=0.25 in such a structure with voltage V, local total current I, and local vacuum wave impedance Z0. The ion current density is enhanced by a factor of approximately 2 over the simple space‐charge limited value. The simulation results are verified in an experiment. An analytical theory is then presented for scaling the results to produce a focused beam of protons with a power of up to 1013 W.

Nonuniform mesh diode simulation code

The nature and time evolution of the ’’surface flashover’’ anode plasma in a magnetically insulated ion diode is studied. Holographic interferometric and spectrographic measurements indicate a plasma with an electron density of 5×1016 cm−3 and a temperature of approximately 5 eV which is created by electric breakdown along a surface parallel to the imposed pulsed electric field. The divergence of the ion beam accelerated from this plasma is governed by the spatial nonuniformities of the plasma. The beam is composed primarily of protons for the experiments studied. Contrary to previous expectations, a substantial C+4 beam component was not observed.

Three‐dimensional particle‐in‐cell simulations of applied‐B ion diodes

A new version of a diode simulation code has been written which allows for nonuniform zoning in both the r and z directions. This new flexibility enables more accurate treatment of crucial areas in the diode such as near emission surfaces and in target regions. The new code also has the ability to treat slanted surfaces such as that found in a tapered cathode diode. An interesting result of the new code is that a ’’parapotential’’ cathode pinches better than the corresponding flat cathode at the same total current.