S. Sampayan

Emission from ferroelectric cathodes

We have recently initiated an investigation of electron emission from ferroelectric cathodes. Our experimental apparatus consisted of an electron diode and a 250 kV, 12 Ω, 70 ns pulsed high voltage power source. A planar triode modulator driven by a synthesized waveform generator initiates the polarization inversion and allows inversion pulse tailoring. The pulsed high voltage power source is capable of delivering two high voltage pulses within 50 μs of each other and is capable of operating at a sustained repetition rate of 5 Hz. Our initial measurements indicate that emission current densities above the Child-Langmuir space charge limit, JCL, are possible. We explain this effect to be based on a non-zero initial energy of the emitted electrons. We also determined that this effect is strongly coupled to relative timing between the inversion pulse and application of the main anode-cathode pulse. We also have initiated brightness measurements of the emitted beam and estimate a preliminary lower bound to be on the order of 109 A/m2rad2. As in our previous measurements at this Laboratory, we performed the measurement using a pepper pot technique. Beamlet profiles are recorded with a fast phosphor and gated cameras. We describe our apparatus and preliminary measurements.

High gradient induction accelerator

A new type of compact induction accelerator is under development at the Lawrence Livermore National Laboratory that promises to increase the average accelerating gradient by at least an order of magnitude over that of existing induction machines. The machine is based on the use of high gradient vacuum insulators, advanced dielectric materials and switches and is stimulated by the desire for compact flash X-ray radiography sources. Research describing an extreme variant of this technology aimed at proton therapy for cancer will be described. Progress in applying this technology to several applications will be reviewed.

Multilayer high gradient insulator technology

We are investigating a novel insulator concept that involves the use of alternating layers of conductors and insulators with periods on the order of 4/spl times/ higher breakdown electric field strength) than conventional insulators in long pulse, short pulse, and alternating polarity applications. A previously defined scaling law of d/sup 0:5/, where d is the insulator length, appears to apply to these new structures when the scaling parameter d is replaced by the layer period d/sub 1/. This observation implies that each layer within the structure behaves independently in the breakdown process. We present our ongoing studies investigating the degradation of the breakdown electric field strength resulting from surface roughness, the effect of gas pressure, and the performance of the insulator structure under bipolar stress. Further, we present our initial work on scaling and modeling studies.

High gradient insulator technology for the dielectric wall accelerator

Insulators composed of finely spaced alternating layers of dielectric and metal are thought to minimize secondary emission avalanche (SEA) growth. Most data to date was taken with small samples (order 10 cm/sup 2/ area) in the absence of an ion or electron beam. We have begun long pulse (>1 /spl mu/s) high voltage testing of small hard seal samples. Further, we have performed short pulse (20 ns) high voltage testing of moderate scale bonded samples (order 100 cm/sup 2/ area) in the presence of a 1 kA electron beam. Results thus far indicate a 1.0 to 4.0 increase in the breakdown electric field stress is possible with this technology.

Optically induced surface flashover switching for the dielectric wall accelerator

Fast, low jitter command triggered switching is key to the successful implementation of the dielectric wall accelerator (DWA). We are studying a UV induced vacuum surface flashover switch for this purpose. We present our initial data using a Nd:YAG (/spl lambda/=0.06 nm) laser incident onto a high gradient insulator surface at 1/spl omega/, 2/spl omega/, 3/spl omega/, and 4/spl omega/. Best 1/spl sigma/ jitter was <1 ns with no degradation of the switch after 500 shots.

Development of a Compact Radiography Accelerator Using Dielectric Wall Accelerator Technology

We are developing an inexpensive compact accelerator system primarily intended for pulsed radiography. Design characteristics are an 8 MeV endpoint energy, 2 kA beam current, a cell gradient of approximately 3 MV/m (for an overall accelerator length is 2-3 m), and <

Multilayer High-Gradient Insulators

1/Volt capital costs. Such designs have been made possible with the development of high specific energy dielectrics (>10J/cm3), specialized transmission line designs and multi-gap laser triggered low jitter (<1 ns) gas switches. In this geometry, the pulse forming lines, switches, and insulator/beam pipe are fully integrated within each cell to form a compact, stand-alone, stackable unit. We detail our research and modeling to date, recent high voltage test results, and the integration concept of the cells into a radiographic system.

High-performance insulator structures for accelerator applications

Multilayer high-gradient insulators are vacuum insulating structures composed of thin, alternating layers of dielectric and metal. They are currently being developed for application to high-current accelerators and related pulsed power systems. This paper describes some of the high-gradient insulator research currently being conducted at Lawrence Livermore National Laboratory

Investigation of UV laser triggered, nanosecond, surface flashover switches

A new, high gradient insulator technology has been developed for accelerator systems. The concept involves the use of alternating layers of conductors and insulators with periods of order 1 mm or less. These structures perform many times better (about 1.5 to 4 times higher breakdown electric field) than conventional insulators in long pulse, short pulse, and alternating polarity applications. We describe our ongoing studies investigating the degradation of the breakdown electric field resulting from alternate fabrication techniques, the effect of gas pressure, the effect of the insulator-to-electrode interface gap spacing, and the performance of the insulator structure under bi-polar stress.

Status of the dual axis radiographic hydrodynamics test (DARHT) facility

Triggered, multi-channel, surface discharges or surface flashover switching have been investigated as a low inductance, low pulse rate switch for conducting large currents. This paper discusses the investigation of UV (355 nm) laser triggered, single channel, low inductance, ns closure and sub-ns jitter switches for applications in switching high dielectric constant, compact pulse forming lines into accelerator loads. The experimental arrangement for evaluating the switch performance and for measuring the high field dielectric constant of the pulse forming lines is presented. Experimental results of delay and jitter measurements versus optical energy on the flashover surface and DC electric field charge.