Juan J. Ramirez

Performance of the hermes-III gamma ray simulator

Hermes III is a 22-MV, 730-kA, 40-ns pulsed power accelerator that drives an electron beam diode/converter to generate an intense pulse of bremsstrahlung radiation. In Hermes III, eighty individual 1.1-MV, 220-kA pulses are generated and added in groups of four to develop twenty 1.1-MV, 730-kA pulses which are then fed through inductively isolated cavities and added in series by a magnetically insulated transmission line (MITL). An extension MITL delivers this surnmed output to the electron beam diode. The construction of Hermes III was completed February 1988 and was followed by an accelerator characterization and performance demonstration phase. Hermes III met all of its performance requirements during this testing period and has gone operational one year ahead or schedule. An overview of the Hermes-III design is presented together with details of the performance obtained for the various subsystems. A discussion of future applications of this technology is also presented.

Sabre, A 10-mv Linear Induction Accelerator

SABRE (Sandia Accelerator and Beam Research Experiment) is a 10-MV, 250-kA, 40-ns linear induction accelerator. It was designed to be used in positive polarity output. Positive polarity accelerators are important for application to Sandias ICF (Inertial Confinement Fusion) and LMF (Laboratory Microfusion Facility) program efforts. SABRE was built to allow a more detailed study of pulsed power issues associated, with positive polarity output machines, MITL (Magnetically Insulated Transmission Line) voltage adder efficiency, extraction ion diode development, and ion beam transport and focusing. The SABRE design allows the system to operate in either positive polarity output for ion extraction applications or negative polarity output for more conventional electron beam loads. Details of the design of SABRE and the results of initial machine performance in negative polarity operation are presented in this paper.

High-power, short-pulse generators based on induction voltage adders

High-power, short-pulse generators based on induction voltage adders were identified by the pulsed power community as the most important recent advance in this field. These systems use linear induction accelerator modules and self-magnetically insulated vacuum transmission lines to generate high-voltage (tens of megavolts), moderate-impedance ( >

A study of low‐current‐density microsecond electron beam diodes

The performance of several electron sources has been investigated and compared for generating low‐current‐density microsecond electron beams with applications to gas laser excitation. A brush cathode made with 10‐μm‐diam carbon filaments gave the best peformance among the various field emitters tested. It emitted well at fields as low as 10 kV/cm. Space‐charge‐limited flow was established in ∼60 ns and apparent gap closure velocities of 1.5 cm/μs were characteristic of this cathode. Two regimes of plasma injected diodes were investigated. First, substantial control of the diode impedence was obtained when the plasma was allowed to fill the entire anode‐cathode volume prior to application of the high‐voltage pulse; however the anode current density was found to be nonuniform with poor reproducibility from shot to shot. Second, more predictable behavior was obtained when the plasma was constrained to be on the cathode surface at the time the high‐voltage pulse was applied. In this case the time integrated a...

The X-1 Z-pinch driver

X-1 is a program initiative to develop the next-generation laboratory X-ray source using fast Z-pinch drivers. X-1 will provide unique applications utility to high-energy density physics, inertial confinement fusion, and radiation effects simulation research. The advent in the 1980s of pulsed power accelerators capable of delivering tens of terawatts to imploding plasma loads led to a quantum improvement in Z-pinch performance by reducing the implosion time to less than 100 ns. Further progress in Z-pinch performance capabilities was achieved in 1996, using a cylindrical wire-array load, and led to the production of 85 TW, 350-500 kJ of X-rays using the Saturn accelerator at Sandia. The PBFA-II accelerator has been converted to drive Z-pinch loads (PBFA-Z), and will provide a factor of 2 increase in current, and 4 in energy, over that provided by Saturn. X-1 is the next step beyond PBFA-Z and, as presently envisioned, represents a factor of 8 increase in energy. It will require a /spl sim/360 TW, /spl sim/100 ns pulsed power generator to impart /spl sim/16 MJ kinetic energy to the reference imploding plasma load. A baseline concept for X-1 has been developed. It utilizes a highly modular, robust architecture with demonstrated performance reliability.

Performance of the hermes-III laser-triggered gas switches

This paper reports the performance of the SF/sub 6/ insulated, multistage, laser-triggered gas switch used in the Hermes-III accelerator.sup 1/ In this accelerator, 20 of these switches are used to transfer energy from intermediate energy storage water dielectric capacitors to the pulse forming lines (PFLs). Approximately 8,000 laser-triggered switch shots have been taken with seven prefires. Nearly 70% of these shots have been at nominal operating parameters. The average first-to last spread in firing times for 20 switches is approximately 8 ns. Removal of systematic differences reduces this spread to /spl tilde/6 ns. This spread implies a one-sigma jitter for a single switch of <2 ns at 70-75% of self-breakdown voltage. Results show that the jitter does not change significantly over an operating range of 70-90% of self-breakdown. In addition, the jitter is insensitive to the factor of two variation in the laser energy delivered to the various switches by the optical system. A detailed summary on the performance, reliability, and maintenance of the switches and optical system is presented. Initial results of a study to investigate the performance of these switches under varying laser trigger conditions is also presented.

Positive polarity voltage adder miti, experiments on helia"'

HELIA is a four stage, 4-MV, 250-kA, 30-ns accelerator used to study the design concepts for Hermes III. The accelerator consists of eight pulse forming lines (PFLs) that deliver 1-MV, 125-kA pulses to four linear induction cavities. The cavities are in series. and the voltage addition is accomplished in a coaxial self magnetically insulated transmission line (MITL). A positive polarity experiment has been performed on Hermes Ill that shows efficient current transport through the system. HELIA has been reconfigured for positive polarity operation, and initial experimental results show operation consistent with Hermes Ill results. These experiments are aimed at obtaining a more complete understanding of the system performance in positive polarity and to investigate the interactions of the cavities, adder MITL, and extensions in this configuration. Initial results and details of these tests are presented in this paper.

Indirect measurement of Hermes III voltage

The peak voltages estimated at the HERMES III diode using the parapotential flow theory, the range of H/sup -/ ions, and the radiation output are 20.5+or-0.4 MV, 18.5+or-0.6 MV, and 18.1+or-0.6 MV, respectively. The values correspond to the mean shot-to-shot variation of about 20 shots. The absolute uncertainty is estimated to be about +or-10% for each of the measurements. Within these uncertainties, the measurements are consistent with each other. Associated measurements of peak voltage along the MITL (magnetically insulated transmission line) support the measured diode voltage and show that HERMES III operates as a balanced system, with each cavity contributing equally to the power pulse at the diode.<<ETX>>

Very high pulse-energy accelerators

The design and performance of the Hermes III are presented and the application of this technology to the light ion beam inertial confinement fusion program is discussed. Hermes III represents a new approach to efficiently generating higher voltages. It is designed to drive a 22-MV, 730-kA, 40-ns electron beam diode and combines conventional, modular pulsed power technology with linear induction acceleration concepts. High-power induction accelerator cavities are combined with voltage addition along a MITL (magnetically insulated transmission line) to generate the desired output. This design differs from a conventional linac in that the voltages are added by the MITL flow rather than by a drifting beam that gains kinetic energy at each stage. The design is a major extrapolation of previous state-of-the-art technology represented by the injector module of the Advanced Test Accelerator and has proven to be efficient and reliable. A key issue for ion beam applications relates to the efficiency of coupling power from the high-voltage MITL to an extraction ion diode. A conceptual design of a light ion beam driver for the LMF (Lase Microfusion Facility) has been developed.<<ETX>>

A conceptual design for an LMF accelerator module

The US Department of Energy has initiated a design study of a Laboratory Microfusion Facility (LMF) that will be used to develop high-gain ICF (inertial confinement fusion) targets. A conceptual design for a light ion beam accelerator, has been developed for a multimodule LMF system based on technology demonstrated on Sandias Hermes-III accelerator. The LMF accelerator module incorporates 32 linear induction cavities, each operating at 1 MV, to produce an output voltage pulse that ramps from 27 to 32 MV with a peak current of 1.2 MA. The power for the accelerator module is derived from 16 Marx generators that drive 128 4- Omega , 54-ns pulse forming lines (PFLs). Four PFL pulses are combined in each inductive cavity. Nanosecond synchronization of the PFL output pulses is accomplished using KrF-laser-triggered output gas switches and low jitter, self-breakdown water switches. The outputs of the cavities are added in a magnetically insulated transmission line and then delivered to an extraction ion diode. Singly ionized lithium ions are accelerated in the diode. Voltage ramping is used to achieve power compression of the ion beam when ballistically drifted to the ICF target. An equivalent circuit has been developed to model the module from the Marx generator to the ion diode. The timing performance of the gas and water switches has been included in the model to calculate the resulting output waveform and system jitter.<<ETX>>