L.F. Bennett

Circuit modeling for ZR

In the Z Refurbishment (ZR) project, the capabilities of the Z accelerator is expanded [D. H. McDaniel et al., June 2002]. Most of the pulsed power hardware is being redesigned for operation with higher reliability, a greater shot rate, and increased pulse energy. The topology of the ZR pulsed power circuit is similar to that of Z with a few significant changes. The initial baseline circuit design for ZR was developed using Screamer [M. L. Kiefer and M. M. Widner, 1985] which has been the primary pulsed power circuit design tool employed at SNL for years. Screamer is, however, limited in the topologies it can simulate. For ZR there are several circuit features which cannot be modeled adequately in Screamer. The most significant of these is the need to allow for a high degree of independence in module firing times so that a prescribed temporally shaped pulse can be generated for isentropic compression experiments (ICE) [3]. For this reason, the baseline circuit design for ZR is now being developed using Bertha [W. N. Weseloh, 1989]. Bertha is a NRL developed T-line circuit simulation code that is not topology limited and runs fast. In this paper, a discussion of the circuit modeling strategy for the ZR project along with a discussion of the current status of the ZR baseline circuit model is presented.

Design and code validation of the Jupiter inductive voltage adder (IVA) PRS driver

The proposed Jupiter accelerator is a /spl sim/10 MV, 500 TW system capable of delivering 15 MJ kinetic energy to an imploding plasma radiation source. The accelerator is based on Hermes-III technology and contains 30 identical inductive voltage adder modules connected in parallel. The modules drive a common circular convolute electrode system in the center of which is located an imploding foil. The relatively high voltage of 8-10 MV is required to compensate for the voltage differential generated across the load due primarily to the fast increase in current (L dI/dt) and to lesser extent to the increasing inductance (I dL/dt) and resistive component of the imploding foil. Here, the authors examine the power flow through the device and, in particular, through the voltage adder and long magnetically insulated transmission line (MITL). Analytical models, such as pressure balance and parapotential flow, as well as circuit and PIC codes, were utilized. A new version of the TWOQUICK PIC code, which includes an imploding, cylindrical foil as load, was utilized to compare the power flow calculations done with SCREAMER and TRIFL. The good agreement adds confidence to the Jupiter design. In addition, an experimental validation of the design is under way this year with Hermes III. Long extension MITLs are connected at the end of the voltage adder with inductive and diode loads to benchmark the above design codes. In this paper, the authors outline the accelerators conceptual design with emphasis on the power flow and coupling to the inductive load and include preliminary results of Hermes-III experimental design validation.

Comparison of the performance of the upgraded Z with circuit predictions

Since the completion of the ZR upgrade of the Z accelerator at the Sandia National Laboratories in the fall of 2007, many shots have been taken on the accelerator, and there has been much opportunity to compare circuit-code predictions of the performance of the machine with actual measurements. We therefore show comparisons of measurements, and describe a full-circuit, 36-line Bertha circuit model of the machine. The model has been used for both short-pulse and long-pulse (tailored pulse) modes of operation. We also present the as-built circuit parameters of the machine and indicate how these were derived. We discuss enhancements to the circuit model that include 2D effects in the water lines, but show that these have little effect on the fidelity of the simulations. Finally, we discuss how further improvements can be made to handle azimuthal coupling of the multiple lines at the vacuum insulator stack.

The light ion pulsed power induction accelerator for ETF

The light ion Engineering Test Facility (ETF) driver concept, based on Hermes III and RHEPP technologies, is a scaled-down version of the LMF design incorporating repetition rate capabilities of up to 10 Hz. The preconceptual design presented here provides 250 TW peak power to the ETF target during 8 ns, equal to 2 MJ total ion beam energy. Linear inductive voltage addition driving a self-magnetically insulated transmission line (MITL) is utilized to generate the 36 MV peak voltage needed for lithium ion beams. The /spl sim/3 MA ion current is achieved by utilizing many accelerating modules in parallel. Since the current per module is relatively modest (/spl sim/300 kA), two-stage or one-stage extraction diodes can be utilized for the generation of singly charged lithium ions. The accelerating modules are arranged symmetrically around the fusion chamber in order to provide uniform irradiation onto the ETF target. In addition, the modules are fired in a programmed sequence in order to generate the optimum power pulse shape onto the target. This design utilizes RHEPP accelerator modules as the principal power source.

Experimental validation of the first 1-MA water-insulated MYKONOS LTD voltage adder

The LTD technological approach can result in very compact devices that can deliver fast, high current and high voltage pulses straight out of the cavity without any complicated pulse forming and pulse-compression network. Through multistage inductively insulated voltage adders, the output pulse, increased in voltage amplitude, can be applied directly to the load. Because the output pulse rise time and width can be easily tailored (pulse shaped) to the specific application needs, the load may be a vacuum electron diode, a z-pinch wire array, a gas puff, a liner, an isentropic compression load (ICE) to study material behavior under very high magnetic fields, or a fusion energy (IFE) target. Ten 1-MA LTD cavities were originally designed and built to run in a vacuum or Magnetic Insulated Transmission Line (MITL) voltage adder configuration and, after successful operation in this mode, were modified and made capable to operate assembled in a de-ionized water insulated voltage adder. Special care has been taken to de-aerate the water and eliminate air bubbles. Our motivation is to test the advantages of water insulation compared to the MITL transmission approach. The desired effect is that the vacuum sheath electron current losses and pulse front erosion would be avoided without any new difficulties caused by the de-ionized water insulator. Presently, we have assembled and are testing a two-cavity, water insulated voltage adder with a liquid resistor load. Experimental results of up to 95kV capacitor charging are presented and compared with circuit code simulations.

Low inductance switching studies for Linear Transformer Drivers

We are developing new low-inductance gas switches for Linear Transformer Drivers (LTDs). Linear Transformer drivers are a new pulsed-power architecture that may dramatically reduce the size and cost of future pulsed-power drivers, but which place stringent requirements on gas switches.1 ,2 A typical large LTD may have 10,000 or more gas switches that are DC-charged to 200 kV and that must be triggered with a jitter of 5-ns or less with a very low prefire rate3. We are studying new air-insulated gas switches with total inductances of 68–90 nH. These switches transfer 100 J of stored energy for thousands of shots. Typical 10%–90% current rise-times are less than 50 ns and peak currents are on the order of 40 kA into a matched load. One of the switches has a 1-sigma jitter less than 800 picoseconds. Since triggering 10,000 or more switches is a significant challenge, we are also making a detailed study triggering requirements for these switches. We have previously reported tests of four competing switches for LTDs4. In this article we report new tests on improved versions of two of the four switches described previously.

Design and power flow studies of a 500-TW inductive voltage adder (IVA) accelerator

We present a preconceptual design for a 500-TW pulsed power accelerator capable of delivering 15-MJ kinetic energy into an imploding plasma radiation source (PRS). The HERMES-III technology of linear inductive voltage addition in a self-magnetically insulated transmission line (MITL) is utilized to generate the 8-10 MV peak voltage required for an efficient plasma implosion. The 50- to 60-MA current is achieved by utilizing many accelerating modules in parallel. The modules are connected to a common circular convolute electrode system in the center of which is located an imploding foil plasma radiation source. This accelerator produces no electron beam since the total current from the voltage adders (IVAs) to the inductive load flows on the surface of metallic conductors or nearby in the form of electron sheath. In this paper we outline the accelerators conceptual design with emphasis on the power flow and coupling to the inductive load of the center section of the device.

High voltage high brightness electron accelerator with MITL voltage adder coupled to foilless diode

The design and analysis of a high brightness electron beam experiment under construction at Sandia National Laboratory is presented. The beam energy is 12 MeV, the current 35–40 kA, the rms radius 0.5 mm, and the pulse duration FWHM 40 ns. The accelerator is SABRE [J. Corley, J. A. Alexander, P. J. Pankuch, C. E. Heath, D. L. Johnson, J. J. Ramirez, and G. J. Denison, in Proceedings of the Eighth International IEEE Pulsed Power Conference, San Diego, California, 1991 (IEEE, New York, 1991), p. 920], a pulsed inductive voltage adder, and the electron source is a magnetically immersed foilless diode. This experiment has as its goal to stretch the technology to the edge and produce the highest possible electron current in a submillimiter radius beam.

The light-ion pulsed power induction accelerator for the Laboratory Microfusion Facility (LMF)

In order to initiate ignition and substantial energy yield from an inertial confinement fusion target (ICF), a light-ion pulse of /spl sim/700 TW peak power and 15-20 ns duration is required. The preconceptual design presented provides this power. The HERMES-III technology of linear inductive voltage addition in a self-magnetically insulated transmission line (MITL) is utilized to generate the 25-36 MV peak voltage needed for lithium ion beams. The 15-20 MA ion current is achieved by utilizing many accelerating modules in parallel. The lithium ion beams are produced in two-stage extraction diodes. To provide the two separate voltage pulses required by the diode, a triaxial adder system is incorporated in each module. The accelerating modules are arranged symmetrically around the fusion chamber in order to provide uniform irradiation onto the ICF target. In addition, the modules are fired in a preprogrammed sequence in order to generate the optimum power pulse shape onto the target. In this paper we present an outline of the LMF accelerator conceptual design with emphasis on the architecture of the accelerating modules.<<ETX>>

Low emittance foil diode studies for the recirculating linear accelerator (rla)

In recent RLA experiments, a foil diode was utilized followed by a 1.2-m-long 60-mTorr Argon gas cell or lower pressure classical IFR transport region. In order to improve the beam capture and transport efficiency, a number of diode configurations were studied both experimentally and numerically using JASON and the latest version of MAGIC code. In particular, a conical shank diode was investigated in detail. A study of the beam current was undertaken using apertures of gradually increasing I.D. steps of 6.4 mm. An excellent agreement between experiment and simulations was observed. MAGIC predicted the exact total diode current, the current through most of the apertures, and the beam envelope radius. An optimized diode geometry gave at the anode foil a very cold beam of 1.4 MV, 22.5 kA with an rms beam radius r = 1.5 cm and a geometric emittance /spl epsiv/ = 0.04 rad.cm. The foil and gas transport region lead always to an increase of beam emittance. A similar design for a 4 MV, 10 kA diode is also presented.