David M. Baca

Operational Performance of the Spallation Neutron Source High Voltage Converter Modulator and System Enhancements

The first-generation high-frequency switching megawatt-class high voltage converter modulators (HVCM) developed by Los Alamos national laboratory for the spallation neutron source (SNS) at Oak Ridge national laboratory have been installed and are now operational. Each unit is capable of delivering pulses up to 11 MW peak, 1 MW average power at voltages up to 140 kV to drive klystron(s) rated up to 5 MW. To date, three variations of the basic design have been installed, each optimized to deliver power to a specific klystron load configuration, for a total of fifteen (15) units installed and operational at the SNS. The units have been operated 24/7 to support successful commissioning of the SNS accelerator. Design improvements, with the primary intention of improving system reliability and availability, have been under development since the initial installation of the HVCM units. This paper will examine HVCM reliability operational data of well over 50,000 operational hours, failure modes, and modifications and improvements performed and planned to increase the overall system availability. We will focus on enhancements designed to allow for increased average power operation, as well as discuss development programs underway to provide active component protection

Electrical Design and Operation of the Phelix Pulsed Power System

The Precision High Energy-Density Liner Implosion Experiment (PHELIX) is a pulsed power driver capable of delivering multimegampere currents to cylindrical loads. The PHELIX hardware includes novel design features to provide a high-energy conversion efficiency of approximately 10-MA output current per megajoule of stored energy. This is achieved by a rail-gap switched low-inductance Marx design (resistively damped) driving a multifilar air-core pulse transformer. The Marx output cables form the toroidal transformer that is an integral part of the disc line and removable load cassette assembly. The transformer and disc line uses conformal insulation methods and does not require replacement; after each shot, the transformer is completely reusable. Load cassettes can be easily exchanged to facilitate experimental variation. PHELIX is selfcontained within its own transport container and Faraday cage that can be moved from the maintenance building to the Los Alamos Neutron Science Center 800-MeV proton accelerator facility to perform multipulse proton radiography. This paper details the electrical and mechanical design of the Marx and multifilar transformer assemblies as well as presenting the operational performance achieved to date.

The PHELIX Liner Demonstration Experiment (PLD-1)

The PHELIX Liner Demonstration Experiment (PLD-1) took place in September of 2010 at Los Alamos National Laboratory. The PHELIX machine consists of a ∼500 kJ single-marx capacitor bank cable-coupled to a toroidal 1∶4 current step-up transformer which delivers multi-Mega-Ampere currents to a cm size load. In this experiment the load consisted of a ∼3 cm radius, 0.8 mm thick, ∼3 cm tall aluminum liner, copper glide planes, a thin polyethylene insulator, and a 0.5 cm thick aluminum return conductor. Two independent channels of fiber optic Faraday rotation measured a peak load current > 4 MA with a pulse width of ∼ 10 µs. Four linear Rogowski coils measured the output current of the 4 marx modules. High-resolution flash X-radiography imaged a stable, highly symmetric and uniform liner 14.5 µs after current start. A 12 channel laser Doppler velocimetry (LDV) system tracked the inside surface of the liner throughout the experiment and showed a peak velocity before impact with probes of ∼ 1 km/s. The LDV probes were arrayed axially as well as azimuthally and confirmed the symmetry of the liner trajectory. Surprisingly, the LDV showed distribution of velocities of the inner liner surface late in time. PLD-1 is the first step towards utilizing the PHELIX pulsed-power system at the Los Alamos proton radiography facility.

Final klystron modulator design, testing, and installation plan for the Los Alamos Neutron Science Center accelerator

This paper describes the production design, initial testing, and the installation plan of the 44 totem-pole pair, triode based klystron modulator systems that will be installed on the Los Alamos Neutron Science Center (LANSCE) accelerator RF system. The existing modulators were designed in the late 60s and replacement components are no longer available. That design uses a single triode tube as a saturated switch to develop the required klystron mod-anode voltage. This design dissipates almost 15 kW and requires oil pumps and oil/water heat exchangers to maintain safe oil tank temperatures. The new modulator switches electrostatically, charging and discharging the klystron mod-anode capacitance. Analog feedback control is utilized to ensure the klystron beam current is flat-top regulated. The new totem-pole design also provides faster rise and fall characteristics which reduces klystron collector dissipation. The new modulator is designed to operate the klystrons up to 86 kV with a nominal 32 Amp beam current at a 120 Hz repetition rate and a 15% duty cycle (>400 kW). The on and off deck modulators are of identical design and utilize a cascode connected planar triode, cathode driven with a high speed MOSFET. Voltage divider feedback is connected to the planar triode grid to enable flat-top control. Although modern design approaches suggest solid state designs may be considered, the planar triode (Eimac Y-847B) is very cost effective and has a low power (50 W) matrix cathode which is easy to integrate with the existing hardware. With the high mu gain characteristics, the triode provides a simplified linear feedback control mechanism. The design is very compact and fault tolerant. This paper will review the final design, test parameters, and the expected installation plan for the LANSCE accelerator.

Electrical design and operation of the PHELIX pulsed power system

The Precision High Energy-Density Liner Implosion Experiment (PHELIX) is a pulsed power driver capable of delivering multimegampere currents to cylindrical loads. The PHELIX hardware includes novel design features to provide a high-energy conversion efficiency of approximately 10-MA output current per megajoule of stored energy. This is achieved by a rail-gap switched low-inductance Marx design (resistively damped) driving a multifilar air-core pulse transformer. The Marx output cables form the toroidal transformer that is an integral part of the disc line and removable load cassette assembly. The transformer and disc line uses conformal insulation methods and does not require replacement; after each shot, the transformer is completely reusable. Load cassettes can be easily exchanged to facilitate experimental variation. PHELIX is selfcontained within its own transport container and Faraday cage that can be moved from the maintenance building to the Los Alamos Neutron Science Center 800-MeV proton accelerator facility to perform multipulse proton radiography. This paper details the electrical and mechanical design of the Marx and multifilar transformer assemblies as well as presenting the operational performance achieved to date.

Klystron modulator design for the Los Alamos Neutron Science Center accelerator

This paper describes the design of the 44 modulator systems that will be installed to upgrade the Los Alamos Neutron Science Center (LANSCE) accelerator RF system. The klystrons can operate up to 86 kV with a nominal 32 Amp beam current with a 120 Hz repetition rate and 15% duty cycle. The klystrons are a mod-anode design. The modulator is designed with analog feedback control to ensure the klystron beam current is flat-top regulated. To achieve fast switching whilst maintaining linear feedback control, a grid-clamp, totem-pole modulator configuration is used with an “on” deck and an “off” deck. The on and off deck modulators are of identical design and utilize a cascode connected planar triode, cathode driven with a high speed MOSFET. The derived feedback is connected to the planar triode grid to enable the flat-top control. Although modern design approaches suggest solid state designs may be considered, the planar triode (Eimac Y-847B) is very cost effective, is easy to integrate with the existing hardware, and provides a simplified linear feedback control mechanism. The design is very compact and fault tolerant. This paper will review the complete electrical design, operational performance, and system characterization as applied to the LANSCE installation.