Ion and Plasma Thruster Test Console Based on Three-Phase Resonant Conversion Power Modules

A test console is described for operation of both high power Hall-effect thrusters (HETs) and ion thrusters. The console utilizes three-phase resonant DC conversion power modules. It is believed that three phase resonant conversion (3PRC) of electrical power is ideally suited for EP power systems, and will soon become the premier converter for flight applications. These converters produce the lowest voltage ripple over any known topology. Additionally, they process power continuously and not in pulses as do their single phase predecessors. The absence of power pulses greatly reduces the size and mass of filter components in the three phase converter, and the smooth power transfer has helped it obtain peak efficiency ratings of >97%. Three phase resonant power converters are also wide ranging compared to competing designs. For example first-order CPE designs have exhibited efficiencies of 97% or higher at full power over an output impedance range of 4:1. Recently developed second-order CPE designs have achieved an output impedance range of 25:1 at efficiencies above 96%. A comparison of 3PRC to square-wave and single-phase resonant conversion is presented. A preliminary comparison is also provided of flash x-ray survivability of standard “current-fed” and 3PRC designs where x-ray pulse duration effects are considered. Another advantage of 3PRC modules over other power conversion hardware is their low specific mass. Attendant to high efficiency and low specific mass is the added benefit of relatively easy thermal management with minimal requirements on heat conduction pathways and thermal interfaces. In addition to presenting a review of the 3PRC design and recent prototype performance, a detailed discussion is provided of the test console design and hardware intended for operation of all SEP-based ion and plasma thrusters that are currently available at NASA centers and commercial aerospace companies. The secondary goal of the test console effort is the development of a modular control system that can be interfaced to any number of 3PRC power modules in a plug and play fashion. The full versatility of the 3PRC design can be applied in this approach to (1) maximize circuit design re-use (enabled by the 3PRC wide range capability), (2) minimize system mass (due to low 3PRC specific mass and light thermal interface requirements), and (3) maintain world-class performance. The 3PRC test console development described herein is intended to serve as a guide to the development of standardized architecture sub-systems intended for off-the-shelf flight hardware solutions to future EP applications. 1 President and Director, 120 Commerce, Unit 3 2 Assistant Professor, Mechanical Engineering, 1320 Campus Delivery, AIAA Senior Member 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit 9 12 July 2006, Sacramento, California AIAA 2006-4339 Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 2 1.0 Introduction The Deep Space 1 spacecraft launched on Oct. 24, 1998 utilized electric propulsion for its primary propulsion system. The propulsion system was developed under the NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) project. This system included a 30-cm diameter ion thruster, a propellant feed and storage subsystem, a power processor unit (PPU), and a digital control interface unit (DCIU) [Bond and Christensen, 1999]. The NSTAR propulsion system was successfully operated for over 16 khr [Polk et al., 2001], and the success of this mission enabled JPL researchers to convince DAWN mission planners to utilize a threethruster version of the NSTAR propulsion system for their spacecraft [Brophy et al., 2004]. The Discovery-class DAWN spacecraft will use the ion propulsion system to rendezvous with two near Earth asteroidsVesta and Ceres. Discovery class programs impose higher quality control requirements in comparison to a technology demonstration program like Deep Space I. These constraints have played a part in increasing the cost of the PPU and DCIU as design changes and component upgrades were implemented and difficulties related to these changes were encountered. Some of the root causes of these problems can be better understood by examining Bond and Christensens’ [1999] photographs of the NSTAR PPU with its cover plate removed. Although there are two locations where additional power supply capacity could be installed, there is very little room to implement these options. A more ergonomic design would be to lay out the power supply system in a slice (or modular) packaging scheme. This choice would allow easy access to individual slices for initial assembly and re-work. In addition, the sub-assembly slices could be acceptance tested prior to insertion into the PPU. The output power could also be scaled using a modular/slicebased layout if the chassis is designed for expansion. Finally delta-qualification tests could be performed at the module-level to further reduce the costs of customization required by future interplanetary or asteroid rendezvous missions. The test console approach described herein utilizes a modular layout to take advantage of this design choice. Another basic implementation-related challenge of the NSTAR PPU and DCIU is the physical separation of the two devices. The single-fault tolerant DAWN ion propulsion system requires two DCIUs and two PPUs. Integration of the DCIU into the PPU into a single device would result in a simpler system that would be easier to mate to the future spacecraft. Although desirable, an entire PPU re-design with embedded DCIU functionality is not easy to accomplish under current and foreseeable future financial constraints. And this is especially true when higher performance systems are needed. The goals of the work described herein is to (1) develop a low-cost, ground-based test console system using industry standard approaches that implements SOA power conversion designs, (2) validate the test console through extensive operation of NASA and commercial thrusters, and (3) systematically transition the test console sub-systems into flight qualified designs. Secondary goals of the test console development effort are to standardize and reduce the cost of flight acceptance and qualification performance tests by developing automated test sequences. It is believed that three phase resonant conversion (3PRC) of electrical power is ideally suited for EP power systems, and will soon become the premier converter for flight applications. The benefits of 3PRC are just now receiving attention. These converters produce the least possible voltage ripple over any known topology. Additionally, they process power continuously not in pulses as do their single phase predecessors. The absence of power pulses greatly reduces the size and mass of filter components in the three phase converter. The 3PRC converter exhibits smooth current flow on input and output lines without filtering. The smooth power transfer of this new class of converter has helped it garner the high efficiency ratings of >97% [Kay, 2005] with low output stored energy.