Hall Thrusters Operating in Pulsed Mode

Heretofore, Hall thrusters were perceived as strictly steady state devices, unsuitable for operation in a pulsed mode. We have demonstrated that at least certain types of Hall thrusters can efficiently deliver thrust pulses with a duration as short as few tens of msec. Our 200 W Hall thruster, designated BHT-200, was pulsed nearly 500,000 times delivering precisely controllable, repetitive impulse bits of arbitrary magnitude starting as low as sub mNsec. This was achieved without significant loss of efficiency and a moderate loss of specific impulse due to xenon flow timing. The thruster was driven by both a PPU and by discharging a capacitor in the so called direct drive pioneered by others. This demonstration opens a new field of applicability for the tandem style and possibly other Hall thrusters that was previously reserved to PPTs and cold gas thrusters. This includes attitude control and primary propulsion for microspacecraft that lack sufficient power for continuous operation at 200 W. For spacecraft with sufficient power on board, the same Hall thruster can be used for both primary and ACS propulsion. The paper treats the subject in a generic manner describing the various methods of pulsed mode operation, flow and electrical start-up transients, system requirements and areas of applicability. This description is supported by analytical models and data from the pulsed BHT-200 experiments. Introduction and Pulsed Hall Thruster Applications Heretofore, Hall thrusters were perceived as devices suitable only for steady state operation. As will be described in this paper we have demonstrated that at least some types of Hall thrusters can be pulsed with a pulse duration as short as few tens of msec and deliver nearly steady state performance. Busek's 200 W tandem style Hall thruster demonstrated that it can efficiently produce impulses ranging from a mNsec to nearly infinity, limited only by its lifetime. For the variable impulse it can be powered by a conventional PPU, capacitor discharge thus eliminating conventional PPU, and in some configurations directly from solar panels. With advanced capacitors the necessary energy can be stored in 1-in cube. With these capabilities and lower complexity/cost of some configurations, a whole new range of applications emerges that includes propulsion for power limited micro or nano spacecraft, ACS (attitude control system) propulsion for large spacecraft that already have Xe on board and general, combined steady state and pulsed thrusting using the same Hall thruster on a wide variety of spacecraft. These applications are briefly discussed below

Outcomes of Telephone Medical Care

OBJECTIVES: To document the outcomes of a telephone coverage system and identify patient characteristics that may predict these outcomes. DESIGN: Telephone survey. SETTING: An academic outpatient medical practice that has a physician telephone coverage service. PATIENTS: All patients (483) who called during the 3-week study period to speak to a physician were evaluated, and for the 180 patients with symptoms, attempts were made to survey them by telephone 1 week after their initial telephone call. MEASUREMENTS AND MAIN RESULTS: The mean age of the 180 patients was 41 years, 71% were female, and 56% belonged to commercial managed care plans. In the week after the initial telephone call, the following outcomes were reported: 27% of the patients had no further contact with the practice; 9% filled a prescription medication; 19% called the practice again; 48% kept an earlier appointment in the practice; 3% saw an internist elsewhere; 8% saw a specialist; 8% went to an emergency department; 4% were admitted to a hospital. Of the 180 patients who called with symptoms, 160 (89%) were successfully contacted for survey. Eighty-seven percent of these 160 patients rated their satisfaction with the care they received over the telephone as excellent, very good, or good. In multivariate analysis, patients' own health perception identified those most likely to have symptom relief (p = .002), and symptom relief, in turn, was a strong predictor of high patient satisfaction (p = .006). Thirty-three percent of the 160 patients reported that they would have gone to an emergency department if a physician were not available by telephone. CONCLUSIONS: In the present study, younger patients, female patients, and patients in commercial managed care plans used the telephone most frequently. Also, the telephone provided a viable alternative to emergency department and walk-in visits. Overall satisfaction with telephone medicine was high, and the strongest predictors of high patient satisfaction were symptom relief and patients' own health perception