From nmp.jpl.nasa.gov/ds1/


SOLAR ELECTRIC (ION) PROPULSION

   
   
   DS1 Ion Thruster Compatibility Test taped on February 15, 1998.
   This testing was carried out on the DS1 spacecraft in the Solar
   Thermal Vacuum Chamber at JPL. The testing proved that 
   the integrated ion propulsion subsystem and spacecraft work and
   behave properly. The engine recycling in the video
   was caused by increasing the engine throttle level without increasing
   the available power and shows both the spacecraft and engine operate
   as expected in a fault condition. The engine will provide about 10
   times the specific impulse (ratio of thrust to propellant used) of
   chemical propulsion. DS1 is the first spacecraft to use ion propulsion
   as the primary propulsion system. It is one of the 12 advanced
   technologies to be validated by DS1 during flight. Ion Propulsion
   System
   
   Solar electric propulsion (SEP) offers significant mass savings for
   future deep-space and Earth-orbiting missions with high delta v
   requirements. The objective of the NSTAR (NASA SEP Technology
   Application Readiness) program is to validate low-power ion
   propulsion, which fits well with the New Millennium Program's goals.
   The joint Jet Propulsion Laboratory/Lewis Research Center effort,
   which was started in November 1992, has been building and ground
   testing ion propulsion hardware in parallel with fabricating flight
   hardware for Deep Space 1. Ion Propulsion System diagram 
   
   
   The ion propulsion system (IPS), provided by NSTAR, uses a hollow
   cathode to produce electrons to collisionally ionize xenon. The Xe+ is
   electrostatically accelerated through a potential of up to 1280 V and
   emitted from the 30-cm thruster through a molybdenum grid. A separate
   electron beam is emitted to produce a neutral plasma beam. The power
   processing unit (PPU) of the IPS can accept as much as 2.5 kW,
   corresponding to a peak thruster operating power of 2.3 kW and a
   thrust of 92 mN. Throttling is achieved by balancing thruster and Xe
   feed system parameters at lower power levels, and at the lowest
   thruster power, 500 W, the thrust is 20 mN. The specific impulse
   decreases from 3100 s at high power to 1900 s at the minimum throttle
   level.
   
   Because the purpose of flying NSTAR's IPS is to validate it for future
   space science missions, a comprehensive diagnostic system is also on
   the spacecraft. This will aid in quantifying the interactions of the
   IPS with the spacecraft, including advanced-technology science
   instruments, and validating models of those interactions. The
   diagnostic instrument suite includes a retarding potential analyzer,
   two Langmuir probes, search-coil and fluxgate magnetometers, a plasma
   wave sensor, and two pairs of quartz-crystal microbalances and
   calorimeters. One of these pairs has a direct view of the ion beam,
   while the other is shadowed by spacecraft structure. Measurements will
   include the rate and extent of contamination around the spacecraft
   from the Xe+ plume and the sputtered Mo from the grid, electric and
   magnetic fields, and the density and energy of electrons and ions in
   the vicinity of the spacecraft. In addition, the sensors will be used
   to complement science measurements of Deep Space 1's ion and electron
   spectrometer (see related section), particularly during the
   encounters.