ION-THRUSTER OPERATION

 

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                              ION-THRUSTER OPERATION

           The first electron-bombardment thruster was conceived and tested

       by Dr.  Harold  R. Kaufman in 1959 at the NASA Lewis Research Center

       (ref.  1).  The thruster operates  by  flowing  a gaseous propellant

       into a discharge chamber.

           The propellant  may  be any gas, but mercury,  cesium,  and  the

       noble gases are  the  most  efficient  for  propulsion applications.

       Propellant atoms are ionized in the  discharge  chamber  by electron

       bombardment in a process similar to that in a mercury arc sunlamp.

           This ionization  occurs when an atom in the discharge  loses  an

       electron after bombardment   by   an   energetic  (40-eV)  discharge

       electron.  The electrons  and  the    ions  form  a  plasma  in  the

       ionization chamber.

           The electric field between the screen and the accelerator  draws

       ions from the  plasma.   These ions are then accelerated out through

       many small holes in the screen and  accelerator electrode to form an

       ion beam.

           A neutralizer injects an equal number of electrons  into the ion

       beam.  This beam  of  electrons  allows  the  spacecraft  to  remain

       electrically neutral and is a requirement  for  successful  thruster

       operation.  A  more  complete description of the mercury-bombardment

       ion thruster is given in the appendix.

           Laboratory testing  of  thrusters  must  be done in a moderately

       large vacuum facility in order to simulate the environment of space.

       Facilities are thus required for laboratory testing.

           Typically, these facilities are  capable of simulating altitudes

       of more than 300 kilometers, where the background  air  pressure  is

       less than 1/100 000 000 of sea-level pressure.

           The development   of   the   mercury-bombardment   thruster  has

       continued through the 1960's to the  present time.  Thrusters 2.5 to

       150 centimeters in  diameter have been successfully  tested.   These

       thrusters require power  of  50  watts  to 200 kilowatts and produce

       thrust of 0.4x10(-3) to 4 newtons (0.1x10(-3) to 1 lb).

           Two of the most advanced bombardment  thrusters,  the  8-and 30-

       centimeter-diameter thrusters, are   described   in   the   sections

       AUXILIARY PROPULSION and     PRIMARY    PROPULSION,    respectively.

       Thrusters of these two sizes fulfill the requirements of present-day

       missions.

           Many laboratories in this country, Europe, and Japan have worked

       on a wide  variety  of  electric  thrusters.   These include colloid

       thrusters using a  doped-glycerine   propellant,   a   pulsed-plasma

       thruster using ablation of a Teflon propellant block (ref. 2), and a

       bombardment thruster using cesium propellant.

           In Germany,  France,  and  England,  numerous  laboratories  and

       universities are at  work  on  electric thrusters for both auxiliary

       and primary propulsion.   The  electric  propulsion  effort  by  the

       Soviet Union includes flights of Zond, Meteor, and Yantar spacecraft

       with ion thruster experiments onboard.

           The mercury-bombardment  thruster  technology developed  at  the

       NASA Lewis Research  Center  has  been  used worldwide.  England has

       developed the T-4 thruster based on this technology (ref. 3).

           The T-4 thruster is a 10-millinewton  (2.2mlb) thruster proposed

       as one of  two  possible ion thrusters to be flight  tested  by  the

       European Space Agency in late 1980.  The other thruster, the RIT-10,

       is a radiofrequency  mercury-bombardment  ion  thruster developed by

       Germany.  It has a similar thrust level of 10 millinewtons (2.2 mlb)

       (ref. 4).

           The Lewis technology has also  been used by Japan.  That country

       has built  and tested a 5-centimeter-diameter, 5-millinewton-thrust,

       mercury-bombardment thruster for  possible  flight  qualification in

       1982 (ref. 5).  Both the European  and  Japanese  ion  thrusters are

       proposed for auxiliary electric propulsion applications.

           Two spacecraft have been flown by the United States specifically

       for the purpose  of  testing ion thrusters in space.   These  tests,

       SERT I and SERT II, are described in the next two sections.

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                     If we can be of service, you may contact

                 Jerry at (214) 324-8741 or Ron at (214) 484-3189

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in

       electrically neutral and is a requirement  for  successful  thruster

       operation.  A  more  complete description of the mercury-bombardment

       ion thruster is given in the appendix.

           Laboratory testing  of  thrusters  must  be d--------

                     If we can be of service, you may contact

                 Jerry at (214) 324-8741 or Ron at (214) 484-3189

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