Ion Energy Diagnostics in the Plume of an SPT-100

Transcription

1 Ion Energy Diagnostics in the Plume of an SPT-100 from Thrust Axis

2

3 measurements

4 q ; e K 45 will reach the collector and be recorded as ion current.

5 Even with

6

7 about the thruster. The region behind the thruster at positive angles greater than 110 degrees and negative angles of magnitude greater than -150 degrees represented ion currents less than

8

9 Furthermore,

10 Eqn. 15 where f(ej)

11 In this model, as in actual Hall thruster operation, neutral propellant is injected through the anode. This propellant is then ionized by electron collisions as the neutral atoms travel towards

12 acceleration layer and, thus, well upstream

13 Reactants at Vh Ion Products Electrons transferred Xe2+ + Xe+ Xe+=2Vb with Xe2+=Vb/2 1 Xe2+ + Xe+ Xe3-"=2Vb/3 1 Xe2+ + Xe* Xe3+=Vb/3 2 Xe3+ + Xe+ Xe2+=3Vb/2 with Xe2+=Vb/2 1 Xe3+ + Xe+ Xe4+=3Vb/4 1 Xe3+ + Xe+ Xe+=3 with Xe3+=Vb/3 2 Xe3+ + Xe+ Xe4+=Vb/4 3 Xe3+ + Xe2+ Xe2+=3Vb/2 with Xe3+=2Vb/3 1 Xe3+ + Xe2+ Xe4+=3Vb/4 with Xe+=2Vb 1 Xe3+ + Xe2+ Xe+=3Vb with Xe4+=Vb/2 2 Table 2. Possible reactant and product combinations for CE collisions occurring between two high-energy beam ions, each with voltage Vb. Reactant ion at Vb with 0 V neutral Xe+ + Xe Ion products with Energy > 0 Electrons Transferred none N/A Xe2+ + Xe Xe+=2Vb 1 Xe3+ + Xe Xe2+=3V,/ Xe + Xe + Xe =3Vb 2 Table 3. Possible reactant-product combinations for the CE collision between a beam ion with voltage Vb and a stagnant background neutral. As discussed in Reference 12, momentum transfer collisions are evidenced by monotonically decaying tails on the ion voltage distributions, while CE collisions are represented by "bump-on-tail" types of distributions. The measured ion voltage distributions near centerline at 0.5 m seem to be very consistent with a momentum-transfer elastic collision analysis between plume beam ions. A discussion of these momentum transfer collisions can be found in References 6 and 12. This paper concentrates on the appearance of CE collision signatures within the ion voltage distributions. Many of the measured ion voltage distributions exhibit a "bump on tail" shape, where the voltage of the bump maximum is exactly twice the most probable voltage of the distribution. This can be seen, for example, by examining Figure 5: denoting Vb as the most probable beam voltage, Vb = 235 V at 90 degrees with the bump occurring at V =

14 V. From Table 2 and Table 3 the possible reactions creating products with a voltage of twice the main beam voltage are (1) the CE collision between a Xe

15 data. In this plot the height of the post-collision distribution has been chosen to match the data, but the distribution shape was determined by "cutting and pasting" the original data and

16 than V b will experience a simultaneous growth in amplitude. Furthermore, although the ionization fraction decreases with increasing

17 plasma acceleration region downstream of the thruster exit plane is not accurately explained by current models

18 fabrication.

19 References 1 Absalamov,

20 12 King, L.B., Transport-property

21 V, Field Correction Plates Ion Trajectory etector Figure

22 Ion Energy/q (volts

23 i ' '

24 I '' ' '' '

25 1.2-20

26 --.? 4 - 'o "x < 'o x 2- I I I ; i i QP«ioi M ; om ; o ; ; ; o M M 8 M M 0 ; ; ; xt 2.0- «o 1.5- ^ ~ 2.0- 'o "** w

27

28 160-H

29 Ion Energy/q (volts wrt plasma) Figure 10. Ion current as a function of ion voltage at 0.5 m radius from the SPT-100 for points at 90, -100, -110,

30 x < IDeg < x < co ideg

31 I i i i i 1 i i i i j Ion Energy/q (volts wrt plasma) Figure 12. Ion current as a function of ion voltage at 1.0 m radius from the SPT-100 on the thrust axis in addition to points at 10,20, and 30 degrees off axis. 31

32 Ion Energy/q (volts wrt plasma) Figure 13. Ion current as a function of ion voltage at 1.0 m radius from the SPT-100 for points at 40,50,

33 ,

34 200-i o.o TTTTTTI1I1III11M. I. II III.. rtttttt Ion Energy/q (volts

35 30 j i i i i i i i i i I i i i i i i i i i [TT i

36 0 I ''''''''' I'''''''''

37 600-1 I ' ' ' ' I

38 I ' ' ' ' I ' ' ' ' I ' ' ' ' I

39 0

40

41 x < I o x 0.8H ^._.~~- _.BH I ' ' ' ' I ' ' ' ' I ' ' ' ' I

42 prt rrjt.,,,,.,,,... (...

43

44 OH I

45 Plasma Potential (V wrt cathode) o o m o o o CO I

46 _ 0.6- Proposed pre-collision distribution *" X

47 Xe Product Ions Unaffected Xe and Xe MBMS data Xe + Product ions

48 Complete MBMS data Portion

49 Figure