NEGATIVE ION PLASMAS

1. Plasma Physics Seminar, April 23, 2007 NEGATIVE ION PLASMAS Professor Robert L. Merlino Department of Physics & Astronomy University of Iowa

2. negative ion plasma • a plasma containing electrons, positive ions and negative ions • a fraction of the electrons are attached to negative ions • characterized by the parameter p = n / n+the % of negative ions in the plasma • occur naturally in space and astrophysics and can be produced in the lab

3. OUTLINE • IntroductionA. the chemical physics of negative ion formation B. examples of negative ion plasmas (1) neutral beam sources (2) photosphere of the Sun (3) D region of the ionosphere (4) plasma processing reactors

4. II.Production of negative ion plasmas A. Q machine B. electron attachment cross sections C. Langmuir probe measurements D. comparison of SF6 and C7F14 results III. Waves in negative ion plasmas A. ion acoustic waves B. electrostatic ion cyclotron waves

5. The chemical physics of negative ions A) Negative ion formation mechanisms: (molecule XYZ) attachment radiativestabilization autoionization IVR dissociative attachment

6. B)Negative ion destruction mechanisms mutual neutralization photodetactment collisional detachment

7. The negative hydrogen ion H • one of the most important negative ions in the universe! • It exists, electron affinity (binding energy of the extra electron) = 0.75 eV • why does it exist? first electron in H only partially shields the nuclear charge • QM calculations confirm this • responsible for most of the continuum opacity of the photosphere

8. Negative ion sources for neutral beam systems • magnetically confined fusion plasmas are heated by neutral beam injection (150 keV D+) • cannot accelerate neutral atoms • accelerate H+ then neutralize by charge exchange  inefficient at >100 keV • however, with H-, the neutralization efficiency remains high out to 500 keV. • now use negative-ion based neutral beam systems capable of producing multiampere beams of H and D negative ions

9. H in the photosphere • photosphere - what you see when you look at the sun • about 400 km thick, cool ~ 4400K – 5800K, mostly H • remarkably opaque at infrared and shorter wavelengths • most H in ground state and thus does not contribute much to absorption • need 13.6 eV (121.6 nm) to get H in first excited state • 1939- about one in 107 H’s are H–, and need only 0.75 eV to remove extra electron  1653 nm (Saha relation) • so H–can account for absorption down to very long wavelengths • negative H makes photosphere as opaque as a dense object, therefore it radiates like a blackbody

10. negative ions in the earth’s ionosphere Data from rocket borne instruments • negative ions (O2–) are generally present in the lower ionosphere (D region) 60 – 90 km • they may play a role in the creation and destruction of the ozone layer observed at 76 km in the polar region

11. Effect of rocket exhaust on the ionospheric plasma artificially induced airglow caused by Challenger engine burn on 29 July 1985

12. electric field mV/m electron density cm-3 time (sec) electron depletion experiments in space • electron density changes recorded on a Langmuir probe onboard a rocket payload when 30 kg (1026 molecules) of CF3Br) triflouromethyl bromide (was released at 309 km. • in less than 0.1 sec, the electron density was reduced from 105 cm-3 to less than 15 cm-3 • CF3Br + e–  Br– + CF3

13. negative ions in plasma processing A typical rf processing reactor in which reactive radicals, positive and negative ions, neutrals and molecules are produced when a glow discharge is formed by a continuous flow of feed gas. • Plasma Assisted Chemical Vapor Deposition (PECVD) systems use silane (SiH4) for deposition of amorphous silicon (a-Si:H) for solar cell fabrication • positive and negative ions are formed: SiH4 + e–  SiH3+ + H + 2e–(dissociative ionization)SiH4 + e–  SiH3– + H (dissociative attachment) • chemical reactions among the various species then lead to the formation of bigger particles (nm) which are deposited on a substrate as a thin film.

14. Interest in negative ion plasmas • much or ordinary plasma behavior is dominated by the fact that me << m+ • but in a negative ion plasma we havene << n+, so the plasma has m– m+ • electron induced ambipolar fields no longer dominate • shielding of low frequency electric fields by electrons is less important • effect on low frequency plasma waves due to the quasineutrality condition n+ = ne + n–

15. plasma potential sheaths position e.g. sheaths in a plasma • typically ve,th >> v+,th  electrons leave first • plasma potential adjusts to maintain quiasi-neutrality  SHEATH

16. Production of negative ion plasmas • introduce an electronegative gas into a plasma, e.g., SF6 • attachment cross sections are highly energy dependent • F is highly corrosive

17. Q machine SF6 grid for launching IA waves K+ or Cs+ plasmas, nearly fully ionized Te = T+ 0.2 eV n+ ~ 108 – 1011 cm-3

18. IQ-3

19. Attachment cross sections SF6 sulfur hexafluoride C7F14 perfluoromethylcyclohexane Low energy cross sections

20. reduction in the electron density asthe SF6 pressure is increased • the Langmuir probe is used to observe the reduction in electron density • the negative ion contribution to the probe current is much smaller than the electron current since m– >> me • the reduction in electron current can be used to estimate n–/n+

21. comparison of results in SF6 andC7F14 in C7F14 can achieve ne/n+ < 10–3

22. I V Vp Vf Langmuir probe floating potential

23. Ion acoustic waves in a negative ion plasma • An e– /+ ion plasma supports low frequency (f << fp+) ion sound waves in the same way that a gas supports ordinary sound waves • the ions provide the inertia for the wave and the electrons the pressure which is communicated to the ions via the electric field • a negative ion plasma supports 2 ion acoustic modes – a ‘slow’ mode and a ‘fast’ mode.

24. p = Ion acoustic waves in a negative ion plasma Fast Mode Slow Mode Notice that for the fast mode, the phase speed is >> ion thermal speed for large values of the negative ion percentage  this reduces, considerably the effects of ion Landau damping on the wave.

25. IAW in plasma with negative ions Phase velocity wave damping

26. electrostatic ion cyclotron (EIC) waves in a plasma with negative ions • EIC waves are fundamental low frequency (ion) modes of a magnetized plasma • they propagate nearly  to B, but with a finite • the mode frequency is just above the ion-cyclotron frequency, Wc+: • it is excited by an electron drift ved ~ (10-20) v+,th along the magnetic field • the critical electron drift speed needed to excite the mode is reduced in a negative ion plasma

27. 100 ms you cannot draw a dc current in a magnetized plasma electron current

28. Te = T+ = 0.2 eV, T- = 0.03 eV EIC modes in a plasma withK+ ions, electrons and C7F14– Negative ion EIC mode can be used as a diagnostic for the relative concentration of the negative ion.

29. B = 0.36 T P(C7F14) = 0 fo, + 10 dB f1, + 0 200 400 Frequency (kHz) B = 0.36 T P(C7F14) = 610-7 Torr fo,– f1,– f1, + fo, + Power spectra of EIC modes in a plasma with C7F14 POWER SPECTRA OF EIC MODES No C7F14 with C7F14 0 200 400 FREQUENCY (kHz)

30. C7F14 mode frequencies vs. B

31. At three minutes and four seconds after 2 AM on the 6th of May this year, the time and date will be 02:03:04 05/06/07. This will never happen again.