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Plasma Physics Seminar, April 23, 2007 NEGATIVE ION PLASMAS Professor Robert L. Merlino Department of Physics & Astronomy University of Iowa Acknowledgements • • • • • • Nick D’ Angelo Dave Suszcynsky Bin Song Jackson Loomis Tao An Q-Z Luo • • • • • • Su-Hyun Kim Ross Fisher Mike Miller V. Nosenko ONR, NASA NSF, DOE 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 WebMD Negative Ions Create Positive Vibes There's something in the air that just may boost your mood -- get a whiff of negative ions. Once they reach our bloodstream, negative ions are believed to produce biochemical reactions that increase levels of the mood chemical serotonin, helping to alleviate depression, relieve stress, and boost our daytime energy Negative ions make goldfish grow ! NI OUTLINE I. Introduction A. the chemical physics of negative ion formation B. examples of negative ion plasmas (1) (2) (3) (4) neutral beam sources photosphere of the Sun D region of the ionosphere plasma processing reactors 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 The chemical physics of negative ions A) Negative ion formation mechanisms: (molecule XYZ) XYZ energy XYZ e attachment ( XYZ )* radiative stabilization XYZ autoionization XY Z dissociative attachment IVR B) Negative ion destruction mechanisms X Y X Y * X h X e * X Y X Y e mutual neutralization photodetactment collisional detachment 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 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 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 negative ions in the earth’s ionosphere • 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 Data from rocket borne instruments N N Effect of rocket exhaust on the ionospheric plasma artificially induced airglow caused by Challenger engine burn on 29 July 1985 electron depletion experiments in space • electric electron density cm-3 field mV/m 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 time (sec) 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. 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 have ne << 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– plasma potential e.g. sheaths in a plasma sheaths position • typically ve,th >> v+,th electrons leave first • plasma potential adjusts to maintain quiasi-neutrality SHEATH Production of negative ion plasmas • introduce an electronegative gas into a plasma, e.g., SF6 SF SF6 e SF6* 6 SF5 F • attachment cross sections are highly energy dependent • F is highly corrosive 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 IQ-3 Attachment cross sections SF6 sulfur hexafluoride C7F14 perfluoromethylcyclohexane Low energy cross sections Source: Asundi and Craggs Proc. Phys. Soc. 83, 611, (1964) 10 10-14 -15 CF 10 10-15 7 14 -16 C7F 14 10 10 -17 -16 SF 6 SF 6 10 10 10-17 -18 10-18 0.001 -19 0 5 10 15 20 0.01 0.1 25 Electron energy (eV) Energy (eV) SF6_C7F14_XS_dat 1 reduction in the electron density as the 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+ comparison of results in SF6 and C7F14 in C7F14 can achieve ne/n+ < 10–3 Langmuir probe floating potential I V Vf Vp 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 kTe 2n 1 2n 2 2 , where Cs is the ion acoustic speed 2 x Cs t M • 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. Ion acoustic waves in a negative ion plasma Fast Mode Slow Mode p= 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. IAW in plasma with negative ions Phase velocity wave damping n ~ ei ( kx t ) , with k kr iki , and real so that n ~ e ki x ei ( kr x t ) 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 k • the mode frequency is just above the ion2 2 2 2 cyclotron frequency, Wc+: Wc kCs • 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 electron current you cannot draw a dc current in a magnetized plasma 100 ms n 30% n EIC modes in a plasma with K+ ions, electrons and C7F14– Te = T+ = 0.2 eV, T- = 0.03 eV Negative ion EIC mode can be used as a diagnostic for the relative concentration of the negative ion. Power spectra of EIC modes in a plasma with C7F14 POWER SPECTRA OF EIC MODES B = 0.36 T P(C7F14) = 0 No C7F14 fo, + 10 dB f1, + 0 fo,– f1,– B = 0.36 T -7 P(C 200 7F14) = 610 Torr 400 Frequency (kHz) fo, + 0 200 FREQUENCY (kHz) f1, + 400 with C7F14 C7F14 mode frequencies vs. B 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.