NEGATIVE ION PLASMAS

1
NEGATIVE ION PLASMAS
Plasma Physics Seminar, April 23, 2007

• Professor Robert L. Merlino
• Department of Physics Astronomy
• University of Iowa

2
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

3
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

4
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
5
Negative ions make goldfish grow !
6
OUTLINE

1. 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

7
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
8
The chemical physics of negative ions
A) Negative ion formation mechanisms (molecule
XYZ)
attachment
radiative stabilization
autoionization
IVR
dissociative attachment
9
B) Negative ion destruction mechanisms
mutual neutralization
photodetactment
collisional detachment
10
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

11
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 gt100 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

12
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 Hs 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

13
negative ions in the earths 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
14
Effect of rocket exhaust on the ionospheric plasma
artificially induced airglow caused by Challenger
engine burn on 29 July 1985
15
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

16
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-SiH) 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.

17
Interest in negative ion plasmas

• much or ordinary plasma behavior is dominated by
  the fact that me ltlt m
• but in a negative ion plasma we havene ltlt 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

18
e.g. sheaths in a plasma

• typically ve,th gtgt v,th ? electrons leave first
• plasma potential adjusts to maintain
  quiasi-neutrality ? SHEATH

19
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

20
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
21
IQ-3
22
Attachment cross sections
SF6 ? sulfur hexafluoride C7F14 ?
perfluoromethylcyclohexane
Low energy cross sections
23
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 gtgt me
• the reduction in electron current can be
  used to estimate n/n

24
comparison of results in SF6 and C7F14
in C7F14 can achieve ne/n lt 103
25
Langmuir probe floating potential
26
Ion acoustic waves in a negative ion plasma

• An e / ion plasma supports low frequency (f ltlt
  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.

27
Ion acoustic waves in a negative ion plasma
Fast Mode
Slow Mode
Notice that for the fast mode, the phase speed is
gtgt ion thermal speed for large values of the
negative ion percentage ? this reduces,
considerably the effects of ion Landau damping on
the wave.
28
IAW in plasma with negative ions
Phase velocity
wave damping
29
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

30
you cannot draw a dc current in a magnetized
plasma
electron current
31
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.
32
Power spectra of EIC modes in a plasma with C7F14
POWER SPECTRA OF EIC MODES
No C7F14
with C7F14
0 200
400
FREQUENCY (kHz)
33
C7F14 mode frequencies vs. B
34
At three minutes and four seconds after 2 AM on
the 6th of May this year, the time and date will
be 020304 05/06/07. This will never happen
again.