Download Plasma Seminar 4/23/07 "Negative Ion Plasmas"

Plasma Physics Seminar, April 23, 2007
NEGATIVE ION PLASMAS
Professor Robert L. Merlino
Department of Physics & Astronomy
University of Iowa
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
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
e
i ( 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   kCs
• 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) = 610 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.