NEGATIVE ION PLASMAS

Professor Robert L. Merlino

Department of Physics & Astronomy

University of Iowa

Plasma Physics Seminar, April 23, 2007

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) neutral beam sources (2) photosphere of the Sun (3) D region of the ionosphere

(4) 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 e XYZ attachment

autoionization

XYZ energy

XYZ

radiative stabilization

IVR

XY Z dissociative attachment

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

N

N

Data from rocket borne instruments

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 • 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)

ele

ctro

n

den

sity

ele

ctri

c fie

ld

mV

/mcm

-3

negative ions in plasma processing

• 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.

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.

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–

e.g. sheaths in a plasma

• typically ve,th >> v+,th electrons leave first

• plasma potential adjusts to maintain quiasi-neutrality SHEATH

plas

ma

pote

ntia

l

sheaths

position

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

6*

6 6

5

SFSF e SF

SF F

Q machine

SF6

grid for launching IA waves

K+ or Cs+ plasmas, nearly fully ionizedTe = T+ 0.2 eV

n+ ~ 108 – 1011 cm-3

IQ-3

Attachment cross sections

10-19

10-18

10-17

10-16

10-15

0 5 10 15 20 25

SF6_C7F14_XS_dat

Energy (eV)

SF6

C7F

14

Source: Asundi and Craggs Proc. Phys. Soc. 83, 611, (1964)

Low energy cross sections

10-18

10-17

10-16

10-15

10-14

0.001 0.01 0.1 1

C7F

14

SF6

Electron energy (eV)

SF6 sulfur hexafluorideC7F14 perfluoromethylcyclohexane

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+

comparison of results in SF6 and C7F14

in C7F14 can achieve ne/n+ < 10–3

Langmuir probe floating potential

V

I

VpVf

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.

2 2

2 2 2

1, e

s

s

kTn nwhere C is the ion acoustic speed

x C t M

Ion acoustic waves in a negative ion plasma

p =

Fast Mode

Slow Mode

Notice that for the fast mode, the phase speed is >> ion thermalspeed for large values of the negative ion percentage thisreduces, considerably the effects of ion Landau damping on the wave.

IAW in plasma with negative ionsPhase velocity wave damping

( )

( )

~ , ,

~ i r

i kx t

r i

k x i k x t

n e with k k ik and real

so that n e e

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, c+:

• 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

k

2 2 2 2

c sk C

you cannot draw a dc current in a magnetized plasma

100 s

30%n

n

elec

tro

n curr

ent

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.

Te = T+ = 0.2 eV, T- = 0.03 eV

Power spectra of EIC modes in a plasma with C7F14

No C7F14

with C7F14

0 200 400 FREQUENCY (kHz)

POWER SPECTRA OF EIC MODES

B = 0.36 T P(C7F14) = 0

B = 0.36 T P(C7F14) = 610-7 Torr fo,– f1,–

fo, + f1, +

fo, +

f1, +

10 dB

0 200 400 Frequency (kHz)

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.