Lecture 6.1Lecture 6.1ADVANCED PLASMA ADVANCED PLASMA

DIAGNOSTICDIAGNOSTICTECHNIQUESTECHNIQUES

Fri 23 May 2008, 1 pm LT5 Presented by Dr Ian Falconer

i.falconer@physics.usyd.edu.au

Room 101

ITER

Langmuir probes

Selected ITER diagnosticsSelected ITER diagnostics

DiagnosticDiagnostic MeasuresMeasuresMagnetic diagnostics Plasma current, position, shape, waves ..

Spectroscopic & neutral Ion temperature, He & impurity particle analyser systems density, ..........

Neutron diagnostics Fusion power, ion temperature profile, ….

Microwave diagnostics Plasma position, shape, electron density, profile, …..

Optical/IR(infra-red) systems Electron density (Line-average & profile, electron temperature profile, ….

Bolometric diagnostics Total radiated power, ….

Plasma-facing components & Temperature of, and particle flux operational diagnostics to First Wall, …..

Neutral beam diagnostics Various parameters

Processing plasmas

Selected low temperature Selected low temperature plasma diagnosticsplasma diagnostics

DiagnosticDiagnostic MeasuresMeasuresLangmuir probes Plasma potential, electron temperature

& density

Magnetic diagnostics Plasma current, plasma waves, ….

Spectroscopic Plasma composition, ion temperature & drift velocity, …….

Microwave diagnostics Plasma electron density, density profile, ….

Laser diagnostics Density etc.of various species in plasma

• Electrostatic probes (Langmuir probes)

• Magnetic probes

• Microwave and optical interferometry

• Spectroscopic techniques

• Particle analysis

• Thomson scattering

• Nuclear radiation detection

• Laser diagnostics of processing plasmas

PLASMA DIAGNOSTICS

General characteristics of a useful plasma diagnostic

• The diagnostic must not perturb the plasma – i.e. it must not change the conditions within the plasma

• Plasma diagnostics generally do not give the parameter(s) directly. An understanding of the physics of the processes involved in interpreting diagnostic results is essential

Electrostatic probes (Langmuir probes)

A short length of fine wire, inserted in a plasma can give valuable information about the plasma properties at a point in the plasma.

A Langmuir probe consists of such a short , thin wire inserted into the plasma: the current to/from the probe is measured as its potential is changed.

A sheath forms around the probe of thickness ~ Debye length

Current to sheath

For a Maxwellian velocity distribution

r S

r

S

j A

j

A

where random current density

surface area of sheath

1 2

1 124

2v e

re

kTj n e ne

m

But this applies ONLY if the potential of the probe is the same as that of the plasma.

How will the current to a Langmuir probe change if we use an external voltage source to change the probe’s potential?

A “typical” Langmuir probe characteristic

Typical probe characteristic: 1

A. VS is the space or plasma potential (the potential of the plasma in the absence of a probe). There is no E. The current is due mainly to the random motion of electrons (the random motion of the ions is much slower).

B. If the probe is more positive than the plasma, electronsare attracted towards the probe and all the ions are repelled. An electron sheath is formed and saturation electron current is reached.

X

Typical probe characteristic: 2

C. If the probe is more negative than the plasma, electrons are repelled (but the faster ones still reach the probe) and ions are attracted. The shape of this part of the curve depends on the electron velocity distribution. For a Maxwellian distribution with Te

> Ti, the slope of ln Ip

plotted against Vs is

e

e

kT

D. The floating potential Vf (an insulated electrode wouldassume this potential)The ion flux = the electron fluxso Ip = 0.

Typical probe characteristic: 3

E. All the electrons are repelled. An ion sheath is formed and saturation ion current is reached.

Probesurface

Magnetic probes

A voltage is induced by the changing magnetic field through this coil

dB

V NAdt

Integrating this voltage gives

0 NAB

VRC

Rogowski coil: measures plasma current

Voltage induced in this toroidal coil by the magnetic field passing through area A

0dI

V NAdt

Integrating 0 IV V dt NA I

Voltage loop: typically used to give the voltage induced in the plasma by the Ohmic heating transformer

A voltage is induced between the (open) ends of a (usually) single-turn loop adjacent to the plasma current. This voltage gives the voltage induced in the plasma by the transformer.

Measurement of induced voltage in plasma enable calculation of plasma conductivity – and hence temperature

Monitoring plasma position.

Coils inside and outside the plasma, and voltage loops above and below the plasma, give the position of the plasma within the toroidal vacuum vessel. Signals from these sensors are used for feedback control of the plasma position. But only for toroidal plasmas with a circulating current

– tokamaks.

Interferometry

1 0 2 0

0

sin sin

2 sin 2 cos 2t

E E t E E t

E E t

Consider these two beams of electromagnetic radiation

and

When combined with a phase difference they give a resultant electric field

When these combine

202 1 cosoutV E

d beams fall on a square-law detector the output

of the detector

higher-order terms

0 0

00 0

2

1

The phase shift of a beam of EM radiation passing through a plasm

where

The phase difference measured by an interferometer

Now for a plasma

a

plasma

k d d kc

k k d dc

2 2 2 20

2 20

2 2 2 20 0

1 1

2 2

2 1 1

1

1

now for (usual case for this diagnostic)

so that

p e

e

e e

n e m

n e m

n e m n e m

Thomson scattering

Thomson scattering is scattering off free electrons in the plasma. The electrons are set oscillating by the incoming laser beam, and then radiate as dipole radiators.

The intensity of the scattered radiation gives the electron density, the double-Doppler broadening of the scattered radiation gives the electron temperature.

Layout of a typical Thomson scattering experiment

The ITER LIDAR Thomson scattering system

• An array of non-perturbing diagnostic techniques has been developed to probe both fusion and “processing” plasmas

• Selection of an appropriate diagnostic depends on the nature of the plasma – and the relative cost of the diagnostics available

• Effective use of a diagnostic technique depends on a thorough knowledge of the physics of both the plasma and the diagnostic technique adopted

Conclusion