Transcript PlasmaPhys08OHeads D..

Lecture 6.1
ADVANCED PLASMA
DIAGNOSTIC
TECHNIQUES
Fri 23 May 2008, 1 pm LT5
Presented by Dr Ian Falconer
[email protected]
Room 101
ITER
Langmuir probes
Selected ITER diagnostics
Diagnostic
Measures
Magnetic diagnostics
Plasma current, position, shape, waves ..
Spectroscopic & neutral
particle analyser systems
Ion temperature, He & impurity
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 &
operational diagnostics
Temperature of, and particle flux
to First Wall, …..
Neutral beam diagnostics
Various parameters
Processing plasmas
Selected low temperature
plasma diagnostics
Diagnostic
Measures
Langmuir 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
PLASMA DIAGNOSTICS
•
Electrostatic probes (Langmuir probes)
•
Magnetic probes
•
Microwave and optical interferometry
•
Spectroscopic techniques
•
Particle analysis
•
Thomson scattering
•
Nuclear radiation detection
•
Laser diagnostics of processing plasmas
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
where jr
AS

jr AS
 random current density
 surface area of sheath
For a Maxwellian velocity distribution
12
jr

1 nve
4

1 n e  2kTe 


2
m

e

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, electrons
are 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
kTe
D. The floating potential Vf
(an insulated electrode would
assume this potential)
The ion flux = the electron flux
so Ip = 0.
Typical probe characteristic: 3
E. All the electrons are repelled. An ion sheath is
formed and saturation ion current is reached.
Probe
surface
Magnetic probes
A voltage is induced by the changing
magnetic field through this coil
V

NA
dB
dt
Integrating this voltage gives
V0

NAB
RC
Rogowski coil: measures plasma current
Voltage induced in this toroidal coil by the magnetic field
passing through area A
dI
V  NA0
dt
Integrating
VI

 V dt

NA0 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
Consider these two beams of electromagnetic radiation
E1
 E0 sin  t  and
E2
 E0 sin  t   
When combined with a phase difference  they give a resultant electric field
Et
 2 E0 sin  t   2  cos  2 
When these combined beams fall on a square-law detector the output
of the detector
Vout
 2 E02 1  cos  

higher-order terms
The phase shift of a beam of EM radiation passing through a plasm a
 

0
kd


0


c
d
where k

2

The phase difference measured by an interferometer


 k
0
plasma
 k0  d


   1 c d
0
Now for a plasma

2
now for
 1   2p  2
 1  ne e 2  2 m 0
ne e 2  2 m 0
1
 1
  1  ne e 2  2 m 0
2
(usual case for this diagnostic)
so that


1
2
ne e 2  2 m 0
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
Conclusion
•
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