Transcript OpticalDiagnostics-Interferometry

EFTS/EODI Training week 12th June 2009
Far Infrared diagnostics for fusion plasma
experiments
Alexandru Boboc
EFDA-JET, UKAEA, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK
12 June 2009
A.Boboc - FIR diagnostics for fusion plasma experiments
OUTLINE
 Principle of interferometry/polarimetry
 Why a FIR plasma diagnostic ?
 Large FIR devices around the world
 Requirements for future FIR diagnostics
 Development of FIR systems
 Components of a FIR system
 Operation and Maintenance
 Conclusions
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A.Boboc - FIR diagnostics for fusion plasma experiments
Optical properties of
magnetically confined plasmas
Refractive index
The refractive index (or index of refraction) of a medium is a measure
of how much the speed of light is reduced inside the medium
Optical activity
The property of a medium to rotate the polarisation plane of a
polarised light beam that propagates through that medium.
In the presence of magnetic fields all molecules have optical activity.
Birefringence (double refraction)
A medium is called birefringent if has two different indices of
refraction in different directions.
A light beam passing through this medium can be divided into two
components (an "ordinary" and an "extraordinary ray" ) that travel at
different speeds through that medium.
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A.Boboc - FIR diagnostics for fusion plasma experiments
Plasma frequency
A plasma has a characteristic frequency which can be understood by considering a
displaced sheet of electrons and the resulting electric field as shown below
The resulting motion of the electron constitutes plasma oscillations with a
characteristic plasma frequency wp
2
ne e
wp 
 0 me
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
GHz
range
where ne is the electron density,
me is the electron mass
e the elementary charge
0 dielectric constant of a vacuum
A.Boboc - FIR diagnostics for fusion plasma experiments
Cuf-off plasma density
For a fixed probing frequency w the critical (cut-off) density nc is defined as
the density which the probing frequency equals the plasma frequency and is
given by
nc 
 0 mew 2
e2
For densities below this critical value the medium acts as a nearly transparent
dielectric for the probing beam but the higher densities it is opaque and highly
reflecting
The angular frequency of an electron in a magnetic fields is called electron
cyclotron frequency.
wc 
qB
m

GHz
range
where q and m are the charge and mass of the particle and B the magnetic field.
An electromagnetic wave of this frequency injected in the plasma will be
resonantly absorbed.
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A.Boboc - FIR diagnostics for fusion plasma experiments
Why a FIR plasma diagnostic
Diagnostics using FIR beams are non-invasive as FIR wavelengths (100mm equiv.
with freq. of 3 THz) are far away from the plasma frequencies(GHz region) and the
beams do not disturb the plasma.
Plasma density and Magnetic fields inside a plasma via
interferometry and polarimetry techniques
Essential for safety of the plant, plasma
performances and real-time control of the plasma
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A.Boboc - FIR diagnostics for fusion plasma experiments
JET interferometer
Shifted
frequency *
Reference
Detector
Probe Detector
w
Reference
signal
z2
Plasma
ww
S probe  cos( wt  j (t ))
S reference  cos( wt )
t
z1
Probe
signal
w
time
Simple schematic of an interferometer
* the shift in frequency of the laser beam can obtained by using a Veron grating wheel or two
laser cavities with different lengths for example.
The probe laser beam that pass through the plasma suffers a phase shift j variation in
time. By subtracting the phase shift of the reference beam (due to vibrations) one can
obtain the phase shift due only to the plasma effects.
This phase shift is proportional to the line-integrated electron density ne along the
propagation direction inside the plasma. The phase change is usually many multiples of
2.What the diagnostic delivers is the number of fringes F of interference (1 fringe
represents 2 phase changes) .
z2
j
F
 C   n( z ) dz
2
z
1
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A.Boboc - FIR diagnostics for fusion plasma experiments
JET polarimeter
Faraday Rotation effect
The plane of linearly polarised light passing through a plasma is
rotated when a magnetic field is applied PARALLEL to the direction of
propagation.
Faraday Rotation angle
  2  ne B p|| dz
Where
ne plasma electron density
B, B|| magnetic fields components
Ip plasma current
FIR beam wavelength
12 June 2009
A.Boboc - FIR diagnostics for fusion plasma experiments
JET polarimeter
Cotton-Mouton effect
The ellipticity acquired by a linearly polarised light passing through a
plasma is dependent on the magnetic field PERPENDICULAR to the
direction of propagation.
Cotton-Mouton angle
    ne Bt dz
2
3
Where
ne plasma electron density
B, B|| magnetic fields components
Ip plasma current
FIR beam wavelength
At JET, for the vertical channels, Bt being largely constant along the line of sight is reducing the previous
equation in
   Bt
3
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2
 n dz
e
A.Boboc - FIR diagnostics for fusion plasma experiments
JET polarimeter
The half-wave plate at the entrance window is used
to set the required direction of the linear input
polarisation and, rotated to provide a calibration
measurement before each discharge.
The amplitudes of the beat signals are proportional to
the corresponding electric field vector amplitudes of
the electromagnetic wave in the local co-ordinate
system defined by the orientation of the wire grid in
front of the detectors:
p (t )  E y cos(w t  j )
(0)
i (t )  E x cos(w t )
(0)
PSD  p (t )  i (t )
RMS  i (t )  i (t )
PSP  p (t )  i (t )
RMP  i (t )  i (t )
i (t )  E x sin(w t )
(0)
is generated by phase shifting i (t ).
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PSD
 C 1 tan  cosj
RMS
PSP
R 
 C 1 tan  sin j
RMS  RMP
R 

j
A.Boboc - FIR diagnostics for fusion plasma experiments
JET polarimeter
Geometrical parameters  and c
(polarisation angle  and ellipticity tan c)
which describe the polarisation state of light
can be represented completely in function of
the characteristics of the electric field vector,
i.e. amplitude ratio tan  and phase shift
angle j that we measure.

j
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+
cos 2  cos 2c cos 2
tan   tan 2c / sin 2
tan 2  tan 2 cos
sin 2 c  sin 2 sin 

c
A.Boboc - FIR diagnostics for fusion plasma experiments
JET FIR
Interferometer / Polarimeter
•Optical path =80 m
•Thousands of optical
components
•FIR power = 200mW
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A.Boboc - FIR diagnostics for fusion plasma experiments
JET FIR Channels
Vacuum vessel
Measurements at JET
Magnetic flux distribution
1
2 3
4
5
6
7
8
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A.Boboc - FIR diagnostics for fusion plasma experiments
Measurements at JET
FIR interferometer/polarimeter
main parameters
Laser wavelengths: 195mm and 119 mm
FIR power: 200mW and 120mW
No channels: 8 (4 vertical, 4 lateral)
Time on: 16h/day during campaigns
Interferometer: Range: 1018 - 4x1022 m-2
(ne)
Accuracy: 3x1017 m-2
Time resolution : 1ms
10 ms(new)
Polarimeter:
(FAR)
Range: 0-70 deg
Accuracy:0.2 deg
Time resolution: 1 ms
Example of line-integrated density (ne) and Faraday
rotation angle measurements at JET
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A.Boboc - FIR diagnostics for fusion plasma experiments
Measurements at JET
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A.Boboc - FIR diagnostics for fusion plasma experiments
Measurements at JET
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A.Boboc - FIR diagnostics for fusion plasma experiments
Measurements at JET
MHD events by Fast data Interferometry (1MHz)
Example of detail obtainable for
core localised TAEs
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Signature of the active TAE antenna (3rd harmonic
frequency) of the TAE amplifier. In this example the
density fluctuations are approximately dn/n = 10-5 (two
order of magnitude smaller than the tornado modes)
A.Boboc - FIR diagnostics for fusion plasma experiments
Future
Next Generations of FIR diagnostics ?
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A.Boboc - FIR diagnostics for fusion plasma experiments
JET versus ITER
Plasma current = 4 MA
Plasma current = 15 MA
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A.Boboc - FIR diagnostics for fusion plasma experiments
Requirements for future FIR diagnostics
 High ambient temperatures
 Long pulse lengths and uninterrupted periods of operation
 High radiation and neutron fluxes
 Strong magnetic fields
 Very low or zero access to some parts of diagnostics
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A.Boboc - FIR diagnostics for fusion plasma experiments
Development of FIR systems
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Off site tasks
On site tasks
Proof of concept
Commissioning
Design
Operation
Manufacture
Maintenance
A.Boboc - FIR diagnostics for fusion plasma experiments
Components of a FIR system
• Wavelength
• Lasers
• Optics
• Modulators
• Detectors
• Mechanical structure
• Atmosphere control
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A.Boboc - FIR diagnostics for fusion plasma experiments
Laser wavelength ()
Long wavelength
Example
= 195 mm
Pros
Better resolution of measurementsFaraday = 5-20 deg (core ch.) (accuracy 0.2 deg)
Cons
Refraction ( ~2)
Cotton-Mouton = 5-20 deg (core ch.)
Refraction of few cm in worst case
Density 1 fringe(360deg phase) = 1x1019/m2
= 119 mm
Short wavelength
Pros
Low refraction
Cons
Mechanical Vibrations issues
Low resolution
Faraday = 1.31 - 3.915 deg (core ch.)
Cotton-Mouton = 0.6-2.7 deg (core ch.)
Refraction of 1-3 mm (good for interferometry)
BAD for JET polarimeter, well suited for ITER
Density 1 fringe(360deg phase) = 1x1019/m2
= 10 mm (CO2 laser)
No refraction –use for machine protection
Density 1 fringe(360deg phase) = 1x1021/m2
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A.Boboc - FIR diagnostics for fusion plasma experiments
FIR lasers
 The choose of the FIR lasers depends with the
wavelength to be used
 Very good stability required as these devices must
operate for long plasma pulses (laser technologies
developed extraordinarily during last 2 decades)
 Considerations at the design phase regarding initial
implementation and installation versus the long term
operation and maintenance
• 119mm FIR methanol laser is a laser with optical pumping that implies
controlling two laser cavities (FIR and pumping), careful optical coupling.
• 195mm FIR DCN laser needs regular and lengthy maintenance
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A.Boboc - FIR diagnostics for fusion plasma experiments
Input and Collection Optics
 Must operate in high ambient
temperatures, radiation, neutron fluxes
and magnetic fields
New materials needed
(at present for development of new mirrors
there are more than 10 institutions in the
fusion community involved)
No magnetic materials employed
Extra shielding required.
 Very low or zero access for handling
to some parts of diagnostics
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Different alignment schemes.
Remote handling scenarios.
Proper evaluation of risk of failures.
A.Boboc - FIR diagnostics for fusion plasma experiments
Input and Collection Optics
 Windows need to be radiation resistant (a lot of work is under progress)
 Wire-grids (wire diameter is around 10 mm) that are widely used for the FIR
devices as polarisers and beam splitters must be away from the heat-sources
 Very high sputtering on the in-vessel components changes the optical
properties (reflection, transmission) and causes damages
JET FIR interferometer in-vessel mirror (new)
4 years later
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A.Boboc - FIR diagnostics for fusion plasma experiments
Input and Collection Optics
Plug for pneumatic motor
(two copper pipes per motor)
 Heavy and robust optical mounts
Resin mirror mount
Tension spring (strong)
Micrometric adjustment screw
Example
Mirror used at EFDA - JET
for the FIR diagnostic
(about 10-20kg in weight)
Still working since 1983 !
Pneumatic motor (brass+copper)
Aluminium mirror holder with 2D movement
Heavy resin holder plate
Thick Mirror (3cm)
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A.Boboc - FIR diagnostics for fusion plasma experiments
Beam modulators
 FIR beams are amplitude or frequency modulated to facilitate detection.
 The higher the modulation, the better the temporal resolution
 There are different modulation techniques
Frequency controlled
choppers
Diffraction
wheels
Rotator stages with air
bearing
Twin-cavity length
modulation
Pros
•Very cheap
•Easy to implement
Pros
•High modulation
(300kHz)
•Very stable
Pros
•Medium modulation
(30kHz)
•Accurate modulation
Pros
•Very high modulation
(MHz region)
Cons
•Low modulation
frequency
Cons
Difficult to make
Break points for
the alignment
Cons
• Vibration and air-leak
control needed
Cons
• Difficult to maintain
the system stability
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A.Boboc - FIR diagnostics for fusion plasma experiments
Detectors
 The choice depends strongly on the modulation frequency and the power of
the laser beams as well as on required accuracy of the measurements
 Pyro-detectors are adequate for low temporal resolution measurements
 At very high modulation frequency or very low beam power the use of
cryogenic detectors becomes essential as they have very low NEP (system
noise equivalent power ) of 10-11WxHz -1/2
• At RFX polarimeter the pyro-detectors are involved as there are only 6
channels (200mW FIR power) and 3kHz modulation via a chopper.
• At Jet for example, the main FIR DCN with 200mW of laser power is
divided in 16 optical branches and the power level of the FIR beams that
reach the detectors are of the order of few mW.
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A.Boboc - FIR diagnostics for fusion plasma experiments
Mechanical structure
 FIR diagnostics require vibration level of the order of 1/10th of the wavelength
(structures must have large mass)
 For short wavelength (10-50 mm) the system needs active compensation
 At longer wavelength (100 -400 mm) the compensation is done generally with
reference channels for interferometer schemes for example.
 In large FIR devices the design of mechanical structures becomes very
complicated due to space constrains limitation and shared area between different
diagnostics.
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A.Boboc - FIR diagnostics for fusion plasma experiments
Atmosphere control
 FIR beams are strongly absorbed by the ambient air due to the presence
of water in particular ( a 200mW 119mm FIR beam is completely absorbed
in normal air after few meters of free propagation).
 Different gasses for purging a sealed optical system are available
(dry-air, nitrogen, argon etc)
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A.Boboc - FIR diagnostics for fusion plasma experiments
Operation and Maintenance
 Alignment/optimisation of the FIR beams is needed from time to
time(every 1-2 years) and often visible beams are used for this task.
On ITER visible beams cannot be used due to no-access areas
 Larger optics and space for temporary optics used for beams alignment
need to be allocated straight from the design phase as well as a clever
strategy for accessing some components for easy repair/removal.
 Duplication of components that needs often and/or lengthy maintenance
Management of Operation and Maintenance of a FIR system have big implications in
long term and sometimes this is not considered properly at the original design phase
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A.Boboc - FIR diagnostics for fusion plasma experiments
Conclusions
 The plasma has optical properties that can be used to measure key
parameters of the magnetically confined plasmas
 The FIR diagnostics technologies are now mature to be used in
ITER-like machines
 Reliable operation of FIR diagnostics will be an important
requirement on the path to the first commercial fusion reactor
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A.Boboc - FIR diagnostics for fusion plasma experiments
Thank you !
Any questions ?
12 June 2009
A.Boboc - FIR diagnostics for fusion plasma experiments