Secondary Electron Emission Detectors for Neutral Beam Characterization on C-2W

Thomson scattering systems on C-2W field-reversed configuration plasma experiment
K. Zhai, T. Schindler, A. Ottaviano, H. Zhang, D. Fallah, J. Wells, E. Parke, M. C. Thompson, and the TAE Team
TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610
1. Overview of C-2W Thomson Scattering
•
•
Thomson scattering experiment
• Thomson scattering process is that when laser fires into plasma, plasma electrons oscillate in the laser field and radiate.
• Te : width of scattered spectrum • ne : intensity of scattered light
Typical laser scattering spectrum at 90° scattering angle for Nd:YAG (1064nm)
C-2W Thomson scattering system overview
• Two sub systems: for FRC plasma in middle plane and for mirror plasma in open field region.
• Whole System is supported from concrete floor, isolated from vessel vibration.
• Entire beam path is enclosed in opaque solid material.
• A temperature controlled room for lasers and beam transportation components.
A temperature controlled room for polychromators.
• Most components, e.g. lasers, collection optics, polychromators, have been bench tested and
calibrated in lab before final installation.
• Rayleigh scattering of argon gas for system intensity calibration.
Jet system: Collection optics
Center system: Collection optics
Fiber bundles duct
Center system: Alignment camera
Laser room
Laser room
Center system: Alignment camera Brewster window Beam dump
Center system Scattering region
Jet system: Alignment camera Beam dump
Jet system: Beam path
Center system: Beam path
Center system: Brewster window 3m focusing lens
Jet system: Alignment camera Brewster window 2m focusing lens
2. System Components, Installation, and Alignment
• Nd:YAGlasersystems
• Temperature controlled laser room for stable laser operation and safety control.
• Switchable beam paths between the central and jet systems.
• GuideSTAR II automatic alignment system
Central laser
Central laser system:
• Nd:YAG at 1064 nm
• 30/6/4 pulses at 1KHz/13kHz/20kHz
• 2J per pulse
• Divergence <= 0.5mrad
• Pointing stability <= 100μrad
Jet laser system:
• Nd:YAG at 1064 nm
• 4 pulses at 100Hz
• 2J per pulse
• Divergence <= 0.5mrad
• Pointing stability <= 100μrad
Jet laser
• Collectionlensandfiberbundles
• ZEMAX model of collection optics
• Collection optics installation
Central TS
Jet TS
Radial profile at r=
[-9.33, -5.33, -1.33, 2.67, 6.67, 10.67, 14.67, 18.67, 22.67, 26.67, 30.67, 34.67, 39.92, 47.92, 55.92, 63.92] cm
[-10 , -5, 0, 5, 10] cm
Collection length
2.5 cm
1.5 cm
Estimated photons
1.6×105
3.5×105
Units are μm. Legend items refer to Wavelengths
Field : 1 2
RMS radius : 67.717 74.692
Scale bar : 1000 Reference : Centroid
Units are μm. Legend items refer to Wavelengths
Field : 1 2
RMS radius : 49.066 96.167
Scale bar : 400 Reference : Centroid
•
•
• • •
Fiber bundle and its back projected image along beam path
Fiber bundle:
• 200/220-core/cladding silica
• 115 fiber in one bundle
• 30 meter long
• AR coating from 850-1070
• NA=0.26+/-0.2
Spectrum dispersion, detection, and data system
Temperature controlled room for polychromators, control units, and digitizers Polychromators are water cooled
12 16bit/1MHz-sampling-rate/64-channel D-tAcq digitizer
Integrated signal with 35 laser pulses from polychromator for measurement at r = -1.33cm Bottom: Expanded view of the TS signal from the 6 burst pulses
3. Data Analysis
• Scatteredsignalanditsvariance
• From the APD preamplifier diagram shown below, the signal from the AC channel Signalpulse
and its variance can be expressed as
!!"#$
!"#$%! = ! ! +!
!
! −! !+! −! !+! !"=! +!
−! =! !"#$%# !"
a)
!!"!!" = !!" ∗ !!!!" in which Sts-ch is TS signal in poly channel ch, Ne-ch is number of photon electrons, Cch is combined gain of APD and the following circuit. are APD quantum efficiency and channel filter transmission
!"#$%
!"#$"%&!!"#$% = !!" + 2!!"#$%#
!"
!"#$%#
!" !"#$%#
!" !"#$%
!"#$%# !"#$%
b) !"#!!"!!" = !!"#$% ∗ !"# !!−!" ∗ !!"! = !!"#$% ∗ !!−!" ∗ !!"! in which VarSts-ch is variance of Sts-ch and Fnoise =4.9 is APD enhanced noise factor
Diagram of APD amplifier unit
c) The spectral calibration of polychromator channel response Rch can be expressed as !!" !! = (! !! ∗ !!"# !! ∗ !!")/(!"/!!) in which Qapd and T
d) Photon electron number can be obtained from TS scattering power spectrum Pts as !!!!" = !!"# ∗ !!" =
!
!!" !! ∗!∗!!! ∗!"∗!∗!! ∗!!"!#$% ∗! !! ∗!!"#
!" !!
• Combined a), b), c), d), and Rayleigh calibration coefficient Cray representing the system efficiency Fsystem,
! ∗Δ! ∗ !
! = ! !! ! ! ! !"#
!"#!"!!"=
!"
!!" !! ∗!!" !! ∗!!" !! !!
!!! ∗!!"#
∗!!"#$%∗
! ∗Δ! ∗ ! ! !
!"#
!"!!!
!" !! !!!"# !!"#!!! !! ∗ !Ω
!
!
! ∗!!!"# !"#−!h 0 !Ω
• With the calibration ratio between the gain of AC and DC channel, DC signal can be converted to equivalent AC channel photon number, from which the signal variance due to background plasma
radiation can be estimated as
!"#$%&#"!" ∗ !" !
!"#$%&#"!" ! ∗ !"
!" !!" ∗ !
! !"#$%#
=
!" !"#
!!"
! !!!"#$#%
=
!"#$%&#"!" ∗ !
!" !"#
!"#$
∗ !"#$%&!" !"
• Maximumlikelihoodalgorithm
• For a test Te and ne, the signal level in each polychromator wavelength channel Sts can be calculated as
!
above. If the experimental signal is S exp , the likelihood of test Te and ne being the actual Te and ne is
givenas
L = ch
(S −Sexp)2 ts ts ts
1 e2*Variancepulse , and total likelihood is Ltotal = Πch Lch . (2π *Variancepulse )1 2
•
Te and ne can be obtained when maximizing Ltotal
Error bar is obtained when Ltotal is reduced by e1/2 which is 1-σ point
An example
• Te and ne profiles evolution in one shot
• Profile evolution with laser firing 29 pulses at 1 kHz combined with one burst of 6 pulses at 13 kHz
From 2 days of experimental time down to single shot
4. Summary
• Central TS system measures Te and ne profiles at 16 radial locations with up to 35 temporal snapshots at up to 20 kHz. The system has now been installed, commissioned, and operational.
• A maximum likelihood algorithm has been applied for data analysis. Te and ne profiles evolution data are available after each shot.
• Jet TS system construction is underway.