design of an infrared prism spectrometer for ultra-short bunch … · 2013-03-12 · design of an...
TRANSCRIPT
Design of an Infrared Prism Spectrometerfor
Ultra-Short Bunch Length Diagnosis.
Christopher Behrenson behalf of
Joe Frisch, Sasha Gilevich, Henrik Loos, and Jen Loos
SLAC National Accelerator Laboratory & Deutsches Elektronen-Synchrotron (DESY)
APE meeting, 30-11-2010
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 1 / 18
Outline
1 MotivationUltra-Short Electron Bunches and FEL PulsesDiagnostics and Limitations
2 Radiation Generation and DetectionCoherent Radiation DiagnosticsPyroelectric Detectors
3 Prism SpectrometerBasics and FormulasPotential MaterialsConfiguration and OpticsWavelength Calibration
4 Radiation Input CouplingViewport and Radiation TransportConfiguration and Optics
5 Summary and OutlookSummaryOutlookThe End
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 2 / 18
Motivation Ultra-Short Electron Bunches and FEL Pulses
Ongoing Tendency of Getting Shorter Electron Bunches• FEL science (user): shorter FEL pulses ⇒ better time resolution.
• Accelerator physics: lower charge ⇒ smaller emittance and less wake fields.
Efforts at Different Facilities• Theoretical Studies and Beam Dynamics Simulations: LCLS, E-XFEL, FLASH, ....
• University of Hamburg started a dedicated project on this topic.
• Experimental Studies: LCLS and FLASH.
• LCLC is already running in short pulse mode with 20 pC and 40 pC.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 3 / 18
Motivation Diagnostics and Limitations
It’s working in simulations and experiment, but ...• does it also work as predicted?
• how long are the electron bunches (FEL pulses)?
• what is the shape of the electron bunches?
Diagnostics for Short Bunch Lengths• Transverse Deflecting Cavities ’TCAV’: time domain.
• Resolution with S-band TCAV is not much better than 10 fs.
• X-band TCAV will give better resolution (higher frequency and gradient).
• Special diagnostics like the A-line experiment by Z. Huang et al. at SLAC/LCLS.
• Coherent radiation diagnostics ’CRD’: frequency domain.
Coherent Radiation Diagnostics• Bunch length comparable to emitted wavelength ⇒ coherent emission.
• Density modulation comparable to emitted wavelength ⇒ coherent emission.
• Coherent radiation contains information on the longitudinal bunch profile (form factor).
• Coherent Sources ’CxR’: CSR, CTR, CDR, CER, CUR, CSPR. (in the optical: ’COxR’)
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 4 / 18
Radiation Generation and Detection Coherent Radiation Diagnostics
Spectrum and Form Factor• Spectrum of single electron (’xR’): U0(λ).
• Incoherent spectrum of N electrons (’IxR’): N · U0(λ).
• Coherent spectrum of N electrons (’CxR’): N2 · U0(λ).
• In general: (N − N · |F(λ)|2 + N2 · |F(λ)|2) · U0(λ)
• Form factor F: Fourier transfrom of the normalized charge density (current) and 0 ≤ |F|2 ≤ 1.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 5 / 18
Prism Spectrometer Basics and Formulas
Prism and Geometry• Index of refraction ’n’ (wavelength dependency).
• Apex angle ’ǫ’.
• Incoming/outgoing angle ’α/β’.
• Total deflection angle ’γ’.
Prism Spectroscopy =̂ Symmetrical Path through Prism.
• γ = 2 · arcsin(n · sin(ǫ2
)) − ǫ
• Independent of incoming angle ’α’ and ’γ’ is a function of ’n’, i.e. of wavelength ’λ’.
• α1 = α2, β1 = β2, β = ǫ/2, and α = γ+ǫ
2
Dispersion =̂ Wavelength-Dependency
• Material dispersion: Dm = dndλ .
• Angular dispersion: Da = dγdλ = dγ
dndndλ = dγ
dn · Dm.
• Linear dispersion: Dl =dxdλ = dx
dγdγdλ = f · Da = f · dγ
dn · Dm. (focal length of a lens ’f ’)
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 6 / 18
Prism Spectrometer Basics and Formulas
Resolution by Diffraction Limitation• Beam waist at prism ’w0’.
• Prism base ’B’.
• Resolution power R = λ∆λ
= k∆k
• R = w0 · Da = w0 · dγdn · Dm = B · Dm
Wavelength Coverage• Detector range ’Ld ’.
• Wavelength integration {λmin, λmax} of Dl yields Ld = f · Bw0
· (n(λmin)− n(λmax)) = f · Bw0
·∆n.
• Wavelength coverage : ∆n(λmin, λmax) =Ldf · w0
B
Remarks Concerning Infrared Spectroscopy• Each application has its own best prism material.
• Infrared applications need special optics, detectors, and environment.
Be aware of abberation effects ⇒ needs corrections!
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 7 / 18
Prism Spectrometer Potential Materials
Transmission Characteristics
Zinc Selenide (ZnSe):• Transmission range: 0.6µm - 18µm
• Standard material for infrared applications.
• Standard viewports and prisms available.
Thallium Bromiodide (KRS-5):• Transmission range: 0.6µm - 40µm
• Standard material for infrared applications.
• Standard prism available.
• Custom viewports available, but ...
• high expansion rate (temperature) and
• senstive to humidity.
KRS-5 viewport
Have to check the exact working conditions (temperature and humidity) and vacuum safety issues!
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 8 / 18
Prism Spectrometer Potential Materials
Index of Refraction n(λ): ZnSe and KRS-5
• Wavenumber ’k’ is defined here as ’1/λ’.
• Index of refraction can be described by the empirical Sellmeier equation.
• Sellmeier equation: n2(λ) = 1+∑
iBiλ
2
λ2−Ci
.
• Sellmeier coefficients: Bi and Ci.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 9 / 18
Prism Spectrometer Potential Materials
Material Dispersion Dm = dndλ : ZnSe and KRS-5
Wavelength Ranges with higher Dispersion and better Resolution
• Short wavelenghts (. 2.5µm or & 4000 cm−1): KRS-5.
• Long wavelenghts (& 2.5µm or . 4000 cm−1): ZnSe.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 10 / 18
Prism Spectrometer Potential Materials
Normalized Resolution ∆λλ · w0 = D−1
a : ZnSe and KRS-5
Better Resolution ⇔ Smaller Wavelength Coverage• What do we need and want?
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 11 / 18
Prism Spectrometer Potential Materials
Wavelength Coverage (λmin, λmax): ZnSe and KRS-5
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
10
20
30
40
λmin
(µm)
λ max
(µm
)
Assumptions: f = 177.8mm, Ld = 12.8mm
ZnSe: 10.0°
ZnSe: 15.0°
KRS−5: 10.0°
KRS−5: 15.0°
Good tranmission starts at about 0.6µm
Possibilities• cover optical wavelengths (microbunching effects)?
• be flexible for longer bunch lengths or dedicated for short bunches?
• flexible design: change prism w/o changing optics?
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 12 / 18
Prism Spectrometer Configuration and Optics
Basic Design: by Sasha Gilevich (ZEMAX Simulation)
Components and Configuration• Using entrance slit.
• Off-axis parabolic mirrors (f = 177.8 mm).
• Prism: KRS-5 or ZnSe
• Detector: Pyroelectric line array with128 channels (pitch of 100µm).
• Detector tilt in order to correct abberation.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 13 / 18
Prism Spectrometer Configuration and Optics
Different Ranges and Focal Length: Apex of 10◦ (ZEMAX Simulation)
Wavelenght Ranges for 10◦ Apex• ZnSe: 0.6µm - 18µm.
• KRS-5: 0.8µm - 39µm.
Different Focal Length f1• First mirror: f1 = 101.6 mm.
• Collect more light from slit.
• Same or even better abberation correction.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 14 / 18
Prism Spectrometer Wavelength Calibration
Wavelength Calibration• Source with known wavelength and small spectral width: Laser (diode).
• Broadband source (thermal source or CTR) and filters: intensity ratio =̂ filter curve.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 15 / 18
Radiation Input Coupling Viewport and Radiation Transport
CVD Diamond as Alternative to KRS-5• Large range of good and flat transmission.
• Some absorption between 3µm and 6µm (interesting spectral range!).
• Not critical in terms of vacuum safety.
Water Absorption• Strong absorption in the interesting spectral range (useful for calibration!?!).
• Flushing with dry air or nitrogen (or vacuum but complex) helps a lot.
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 16 / 18
Radiation Input Coupling Configuration and Optics
Radiation Input Coupling: 2” mirrors and f1 = 101.6 mm
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 17 / 18
Summary and Outlook The End
Thanks for your Attention!
C. Behrens (SLAC & DESY) Infrared Prism Spectrometer. APE meeting 18 / 18