Design of an Infrared Prism Spectrometer for Ultra-Short Bunch … · 2013-03-12 · Design of an...

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


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.


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’)


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.


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 ’)


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!


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!



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.



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.



Normalized Resolution ∆λλ · w0 = D−1

a : ZnSe and KRS-5

Better Resolution ⇔ Smaller Wavelength Coverage• What do we need and want?



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?


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.


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.


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.


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.


Radiation Input Coupling Configuration and Optics

Radiation Input Coupling: 2” mirrors and f1 = 101.6 mm


Summary and Outlook The End

Thanks for your Attention!


Design of an Infrared Prism Spectrometer for Ultra-Short Bunch … · 2013-03-12 · Design of an...

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