Experimental Optimization of Electron Beams for …pitz.desy.de/sites2009/site_pitz/content/e145700/e237560/e237561/...Experimental Optimization of Electron Beams for Generating THz

Prach Boonpornprasert

DPG-Frühjahrstagung 2017

TU Dresden, Dresden

22.03.2017

Experimental Optimization of Electron Beams for

Generating THz CTR and CDR with PITZ

Outline

► Introduction

► Optimization of Electron Beams

► Calculations of CTR/CDR

Pulse Energy

► Summary & Outlook

Experimental Optimization of Electron Beams for Generating THz CTR and CDR with PITZ

Prach Boonpornprasert | DPG-Frühjahrstagung 2017 | 22.3.2017 | Page 2

PITZ Facility

Photocathode

RF Gun Booster (Linac)

Deflecting Cavity

0 m

PITZ Beamline Layout

~21.7 m

► The Photo-Injector Test facility at DESY

Zeuthen site (PITZ).

► Develop, study and optimize high

brightness electron sources for linac-

based FELs.

► Working closely with FLASH and the

European XFEL.

► 2 UV photocathode laser systems

■ Cylindrical pulse shape

(Gaussian, flat-top, comb-like)

■ 3D-ellipsoidal pulse shape

Important Parameters

Beamline length ~22 m

Cathode laser pulse

duration

few ps to ~22 ps

(FWHM)

Electron bunch charge sub pC to > 5 nC

Maximum electron

beam momentum ~24 MeV/c

Quadrupole magnet

Dipole magnet

Screen

~7 MeV/c ~22 MeV/c

HEDA2

Adjustable by using laser pulse shaper



Motivations for Studies of IR/THz Production at PITZ

IR/THz sources

• Conventional laser

• Accelerator based

Pump & Probe

experiment

X-ray

IR/THz

Requirements of the IR/THz Pulses

Same time structure and repetition rate

as those of the X-ray pulses

(XFEL 27000 pulses / sec)

Possible to have precise synchronization

with each of the X-ray pulses

High stability (intensity and phase)

High (various) pulse energy (µJ - mJ)

Wide wavelength tunability:

from 15 µm (20THz) to 3 mm (0.1 THz)

Variety of temporal and spectral patterns

Variety of polarizations

PITZ is an ideal facility for

the development of a prototype of

such accelerator based IR/THz source

Strengths

Over a decade of experience in high brightness photo

injector research and development.

PITZ has same type of electron source as EXFEL

same time structure of radiation pulse merit for precise

synchronization.

The site of a PITZ-like setup is small enough to fit in the

experimental hall for the EXFEL users. short IR/THz

transport

Born to be a test facility, the beam time and the

accelerator are adaptable to new research ideas and

proposals.

3 methods of radiation production have been studied.

Single-pass FEL SASE FEL for λrad ≤ 100 µm (f ≥ 3 THz)

Coherent Transition Radiation(CTR) for λrad ≥ 100 µm (f ≤ 3 THz)

Coherent Diffraction Radiation(CDR) for λrad ≥ 100 µm (f ≤ 3 THz)

Progresses on CTR and CDR studies

are presented in this presentation.



CTR and CDR

E-beam

CTR radiator

𝜃

Backward

transition

radiation

𝜃

E-beam

CDR radiator

Backward

diffraction

radiation

Coherent Transition Radiation

(CTR) Coherent Diffraction Radiation

(CDR)

► Backward TR energy (𝑼𝑻𝑹) emitted in the

frequency range 𝒅𝝎 into the solid angle 𝒅𝛀

can be calculated by Ginzburg-Frank Formula.

► CTR energy from an electron bunch can be

expressed as

𝒅𝟐𝑼𝑪𝑻𝑹𝒅𝝎𝒅𝜴

𝒃𝒖𝒏𝒄𝒉

∝ 𝑵𝟐𝑭𝒍𝒐𝒏𝒈 𝝎𝒅𝟐𝑼𝑻𝑹𝒅𝝎𝒅𝜴

𝟏−𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒏

where 𝑵 is number of electrons in the bunch and

𝑭𝒍𝒐𝒏𝒈 𝝎 = 𝝆𝒍𝒐𝒏𝒈 𝒕 𝒆−𝒊𝝎𝒕𝒅𝒕

+∞

−∞

𝟐

is longitudinal form factor of the electron bunch.

► The DR energy (𝑼𝑫𝑹) from a circular aperture

radius 𝑟 can be calculated by

𝒅𝟐𝑼𝑫𝑹𝒅𝝎𝒅𝜴


= 𝑫 𝝎, 𝒓𝒅𝟐𝑼𝑻𝑹𝒅𝝎𝒅𝜴


where 𝑫 𝝎,𝒓 is the correction for DR.

► CDR energy from an electron bunch can be

expressed as

𝒅𝟐𝑼𝑪𝑫𝑹𝒅𝝎𝒅𝜴

𝒃𝒖𝒏𝒄𝒉

∝ 𝑵𝟐𝑭𝒍𝒐𝒏𝒈 𝝎𝒅𝟐𝑼𝑫𝑹𝒅𝝎𝒅𝜴




Optimization of Electron Beams

Machine Parameters

Laser diameter on the cathode 2.0, 2.5 mm

Peak E-field in gun 60 MV/m

Peak E-field in booster 17.2 MV/m

Gun phase* 0 degree

Booster phase* 0 to -60 degree

*w.r.t. Maximum Mean Momentum Gain phase

Short Gaussian Beam Optimization

► Illuminated the cathode with a Gaussian

laser pulse with temporal length of ~2.4 ps

FWHM.

► Velocity bunching Went off-crest

booster phase for lower momentum at the

head and higher momentum at the tail of the

electron bunch.

► Measured the longitudinal profile by using

the deflecting cavity,

► Experiments were done with bunch charges

of 20 pC, 100 pC and 250 pC.

Comb-like Beam Optimization

► Illuminated the cathode with a comb-like

laser pulse. In this experiments, the laser

pulse shaper was adjusted for a comb-like

laser pulse with 6 teeth.

► Also went off-crest booster phase for

velocity bunching.

► Measured the longitudinal profile by using

the deflecting cavity,

► Experiments were done with bunch charges

of 100 pC, 250 pC and 500 pC.

Corresponding

<Pz> ~ 22 to 15 MeV/c



Examples of Optimized Bunch Profiles

Longitudinal Bunch Profiles

Corresponding Form Factors

Short Gaussian Beams Comb-like Beams

Longitudinal Bunch Profiles

Corresponding Form Factors

<Pz> ~ 14MeV/c <Pz> ~ 21MeV/c



Calculations of CTR/CDR Pulse Energy

Radiation Pulse Energy vs Ω vs f

Short Gaussian beam, 250 pC

Comb-like beam, 250 pC

CTR

CTR

CDR

CDR

CTR

CDR

Short Gaussian beam

Comb-like beam

Total Radiation Pulse Energy



Design of the CTR/CDR station

P = off-axis parabolic mirror

F = flat mirror

► The station is placed ~13 m downstream

from the cathode.

► Acceptance angle from the radiator to

the viewport is 0.4 rad.

► The viewport is made of z-cut crystal

quartz.

► THz radiation diagnostics system is

placed in the tunnel, normal room

environment.

► The system will be used to measure:

■ Radiation energy/power

■ Radiation spatial profile

■ Radiation polarization

■ Radiation spectrum (Michaelson

interferometer)

► The detectors is pyroelectric detector. Acknowledgement: G.Koss, S.Philipp



Summary and Outlook

Summary

►Optimizations of electron beams (Short Gaussian and Comb-like) for

CTR/CDR were done.

►The calculated radiation has pulse energy in the range of sub µJ within

the frequency range of 0.01-0.4 THz.

►The bunch lengths are still too long to cover THz frequency.

►Design, machining and installation of CTR/CDR station is ongoing.

Outlook

►First experimental generation of (sub)THz CTR/CDR at PITZ is planned

to take place in May-Jun 2017

►Ways to achieve shorter bunch length

■ Sub-ps cathode laser pulse from the 3D-ellipsoidal laser system

■ Bunch compressor



Backup Slides



Experiments of Velocity Bunching



Comb Beams: Profiles and Form Factors

Fo

rm f

acto

r F

orm

fa

cto

r F

orm

fa

cto

r

FFT

FFT

FFT



Photo Injector: RF-Gun

RFgun:

L-band (1.3 GHz)

nc (copper)

standing wave

1½-cell cavity

Main solenoid,

Bz_peak~0.2T

Bucking solenoid

Photo cathode

(Cs2Te)

QE~0.5-5%

Coaxial RF

coupler

Cathode laser

262nm

20ps (FWHM)

Vacuum

mirror

Electron bunch

<1 pC – 5 nC,

~5-7MeV

UHV

RF gun

• L-band 1.6-cell copper cavity

• Cs2Te photocathode (QE~5-10%)

• Dry ice cleaning low dark current (<100uA@6MW)

• LLRF control for amplitude and phase stability

• Solenoid for emittance compensation



Variation of the pulse shape by using a single different

Lyot filter (UV, measured with OSS)

Edges of the flat-top pulses are slightly shorter than

FWHM of the Gaussian pulse (measured without shaper)

“Smoothening” of the Modulations in the flat-top

region of the pulse

Lyot: 6 mm YLF Lyot: 4 mm YVO4 no Lyot filter

FWHM

= 2.5 ps

FWHM

= 4 ps

FWHM

= 7 ps

FWHM

~ 16 ps

edges ~ 2ps edges ~ 3ps

2 4 ps

2 5 ps 2 5 ps 2 5 ps 2 5 ps

2 5 ps 2 5 ps 2 5 ps 2 5 ps

with

shaper

(13

crystals)

without

shaper

Lyot: 16 mm YLF

~ 2 3 ps

edges ~ 6 ps

~ 19 ps

~ 14 ps

I. Will, G. Klemz „Increasing the flexibility in pulse shape of a Yb:YAG

photocathode laser” 20.06.2009

V

Laser temporal profile for high TR PWA experiment



Characterizations of 4 nC Beams for SASE FEL

Options (11.2016)

Measured slice emittance

Calculated SASE FEL radiations

using the measured beam parameters

Measured longitudinal phase

space (LPS) Measured slice

momentum spread

Helical undulator with

period length of 40 mm

<Pz>~15MeV/c

<Pz>~22MeV/c

► The measurements were done with beam momenta of about 15 and 22 MeV/c

for λFEL of 100 and 20 µm, respectively.

► Longitudinal profiles were measured by using transverse-deflecting cavities.

► The measured parameters were used as input of the GENESIS1.3 code for

FEL calculations.

Experimental Optimization of Electron Beams for …pitz.desy.de/sites2009/site_pitz/content/e145700/e237560/e237561/...Experimental Optimization of Electron Beams for Generating THz

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