Possibility of THz Light Generation by using SW/TW Hybrid Photoinjector

11/16-19, 2009, HBEB, MauiAtsushi Fukasawa, James Rosenzweig,

David Schiller, UCLA, Los Angeles, CA, USA

Hybrid of Standing-Wave and Travelling-Wave Structures

Produce short bunches without a magnetic chicane.

Bunching vs. Injection Phase

Peak Current Bunch Form Factor @1THz

EmittanceQ = 500pC

-The peak current reaches as high as 2.3 kA.- The bunch form factor at 1 THz is 0.43.- The emttance will get worse by increasing bunching force.

Full Bunching CaseCharge 500 pC

Energy 21.04 MeV

Xrms, Yrms 1.48, 1.48 mm

Emitx,y 2.1, 2.1 mm.mrad

Trms (T_FWHM),Erms

210 fs (54 fs), 1.3%

Bunch Form Factor (1 THz) 0.18

Emit (t-bg) 2.74 ps

Strong energy modulation by the space charge.

Strong spike.(54 fs, FWHM) Bad around the spike.

Large energy modulation.

“Swan” shape shows strong modulation due to space charge.

3.7 mm.mrad

Bunch shape

Slice emittance

t-E phase space Energy Spectrum

Bunch Form Factor

Coherent Cherenkov Radiation

- The mode whose vph = vb will be excited well.

- b-a is most important to determine the resonant frequency.

- The energy of the beam does not matter.

CCR Experiment at UCLA

a = 250 mm

Fourier Transform

Beam: Q=200pC, st=270fs, E=10MeVDielectric tube: er=3.8 (SiO2), L=1cm

OOPIC SimulationsParameters: 1 THz case Parameters: 1.5 THz

case

Rms beam length 100 μm Rms beam length 50 μm

Rms beam radius 30 μm Rms beam radius 22 μm

Beam total charge Q 1 nC Beam total charge Q 0.5 nC

Fundamental frequency 1 THz Fundamental frequency 1.5 THz

Bunch Form Factor 0.012 Bunch Form Factor 0.085

Outer radius b 115 μm Outer radius b 84.6μm

Inner radius a 77 μm Inner radius a 60.2μm

Peak power 21 MW Peak power 52 MW

Peak long. electric field 294 MV/m Peak long. electric field 619 MV/m

Pulse length 69 ps Pulse length 60 ps

Total CCR energy 1.47 mJ Total CCR energy 1.55 mJ

Scaled to the beam from the hybrid photoinjector

Beam total charge Q 0.5 nC Beam total charge Q 0.5 nC

Fundamental frequency 1 THz Fundamental frequency 1.5 THz

Bunch Form Factor 0.18 Bunch Form Factor 0.042

Peak power 79 MW Peak power 26 MW

Peak long. electric field 570 MV/m Peak long. electric field 310 MV/m

Total CCR energy 5.5 mJ Total CCR energy 1.1 mJ

Coherent Edge Radiation

- The edge radiation is produced at the interface of the dipole field: the entrance and the exit. - The property of the edge radiation is similar to that of the transition radiation; the radial polarization and the hollow distribution.

QUINDI

Flow chart of QUINDI

QUINDI is a first principles beam diagnostics simulator which calculates the radiative spectrum from a relativistic electron bunch passing through a magnetic array. (From QUINDI’s home page.)

Postprocess on Matlab.

Lienard-Wiechelt potentials

Parallelized with MPI

HDF5

QUINDI Example

G. Andonian et al., ”Observation of coherent terahertz edge radiation from compressed electron beams”, Phys. Rev. ST AB 12, 030701 (2009)

peaks coincide.

CER spectra

CER, coherent edge radiation

CSR, coherent synchrotron radiation

Coherent THz Edge Radiation

- Measured at BNL ATF.- Used vertical four-bend magnet array.(f = 20o and r = 1.2 m, or B = 1.7kGauss at 61 MeV)- Bunch length = 100 – 150 fs.- Initial distribution for QUINDI input: UCLA PARMELA and Elegant

Vertical Chicane

QUINDI Example (continued)Polarization of CER + CSR

p-polarization of CER

Far-field radiation intensity distribution of CER + CSR with various polarizer’s angles.

Dots: measured.Solid line: QUINDI.

Color map: measured.Black contour line: QUINDI.

Modest agreement.

G. Andonian et al., ”Observation of coherent terahertz edge radiation from compressed electron beams”, Phys. Rev. ST AB 12, 030701 (2009)

CSR: s-polarizationCER: s- + p-polarization

CER at NEPTUNE

PMQ (Triplet)

PM Dipole (90 deg)

- 90-deg bending (r = 4 cm) enables to watch the radiation mainly from CER.- CER signal was as large as Cherenkov radiation at the beam dump.- Radial polarization, which is characteristic to CER was observed.

CER Spectrum

Polarization property

CER

CER + CSR

Beam (11 MeV)

CER

Other Method

Beam Lorentz factor g 44Rms bunch length 100 mmRms bunch width 200 mmUndulator wavelength 6.9 cmNumber of undulator periods 15Undulator strength K 5.6On-axis radiation wavelength 299 mm (1 THz)

Super-radiant FEL

QUINDI is updating to solve this problem.

Summary

- SW/TW hybrid photoinjector can produce the high brightness beam.Q = 500 pC, ex=2.1 mm.mrad, trms = 210 fs, bunch form factor @1THz = 0.18.

- Coherent Cherenkov radiation was simulated. (Scaled from OOPIC results)@1THz: P = 79 MW (5.1mJ), Ez = 570 MV/m@1.5THz: P = 26 MW (1.1mJ), Ez=360MV/m

- Coherent edge radiation was being investigated from the experiment and simulation.

- QUINDI is being developed.- Demonstrated at BNL ATF, and being commenced at Neptune, UCLA.

- Super-radiant FEL will be simulated on QUINDI.