^{1}, M. Bellaveglia

^{1}, P. Calvani

^{2}, M. Castellano

^{1}, L. Catani

^{3,4}, A. Cianchi

^{3,4}, G. Di Pirro

^{1}, M. Ferrario

^{1}, G. Gatti

^{1}, O. Limaj

^{2}, S. Lupi

^{2,5}, B. Marchetti

^{3}, A. Mostacci

^{5,6}, E. Pace

^{1}, L. Palumbo

^{5,6}, C. Ronsivalle

^{7}, R. Pompili

^{1,3}and C. Vaccarezza

^{1}

### Abstract

The linac driven coherent THz radiation source at the SPARC-LAB test facility is able to deliver broadband THz pulses with femtosecond shaping. In addition, high peak power, narrow spectral bandwidth THz radiation can be also generated, taking advantage of advanced electron beam manipulation techniques, able to generate an adjustable train of electron bunches with a sub-picosecond length and with sub-picosecond spacing. The paper reports on the manipulation, characterization, and transport of the electron beam in the bending line transporting the beam down to the THz station, where different coherent transition radiation spectra have been measured and studied with the aim to optimize the THz radiation performances.

This work profited from the discussion and the help of the whole SPARC team.

I. INTRODUCTION AND MOTIVATION

II. THE SPARC-LAB TEST FACILITY

A. THz source experimental layout

III. THEORETICAL BACKGROUND

A. Coherent transition radiation

B. Broadband generation

C. Narrowband generation

D. Dogleg longitudinal dynamics

E. Frequency-domain technique for the characterization of longitudinal distributions

IV. THE SPARC-LAB THz SOURCE

A. Broadband THz radiation

B. Narrowband THz radiation

1. Two-bunches train

2. Four-bunches train

C. Main features

V. CONCLUSIONS

### Key Topics

- Terahertz radiation
- 44.0
- Terahertz radiation sources
- 17.0
- Compressors
- 16.0
- Electron beams
- 16.0
- Form factors
- 15.0

## Figures

SPARC schematic layout with the THz source placed at the end of the by-pass line.

SPARC schematic layout with the THz source placed at the end of the by-pass line.

Experimental layout for extraction (top) and detection (bottom) of THz radiation.

Experimental layout for extraction (top) and detection (bottom) of THz radiation.

Single electron TR energy density calculated for 100 MeV energy in case of an infinite screen (solid black line), in comparison with a 30 × 30 mm (dashed red line) and a 20 × 20 mm (dotted blue line) target.

Single electron TR energy density calculated for 100 MeV energy in case of an infinite screen (solid black line), in comparison with a 30 × 30 mm (dashed red line) and a 20 × 20 mm (dotted blue line) target.

Calculated CTR energy density for a 20 mrad acceptance angle, taking into account the 3D form factor for uniform distributed beams, both transversely and longitudinally. A 260 pC charge, 260 fs pulse duration and 100 MeV energy beam is considered. Orange and blue solid curves are obtained through the ideal GF formula, considering a 150 μm and 1.5 mm beam radius, respectively. The red dashed and the navy dotted curves take into account the target screen size, for a transverse beam radius of 150 μm and 1.5 mm, respectively.

Calculated CTR energy density for a 20 mrad acceptance angle, taking into account the 3D form factor for uniform distributed beams, both transversely and longitudinally. A 260 pC charge, 260 fs pulse duration and 100 MeV energy beam is considered. Orange and blue solid curves are obtained through the ideal GF formula, considering a 150 μm and 1.5 mm beam radius, respectively. The red dashed and the navy dotted curves take into account the target screen size, for a transverse beam radius of 150 μm and 1.5 mm, respectively.

Comparison of different longitudinal bunch distributions (left) and corresponding form factors (right) for the same RMS pulse duration, i.e., 0.26 ps, to highlight the impact of the longitudinal shape beyond the bunch length.

Comparison of different longitudinal bunch distributions (left) and corresponding form factors (right) for the same RMS pulse duration, i.e., 0.26 ps, to highlight the impact of the longitudinal shape beyond the bunch length.

(Left) Calculated four-pulses comb train with 1 ps pulse separation and 100 fs rms sub-pulse length (blue solid line), same sub-pulse length, but shorter inter-distance (black dashed-dotted line) and single bunch (red dashed line). (Right) Corresponding form factors.

(Left) Calculated four-pulses comb train with 1 ps pulse separation and 100 fs rms sub-pulse length (blue solid line), same sub-pulse length, but shorter inter-distance (black dashed-dotted line) and single bunch (red dashed line). (Right) Corresponding form factors.

Longitudinal phase space and current profile at the Linac exit (left plot) and at the THz station (right plots) (TSTEP simulation ^{ 53 } ).

Longitudinal phase space and current profile at the Linac exit (left plot) and at the THz station (right plots) (TSTEP simulation ^{ 53 } ).

Longitudinal phase space transport in the dogleg for a two bunches train. (Left plot) linac output; (right plots) THz station at the end of the dogleg. Due to high order chromatics effect in the dogleg and possible off energy operation, each bunch in the train may undergo different chirp, causing a degradation of the longitudinal modulation.

Longitudinal phase space transport in the dogleg for a two bunches train. (Left plot) linac output; (right plots) THz station at the end of the dogleg. Due to high order chromatics effect in the dogleg and possible off energy operation, each bunch in the train may undergo different chirp, causing a degradation of the longitudinal modulation.

Measured form factors for three different compression factors, corresponding to 200 fs, 450 fs, and 1.4 ps RMS bunch length.

Measured form factors for three different compression factors, corresponding to 200 fs, 450 fs, and 1.4 ps RMS bunch length.

CTR energy spectrum in μJ/THz, measured in case of a 500 pC, 500 fs (black squares), and 300 pC, 450 fs beam (red dots).

CTR energy spectrum in μJ/THz, measured in case of a 500 pC, 500 fs (black squares), and 300 pC, 450 fs beam (red dots).

Measured form factor and retrieved bunch profile.

Measured form factor and retrieved bunch profile.

Computed compression curves as function of the RF compressor phase for 180 pC total charge in the operating conditions of the COMB experiment at SPARC.

Computed compression curves as function of the RF compressor phase for 180 pC total charge in the operating conditions of the COMB experiment at SPARC.

Longitudinal phase space for low-charge (180 pC) compressed beam. (Left) compression, −84°. (Middle) maximum compression, −90°. (Right) over-compression, −95.6° . (Top) Measurements. (Bottom) Simulation.

Longitudinal phase space for low-charge (180 pC) compressed beam. (Left) compression, −84°. (Middle) maximum compression, −90°. (Right) over-compression, −95.6° . (Top) Measurements. (Bottom) Simulation.

Simulated longitudinal phase space, for the −84° case, from the end of the linac (lower left) down to the THz station (lower right), where one sub-bunch lengthens (upper left), while the other is compressing (upper right).

Simulated longitudinal phase space, for the −84° case, from the end of the linac (lower left) down to the THz station (lower right), where one sub-bunch lengthens (upper left), while the other is compressing (upper right).

Bunch form factors measured for different RF compressor phases: sub-compression (black solid line), maximum compression (red dashed line), over-compression (blue dotted line and olive dashed-dotted line). The last two curves are for different beam transport channel. 180 pC, 110 MeV beam.

Bunch form factors measured for different RF compressor phases: sub-compression (black solid line), maximum compression (red dashed line), over-compression (blue dotted line and olive dashed-dotted line). The last two curves are for different beam transport channel. 180 pC, 110 MeV beam.

Whole bunch compression curve for the 4 sub-bunches train (TSTEP simulation, ^{ 53 } measurements).

Whole bunch compression curve for the 4 sub-bunches train (TSTEP simulation, ^{ 53 } measurements).

Compression curve of each bunch of the comb beam (TSTEP simulation ^{ 53 } ).

Compression curve of each bunch of the comb beam (TSTEP simulation ^{ 53 } ).

(a) Multi-peak measured interferogram (red dots) and retrieved (solid blue line) from the measured longitudinal profile in the inset. (b) Time separation of the bunches in the train as function of the number of peaks in the interferogram. (c) Retrieved form factor peaked at the comb repetition frequency. (d) Measured CTR spectrum centered at 0.8 THz.

(a) Multi-peak measured interferogram (red dots) and retrieved (solid blue line) from the measured longitudinal profile in the inset. (b) Time separation of the bunches in the train as function of the number of peaks in the interferogram. (c) Retrieved form factor peaked at the comb repetition frequency. (d) Measured CTR spectrum centered at 0.8 THz.

(Top) Current profile measured through the RFD in case of −106° RF compression phase. (Bottom left) Comparison between interferograms measured with the same beam parameters, but different settings of the dogleg transport beamline (*R* _{56}: 110 μm per 1% energy spread (blue squares) and 56 μm per 1% energy spread (red dots)). (Bottom right) Resulting form factors.

(Top) Current profile measured through the RFD in case of −106° RF compression phase. (Bottom left) Comparison between interferograms measured with the same beam parameters, but different settings of the dogleg transport beamline (*R* _{56}: 110 μm per 1% energy spread (blue squares) and 56 μm per 1% energy spread (red dots)). (Bottom right) Resulting form factors.

## Tables

SPARC-LAB THz source performance: radiation parameters.

SPARC-LAB THz source performance: radiation parameters.

SPARC-LAB THz source performance: electron beam parameters.

SPARC-LAB THz source performance: electron beam parameters.

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