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Characterization of the THz radiation source at the Frascati linear accelerator
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Image of FIG. 1.
FIG. 1.

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

Image of FIG. 2.
FIG. 2.

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

Image of FIG. 3.
FIG. 3.

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.

Image of FIG. 4.
FIG. 4.

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.

Image of FIG. 5.
FIG. 5.

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.

Image of FIG. 6.
FIG. 6.

(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.

Image of FIG. 7.
FIG. 7.

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

Image of FIG. 8.
FIG. 8.

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.

Image of FIG. 9.
FIG. 9.

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

Image of FIG. 10.
FIG. 10.

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).

Image of FIG. 11.
FIG. 11.

Measured form factor and retrieved bunch profile.

Image of FIG. 12.
FIG. 12.

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.

Image of FIG. 13.
FIG. 13.

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.

Image of FIG. 14.
FIG. 14.

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).

Image of FIG. 15.
FIG. 15.

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.

Image of FIG. 16.
FIG. 16.

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

Image of FIG. 17.
FIG. 17.

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

Image of FIG. 18.
FIG. 18.

(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.

Image of FIG. 19.
FIG. 19.

(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.


Generic image for table
Table I.

SPARC-LAB THz source performance: radiation parameters.

Generic image for table
Table II.

SPARC-LAB THz source performance: electron beam parameters.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Characterization of the THz radiation source at the Frascati linear accelerator