Radiation transport line for Terahertz Coherent Diffraction Radiation at ERL Test Accelerator in KEK

Coherent radiation from a short bunch of electron beams is expected to be utilized as a terahertz (THz) radiation source. We have conducted an investigation on a terahertz source based on coherent diffraction-radiation (CDR) that possesses a unique characteristic of radial polarization and is potentially useful for certain applications. Particularly, with the high repetition beam of an energy-recovery linac, it is possible to achieve a watt-class high power source. We have designed a CDR setup and THz transport line, and subsequently measured the THz beam profile at the end of the line. The results confirmed the preservation of the distinctive characteristics of CDR within the experimental area.


Introduction
Accelerator-based terahertz (THz) radiation is expected to realize a high-power broadband source [2].Since the bunch length of an electron linac can be shorter than the wavelength, the radiation becomes coherent, and the emission efficiency is greatly enhanced in the THz range.With the modern design of the energy recovery linac (ERL), which uses a photo-cathode electron gun and has a bunch compression arc, a low-emittance and short-bunch beam can be operated at a high repetition rate.
This design may provide a promising solution for a high-power THz source (Fig. 1).Coherent synchrotron radiation (CSR) and coherent undulator radiation (CUR) are the well-established mechanisms for accelerator-based THz sources.On the other hand, there are other mechanisms that have unique characteristics and may be useful for some applications.We have been developing a THz source based on coherent diffraction radiation (CDR) and coherent transition radiation (CTR).These radiations couple with the longitudinal motion of the electron beam and are qualitatively different from CSR or CUR.
By inserting a target in the path of the electron beam, CTR can be produced at the target.The mechanism can work even if the beam does not touch the material directly.In the case where the beam non-destructively passes through a small aperture or near the edge of a target, CDR can be produced.
One of the important characteristics of CDR is that the orientation of polarization has a nonuniform distribution, called radial polarization.Since there is a singularity at the central part of the profile, the intensity profile becomes a donut shape.It can be understood as radiation of higher-order spatial modes.There might be unique applications to utilize the singularity to pump materials in an unusual way or manipulate particle beams.
Figure 2 shows a power estimation of CDR calculated using the generalized Ginzburg-Frank formula given in [3].Watt-class radiation power (corresponds to the pulse energy of ∼10 nJ/pulse) can be realized assuming the target parameters of cERL [4], an ERL test facility in KEK.Note that we assume the transportation of a high-current beam through a small aperture without beam loss, but the technical feasibility of such an operation in CW operation mode needs to be confirmed.characteristics downstream.Here, we describe the design and commissioning of the transport line.

Experimental setup 2.1. Beam operation at cERL
Although cERL can operate a continuous beam of high average current, this experiment was performed in pulsed operation called burst mode for beam tuning.Bunch trains of 130 bunches at 1.3 GHz repetition were produced at the gun at a rate of 5 Hz.The beam energy was 17.5 MeV.The bunch charge was 60 pC.Tuning of the injector was optimized for the bunch charge.The main accelerator and the first arc section were set for bunch compression to realize the bunch length shorter than 1 ps downstream of the arc [5][6].The CDR setup was installed in the straight section where the short bunch beam was realized.To pass through the small aperture of the CDR target, the beam size was minimized using quadrupole magnets.

THz beam line
The CDR target, which is the source of the radiation, is an aluminum-coated silicon substrate.
The target was inserted into the electron beam at a 45-degree angle.The radiation was emitted perpendicular to the beam and was extracted into the air through a 40 mm diameter quartz window.The target can be replaced with a scintillator or screen monitor system to confirm the electron beam size on the target.The target has an elliptical aperture that will be projected as a circular aperture with a diameter of 3 mm for the electron beam to pass through.Figure 3 shows the beam profile measured on a screen monitor located downstream of the target.It shows that the core of the beam passes through the aperture without being destroyed.When we perform the experiment with the CTR setup, the target is set with an offset so that the electron beam hits the target without changing the beam position.The THz transport line consists of two sections.The first section is the tuning section which starts just after the window of the target chamber.This section consists of remotecontrolled lenses and mirrors assembled on a table in the air.The first lens (a Teflon lens with F =300 mm) is for collimating the initial divergence.The second and third lenses (Teflon lenses with F =200 mm) are for optimizing the beam optics for the following section.
The second section is the transport line to the experimental area outside the accelerator shield.The transport line first descends to floor level, crosses the floor to the wall, and then passes through a 1.5 m thick hole prepared in the concrete wall.The transport line was vacuumpumped to avoid attenuation of THz radiation.Quartz windows with an 80 mm diameter were used at the entrance and exit of the vacuum line.The vacuum line consists of mirror chambers and ducts that connect between them.The function of the mirror chamber is to change the direction and to focus the beam at the same time.The triangular path, as shown in Fig. 5, was assembled in the chamber with a concave mirror (F =3000 mm or 4500 mm) and a flat mirror.The effective aperture of the mirrors is 125 mm, which determines the aperture limit of the entire transport line.
The radiation profile was confirmed at the upstream, after the first collimating lens, and at the downstream, just after the exit of the vacuum transport.Figure 6 shows the designed THz beam size calculated as a Gaussian beam and the measured size at two locations.The measurements were performed using a diode detector that is sensitive in the spectral range of 330−440 GHz.The detector was mounted on a 2-dimensional scanning stage for obtaining the spatial profile.Since the detector is sensitive to only one of the linear polarization components, the profile will have a two-lobe shape when the ideal CDR is measured.

THz detection
In order to measure the spatial profile of THz radiation in the experimental area, we used a THz camera (TZCam Premium by i2S [7]).It is based on 320×240 microbolometers with a pixel size of 50 µm, which are sensitive in the spectral range of 0.1∼5 THz with a sensitivity of 20 pW/pixel at 2.5 THz.It can work in external trigger mode at a frame rate of 25 Hz, meaning that the signal is integrated over 40 ms for each frame.We prepared a data acquisition setup to obtain frames during the beam timing and frames off the beam timing sequentially for background subtraction.
The THz camera was placed in the experimental area about 0.4 m downstream from the exit window of the transport line.To obtain a high-resolution image of the transported radiation, we placed a Teflon lens with F =100 mm in front of the camera at a distance of about 70 mm from the camera surface.
Since the signal was too weak to be visible in a single shot, the following analysis procedures were necessary to obtain a clear profile of the THz radiation.Shot-by-shot data of backgroundsubtracted images were recorded in every pulse of the accelerator's operational mode.Then, 140 shots of image data were stacked to statistically increase the signal, and a Gaussian convolution filter was applied to remove discrete noise pixels and emphasize the low spatial frequency structure.

Results
Figure 7 shows the obtained images of CDR in the experimental area using the THz camera.The donut-shaped profile indicates the characteristics of CDR.This suggests that the THz transport line was well-established, without any partial losses due to aperture or wavefront distortion.
In order to confirm the polarization, we placed a wire-grid polarizer just after the exit window of the transport line.Figure 8 shows the data obtained with the THz camera while changing the orientation of the polarizer.The orientation of the two-lobe shape rotates with the orientation of the polarizer, which confirms the radial polarization characteristics.
To compare the CDR scheme with the CTR scheme, which is totally destructive, we measured the images of both under the same beam conditions.Figure 9 shows the results.The profiles had similar donut shapes.The peak intensity was about 40 % higher in the CTR case.Considering that the beam loss at the target was reduced by more than one order of magnitude in the CDR case, this result shows the possibility of the CDR scheme to be a high-power THz source with a high-current beam.

Conclusion
To investigate the feasibility of the CDR THz source, we developed a THz radiation transport line at cERL.The THz beam profile was measured at the end and confirmed that the special characteristics of CDR can be maintained in the experimental area.

Figure 1 .
Figure 1.Concept of the THz source at an ERL facility.

Figure 3 .
Figure 3. Electron beam profile observed at downstream of the CDR system.Most of the beam pass through the aperture except for the halo part of less than 5% of the total charge.

Figure 4 .
Figure 4. Picture of the THz transport line.

14thFigure 5 .
Figure 5. Design of the transport line.

Figure 6 .
Figure 6.Design and the measured size of THz radiation.

14th 7 Figure 7 .
Figure 7. Obtained CDR images.The distance between the camera and the final lens in front of the camera (L) was varied around the waist.

Figure 8 .
Figure 8. Polarization measurements.One of the linear polarization component determined by the orientation of the polarizer was imaged.