Terahertz Free Electron Laser Research Articles

Characterization of RF System for MIR/THz Free Electron Lasers at Chiang Mai University

The establishment of the mid-infrared and terahertz free-electron laser (MIR/THz FEL) facility is ongoing at the PBP-CMU Electron Linac Laboratory (PCELL) in Chiang Mai University. The facility utilizes an S-band radio-frequency (RF) gun and a linear accelerator (linac) to generate and accelerate electron bunches. These electron bunches are accelerated in the RF gun and the linac using RF pulses with a frequency of 2856 MHz. Measuring the RF properties becomes essential, as the RF pulse information can be utilized to estimate the electron beam properties. To achieve the measurement results, we employed an RF measurement system comprising directional couplers, coaxial cables, attenuators, a crystal detector, and an oscilloscope. Prior to conducting measurements, the crystal detector and RF equipment were calibrated and characterized to ensure precise and reliable results. The electron beam energy estimation using the measured RF power was compared with the measured beam energies. The gun and the linac were operated with an absorbed RF power of 1.52 MW and an input power of 1.92 MW, respectively. The estimated electron beam energies were found to be 2.18 MeV and 15.0 MeV, respectively, closely aligning with the measured beam energies of 2.1 MeV and 14.0 MeV after the gun and linac acceleration. These consistent energy values support the reliability of our RF power measurement system and procedure.

Third harmonic generation due to free carrier in InSb using a terahertz free electron laser.

We report on the third harmonic generation (THG) in InSb semiconductor irradiated by a terahertz (THz) free electron laser (FEL). The conversion of 4 THz (wavelength 70 µm) FEL outputs into its third harmonic 12 THz was observed. We found that by tuning the sample temperature to 360 K, high conversion efficiency up to 1% can be obtained and is the highest in the THz and FIR regions below 10 THz. We also discuss the observed intensity dependence of the THG with the nonlinear order lower than 3 when the pumping intensity was high.

Numerical Simulation of an Electron Beam for Magnetic Bunch Compressor Commissioning at PITZ

A THz free electron laser (FEL) prototype has been developed at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) for obtaining high intensity radiation for THz-pump and X-ray-probe experiments at the European XFEL. In this development, a magnetic chicane was recently installed to optimize the THz FEL performance. The aim of this study is to investigate the beam dynamics in the chicane for a trajectory commissioning by tracking the electron beam via ASTRA using a 3-dimensional (3-D) magnetic field of the chicane simulated with CST-EM Studio. The simulated results indicate the possibility to obtain a minimum-momentum dispersion of an electron beam after chicane with the high bunches charge of electron beam transportation.

Photon Diagnostics for the High-Gain THz FEL at PITZ

Research and development of an accelerator-based THz source prototype for pump-probe experiments at the European XFEL are ongoing at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). Proof-of-principle experiments have been performed to generate a high-gain THz Free-electron Laser (FEL) based on the Self-Amplified Spontaneous Emission scheme. The FEL radiation pulses with a central wavelength of about 100 μm (3 THz) and single pulse energy of several tens μm can be generated. In this paper, we present and discuss the photon diagnostic setup for the THz FEL together with examples of diagnostic results, including pulse energy and an FEL gain curve. The upgraded photon diagnostic setup, capable of measuring pulse energy, transverse distribution, and spectral distribution, is expected to be operational in the spring of 2023.

Effective DC acceleration of charged particles in a circular ring and its potential application

A novel scheme is proposed for realizing effective DC acceleration of charged particles in a circular ring. The key is to use an induction acceleration cell with multi-insulating gaps that allow forced short circuit using solid-state switching devices such as SiC-MOSFET or unforced short circuit using diodes. A baseline design of the induction acceleration cell is shown. A coasting beam is accelerated in the circular ring without bunching. As its possible application, a continuous-wave terahertz free-electron laser (FEL) is proposed with numerical parameters, where this effective DC acceleration compensates for energy loss of the electron beam occupying the entire isochronous storage ring due to synchrotron radiation and FEL interaction. As a result, FEL microbunching of a coasting beam can be created, resulting in a high average output power that has not been possible with existing linac-driven FELs or RF storage-ring FELs.

High-intensity single-pulse extraction using a laser-activated GaAs reflective switch for a terahertz free-electron laser

Single-pulse extraction from a terahertz (THz) pulse train is demonstrated for a free-electron laser oscillator using the laser-activated semiconductor switching technique with gallium arsenide (GaAs). Using a GaAs wafer as the switching substrate and a titanium sapphire laser (Ti:sapphire laser), a single THz pulse can be extracted with a high contrast from the pulse train at intervals of 37 ns in a simple experimental setup due to a short decay time of the electron–hole plasma. For a single THz pulse with a duration of a few to a few tens of picoseconds and a nominal wavelength of 70μm, we achieve an extracted single pulse energy of 76 μJ.

Analysis of the beam loading effect in a compact linac with long bunch tails

Beam loading effect is an important issue in all kinds of accelerating structures, especially in a high current case, which might result in a significant difference between the actual output energy and the designed energy. In this paper, we perform simulations and experimental studies on the beam loading effect in an S-band traveling wave (TW) linac with a maximum acceleration gradient of 19.4 MV/m for an electron bunch larger than 200 pC. The TW linac is designed to drive a terahertz free electron laser (THz-FEL) and can produce beam energies varying from 8 ∼ 14 MeV. The main challenge of the beam loading analysis come from the long bunch tail produced by the thermionic cathode gun. The long bunch tail causes considerable beam loss in the linac and brings great difficulties in the beam commissioning, especially in the beam energy tuning. So, the macro-particle tracking (MPT) algorithm is proposed to accurately analyze the beam loading effect of the long tail beams. The MPT algorithm uses energy conservation in each cell of the TW linac to take into account the particle loss and phase shift during the acceleration process. Based on the THz-FEL test facility, beam energies were measured under different loading cases. The experimental results confirmed that the new algorithm could predict the beam energy with a relative error less than 1.4%, while that of the traditional method is 4% ∼ 6%.

Design, construction, and measurement of electromagnetic steering magnet for 25-MeV electron accelerator system

Mid-infrared and terahertz free-electron laser (MIR/THz FEL) facility is under the construction at the PBP-CMU Electron Linac Laboratory (PCELL) in Chiang Mai University. For producing MIR-FEL, electron beams with an energy of 25 MeV accelerated by a radio-frequency linear accelerator (RF linac) are required. Transporting the 25-MeV electron beam from the linac to the MIR-FEL station requires small dipole magnets for steering the electron beams along the beamline. This work focuses on the design, construction, and measurement of an electromagnetic steering magnet for controlling the electron beam trajectory. A computer software, CST Studio Suite 2022 is used for the 3D magnetic field simulation of the magnet. The physical length of the magnet and the diameter of copper coil are decided based on the simulation and calculation results. After the construction, a hall probe with a measuring system controlled by a computer program is used to measure the magnetic field for comparison with the simulation results. From simulated and calculated results, the steering magnet with a length of 40 mm and the coil with a diameter of 1.06 mm was chosen. At an applied current of 1.8 A, this steering magnet can bend the 25-MeV electron beam with a bending angle of 13.7 mrad. The measured magnetic field is consistent with the simulated results. The advantage of this steering magnet is that it can be assembled on a square frame for steering the beam in both horizontal and vertical direction. It can also be used for the THz-FEL beamline, which requires the electron beam with an energy of 10-16 MeV.

Construction and tests of phosphor view screen station for monitoring transverse profile of electron beam at PCELL

Development of an accelerator-based light source for generating coherent terahertz (THz) and mid-infrared (MIR) free-electron laser (FEL) is ongoing at the PBP-CMU Electron Linac Laboratory (PCELL). For producing high quality radiation, suitable electron beam property is necessary. In particle accelerator technology, transverse beam distribution and profile can be measured by using several techniques. The beam monitoring using view screen station is one of popular and effective methods. At PCELL, we use stations consisting of phosphor screen equipped with CCD camera to monitor, measure and record the beam image along the accelerator system and beamlines. The phosphor screen was chosen due to its high spatial resolution. The phosphor film is uniformly coated on a thin aluminium plate. Its green emission when electrons hitting the screen is matched well with the respond of the CCD camera. The screen can be moved in and out from the vacuum chamber using the air pressurized linear actuator. This paper presents about design, construction and tests of the screen stations at PCELL. The resolution of measurements for beam transverse profile and emittance is also be discussed.

Specific Features of the Proteomic Response of Thermophilic Bacterium Geobacillus icigianus to Terahertz Irradiation

Studying the effects of terahertz (THz) radiation on the proteome of temperature-sensitive organisms is limited by a number of significant technical difficulties, one of which is maintaining an optimal temperature range to avoid thermal shock as much as possible. In the case of extremophilic species with an increased temperature tolerance, it is easier to isolate the effects of THz radiation directly. We studied the proteomic response to terahertz radiation of the thermophilic Geobacillus icigianus, persisting under wide temperature fluctuations with a 60 °C optimum. The experiments were performed with a terahertz free-electron laser (FEL) from the Siberian Center for Synchrotron and Terahertz Radiation, designed and employed by the Institute of Nuclear Physics of the SB of the RAS. A G. icigianus culture in LB medium was THz-irradiated for 15 min with 0.23 W/cm2 and 130 μm, using a specially designed cuvette. The life cycle of this bacterium proceeds under conditions of wide temperature and osmotic fluctuations, which makes its enzyme systems stress-resistant. The expression of several proteins was shown to change immediately after fifteen minutes of irradiation and after ten minutes of incubation at the end of exposure. The metabolic systems of electron transport, regulation of transcription and translation, cell growth and chemotaxis, synthesis of peptidoglycan, riboflavin, NADH, FAD and pyridoxal phosphate cofactors, Krebs cycle, ATP synthesis, chaperone and protease activity, and DNA repair, including methylated DNA, take part in the fast response to THz radiation. When the response developed after incubation, the systems of the cell's anti-stress defense, chemotaxis, and, partially, cell growth were restored, but the respiration and energy metabolism, biosynthesis of riboflavin, cofactors, peptidoglycan, and translation system components remained affected and the amino acid metabolism system was involved.

Enhancement of Smith-Purcell radiation from cylindrical gratings by quasi-bound states in the continuum.

Smith-Purcell radiation (SPR) is an important means of generating terahertz waves, and the enhancement of SPR is an attractive topic nowadays. Inspired by the phenomenon of special SPR, where the enhancement is achieved by using a high-duty-cycle grating, we describe a new, to the best of our knowledge, but more effective approach to this challenging problem. By deriving a simple analytical solution for the SPR from an annular electron beam passing through a cylindrical metallic grating, we show that the inverse structure, a low-duty-cycle grating can exhibit rather high SPR efficiencies in the presence of quasi-bound states in the continuum (quasi-BICs). The analytical prediction is supported by particle-in-cell simulations, which show that the quasi-BICs can enhance the superradiant SPR generated by a train of electron bunches by orders of magnitude. These results present an interesting mechanism for enhancing the SPR from metallic gratings, and may find applications in terahertz free-electron lasers.

Nonlinear terahertz transmission spectroscopy on Ga-doped germanium in high magnetic fields

The authors report the observation of cyclotron resonance transitions of holes in the magnetotransmission spectra of gallium-doped germanium at low temperatures, using intense, pulsed terahertz free-electron laser radiation with a photon energy lower than the ionization energy of the Ga dopants. The spectra are very sensitive to THz intensity and provide compelling results about ionization mechanisms and saturation of Landau level transitions. The spectroscopy technique introduced here offers an exciting perspective for researching nonlinear magneto-optical processes in semiconductors.

Single-pass high-efficiency terahertz free-electron laser

Single-pass high-efficiency terahertz free-electron laser

Radioprotective effect of X-ray abdominal FLASH irradiation: Adaptation to oxidative damage and inflammatory response may be benefiting factors.

Ultrahigh dose-rate irradiation (FLASH-IR) was reported to be efficient in tumor control while reducing normal tissue radiotoxicity. However, the mechanism of such phenomenon is still unclear. Besides, the FLASH experiments using high energy X-ray, the most common modality in clinical radiotherapy, arerarely reported. This study aims to investigate the radiobiological response using 6 MV X-ray FLASH-IR or conventional dose-rate IR (CONV-IR). The superconducting linac of Chengdu THz Free Electron Laser (CTFEL) facility was used for FLASH-IR, a diamond radiation detector and a CeBr3 scintillation detector were used to monitor the time structure and dose rate of FLASH pulses. BALB/c nude mice received whole abdominal 6 MV X-ray FLASH-IR or CONV-IR, the prescribed dose was 15Gy or 10Gy and the delivered absolute dose was monitored with EBT3 films. The mice were either euthanized 24 h post-IR to evaluate acute tissue responses or followed up for 6weeks to observe late-stage responses and survival probability. Complete blood count, histological analyses, and measurement of cytokine expression and redox status were performed. The mean dose rate of>150Gy/s and instantaneous dose rate of>5.5 × 105 Gy/s was reached in FLASH-IR at the center of mice body. After 6 weeks' follow-up of mice that received 15Gy IR, the FLASH group showed faster body weight recovery and higher survival probability than the CONV group. Histological analysis showed that FLASH-IR induced less acute intestinal damage than CONV-IR. Complete blood count and cytokine concentration measurement found that the inflammatory blood cell counts and pro-inflammatory cytokine concentrations were elevated at the acute stage after both FLASH-IR and CONV-IR. However, FLASH irradiated mice had significantly fewer inflammatory blood cells and diminished pro-inflammatory cytokine at the late stage. Moreover, higher reactive oxygen species (ROS) signal intensities but significantly reduced lipid peroxidation were found in the FLASH group than in the CONV group in the acute stage. The radioprotective effect of 6 MV X-ray FLASH-IR was observed. The differences in inflammatory responses and redox status between the two groups may be the factors responsible for reduced radiotoxicities following FLASH-IR. Further studies are required to thoroughly evaluate the impact of ROS on FLASH effect.

Prospects of a terahertz free-electron laser for field application

Prospects of a terahertz free-electron laser for field application

Low-loss and small-cross-section waveguide for compact terahertz free-electron laser

The demand for terahertz (THz) sources that can be used in practical implementations has not yet been fully met. Through the development of small-scale free-electron lasers (FEL), we investigated a solution to overcome the low lasing gain of the FEL, which is caused by a small-current accelerator and a short-length undulator. To enhance the FEL gain, the FEL interaction between the electron beam and radiation was increased by reducing the cross-sectional area of the FEL oscillator mode. We developed a waveguide for the FEL oscillator, which has a remarkably small eye-shaped cross section with low wave loss. The mode cross-sectional area was calculated to be just 4 mm2 (full width at half maximum), which is considerably smaller than those of the free-space Gaussian mode and other waveguide modes. Using COMSOL Multiphysics simulation code, we calculated and analyzed the attenuation loss and mode cross-section area for waveguides of different shapes and sizes, and we observed that the dielectric coated eye-shaped waveguide has the lowest attenuation loss, that is, less than 2.5% for 1-m propagation at an operating wavelength of 300–600 µm. These results are in good agreement with our requirements for the tabletop THz FEL. Finally, we showed that the calculated operating wavelength range of the waveguide-mode THz FEL is 300–600 µm using the machine parameters of an accelerator and undulator that were developed by considering the dispersion relations of the eye-shaped waveguide and undulator radiation.

Low-loss and Small-cross-section Waveguide for Compact Terahertz Free -electron Laser

The demand for terahertz (THz) sources that can be used in practical implementations has not yet been fully met. Through the development of small-scale free-electron lasers (FEL), we investigated a solution to overcome the low lasing gain of the FEL, which is caused by a small-current accelerator and a short-length undulator. To enhance the FEL gain, the FEL interaction between the electron beam and radiation was increased by reducing the cross-sectional area of the FEL oscillator mode. We developed a waveguide for the FEL oscillator, which has a remarkably small eye-shaped cross section with low wave loss. The mode cross-sectional area was calculated to be just 4 mm2 (full width at half maximum), which is considerably smaller than those of the free-space Gaussian mode and other waveguide modes. Using COMSOL Multiphysics simulation code, we calculated and analyzed the attenuation loss and mode cross-section area for waveguides of different shapes and sizes, and we observed that the dielectric coated eye-shaped waveguide has the lowest attenuation loss, that is, less than 2.5% for 1-m propagation at an operating wavelength of 300–600 µm. These results are in good agreement with our requirements for the tabletop THz FEL. Finally, we showed that the calculated operating wavelength range of the waveguide-mode THz FEL is 300–600 µm using the machine parameters of an accelerator and undulator that were developed by considering the dispersion relations of the eye-shaped waveguide and undulator radiation.

First demonstration of the FLASH effect with ultrahigh dose rate high-energy X-rays

PurposeThis study aimed to evaluate whether high-energy X-rays (HEXs) of the PARTER (platform for advanced radiotherapy research) platform built on CTFEL (Chengdu THz Free Electron Laser facility) can produce ultrahigh dose rate (FLASH) X-rays and trigger the FLASH effect. Materials and methodsEBT3 radiochromic film and fast current transformer (FCT) devices were used to measure absolute dose and pulsed beam current of HEXs. Subcutaneous tumor-bearing mice and healthy mice were treated with sham, FLASH, and conventional dose rate radiotherapy (CONV), respectively to observe the tumor control efficiency and normal tissue damage. ResultsThe maximum dose rate of HEXs of PARTER was up to over 1000 Gy/s. Tumor-bearing mice experiment showed a good result on tumor control (p < 0.0001) and significant difference in survival curves (p < 0.005) among the three groups. In the thorax-irradiated healthy mice experiment, there was a significant difference (p = 0.038) in survival among the three groups, with the risk of death decreased by 81% in the FLASH group compared to that in the CONV group. The survival time of healthy mice irradiated in the abdomen in the FLASH group was undoubtedly higher (62.5% of mice were still alive when we stopped observation) than that in the CONV group (7 days). ConclusionThis study confirmed that HEXs of the PARTER system can produce ultrahigh dose rate X-rays and trigger a FLASH effect, which provides a basis for future scientific research and clinical application of HEX in FLASH radiotherapy.

First Demonstration of the FLASH Effect With Ultrahigh Dose-Rate High-Energy X-Rays

Purpose/Objective(s)Ultrahigh dose-rate (FLASH) radiotherapy has attracted immense attention because of its tumor control efficiency and healthy tissue protection during preclinical experiments with electrons, kilo-voltage X-rays, and protons. Using high-energy X-rays (HEXs) in FLASH is advantageous owing to its deep penetration, small divergence, and cost-effectiveness. We did this experiment to find out whether PARTER (platform for advanced radiotherapy research) platform built on CTFEL (Chengdu THz Free Electron Laser facility) can trigger FLASH effect.Materials/MethodsSubcutaneous tumor-bearing mice and healthy mice were treated with sham, FLASH, and conventional (CONV) radiotherapy respectively to observe the tumor control and normal tissue damage under different radiation conditions. With a high-current and high-energy superconducting linear accelerator, FLASH with a good dose rate and high penetration was achieved. Breast cancers artificially induced in BAL b/c mice were efficiently controlled, and normal tissues surrounding the thorax/abdomen in C57BL/6 mice were protected from radiation with HEX-FLASH (HEX with FLASH). Theoretical analyses of cellular responses following HEX-FLASH irradiation were performed to interpret the experimental results and design further experiments.ResultsHEX-FLASH was implemented using the superconducting LINAC with the maximum mean dose rate up to over 1000 Gy/s. Tumor-bearing mice experiment showed a good result on tumor control (F = 25.14, P < 0.0001) and significantly different in survival curves (P < 0.005) among the three groups. In the thorax-irradiated healthy mice experiment, there was a statistically significant difference (P = 0.038) in survival among the three groups. The hazard ratio (HR) was 0.19, 95% confidence intervals (CIs) was 0.035–1.010, and P was 0.0486 between the FLASH and CONV groups; therefore, the risk of death decreased by 81% in the FLASH group compared with that in the CONV group. Although the survival time of healthy mice irradiated on abdomen in the FLASH group was undoubtedly higher (62.5 % of mice were still alive when we stopped observation) than that in the CONV group (7 days), the difference in survival between the two groups was not statistically significant (HR 0.369; 95% CI 0.113–1.202; P = 0.0735). Despite this, the survival trend of mice treated with HEX-FLASH radiotherapy was better. We demonstrated theoretical cellular response analysis of the FLASH effect based on the ROD hypothesis and it showed consistent radioprotective effects in normal cells under FLASH irradiation.ConclusionThis study highlights the generation of HEX-FLASH for the first time and its potential in future clinical applications. Ultrahigh dose-rate (FLASH) radiotherapy has attracted immense attention because of its tumor control efficiency and healthy tissue protection during preclinical experiments with electrons, kilo-voltage X-rays, and protons. Using high-energy X-rays (HEXs) in FLASH is advantageous owing to its deep penetration, small divergence, and cost-effectiveness. We did this experiment to find out whether PARTER (platform for advanced radiotherapy research) platform built on CTFEL (Chengdu THz Free Electron Laser facility) can trigger FLASH effect. Subcutaneous tumor-bearing mice and healthy mice were treated with sham, FLASH, and conventional (CONV) radiotherapy respectively to observe the tumor control and normal tissue damage under different radiation conditions. With a high-current and high-energy superconducting linear accelerator, FLASH with a good dose rate and high penetration was achieved. Breast cancers artificially induced in BAL b/c mice were efficiently controlled, and normal tissues surrounding the thorax/abdomen in C57BL/6 mice were protected from radiation with HEX-FLASH (HEX with FLASH). Theoretical analyses of cellular responses following HEX-FLASH irradiation were performed to interpret the experimental results and design further experiments. HEX-FLASH was implemented using the superconducting LINAC with the maximum mean dose rate up to over 1000 Gy/s. Tumor-bearing mice experiment showed a good result on tumor control (F = 25.14, P < 0.0001) and significantly different in survival curves (P < 0.005) among the three groups. In the thorax-irradiated healthy mice experiment, there was a statistically significant difference (P = 0.038) in survival among the three groups. The hazard ratio (HR) was 0.19, 95% confidence intervals (CIs) was 0.035–1.010, and P was 0.0486 between the FLASH and CONV groups; therefore, the risk of death decreased by 81% in the FLASH group compared with that in the CONV group. Although the survival time of healthy mice irradiated on abdomen in the FLASH group was undoubtedly higher (62.5 % of mice were still alive when we stopped observation) than that in the CONV group (7 days), the difference in survival between the two groups was not statistically significant (HR 0.369; 95% CI 0.113–1.202; P = 0.0735). Despite this, the survival trend of mice treated with HEX-FLASH radiotherapy was better. We demonstrated theoretical cellular response analysis of the FLASH effect based on the ROD hypothesis and it showed consistent radioprotective effects in normal cells under FLASH irradiation. This study highlights the generation of HEX-FLASH for the first time and its potential in future clinical applications.

Magnetic field induced rotation of the polarization plane of hydroxyl radical free induction decay in the terahertz region

Effect of the external longitudinal magnetic field on the optical free induction decay (FID) of the hydroxyl radical was investigated in detail. The experiments were performed on a rotational transition of OH (2Π3/2(J = 5/2) ← 2Π3/2(J = 3/2) at 83.8 cm−1) using the terahertz free electron laser. The rotation of the plane of polarization of FID as a function of time after the excitation of the radical was measured for various magnetic fields. The effect of nonmonotonic rotation of FID polarization plane in magnetic field was experimentally discovered. Numerical simulation of FID of the hydroxyl radical in a magnetic field has been carried out.