Beam Transport Research Articles

THz SASE FEL at PITZ: lasing at a wavelength of 100 μm

Development of an accelerator-based tunable THz source prototype for pump-probe experiments at the European XFEL is ongoing at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The proof-of-principle experiments on the THz SASE FEL are performed utilizing the LCLS-I undulator (on loan from SLAC) installed in the PITZ beamline. The first lasing at a center wavelength of 100 μm was observed in the summer of 2022. The lasing of the narrowband THz source was achieved using an electron beam with an energy of ∼17 MeV and a bunch charge up to several nC. Optimization of beam transport and matching resulted in the measurement of THz radiation with a pulse energy of tens of μJ, measured with pyroelectric detectors. The THz FEL gain curves were measured by means of specially designed short coils along the undulator. The results of the first characterization of the THz source at PITZ will be presented.

Single-mode waveguides for GRAVITY

The second generation Very Large Telescope Interferometer (VLTI) instrument GRAVITY is a two-field infrared interferometer operating in the K band between 1.97 and 2.43 µm with either the four 8 m or the four 1.8 m telescopes of the Very Large Telescope (VLT). Beams collected by the telescopes are corrected with adaptive optics systems and the fringes are stabilized with a fringe-tracking system. A metrology system allows the measurement of internal path lengths in order to achieve high-accuracy astrometry. High sensitivity and high interferometric accuracy are achieved thanks to (i) correction of the turbulent phase, (ii) the use of low-noise detectors, and (iii) the optimization of photometric and coherence throughput. Beam combination and most of the beam transport are performed with single-mode waveguides in vacuum and at low temperature. In this paper, we present the functions and performance achieved with weakly birefringent standard single-mode fiber systems in GRAVITY. Fibered differential delay lines (FDDLs) are used to dynamically compensate for up to 6 mm of delay between the science and reference targets. Fibered polarization rotators allow us to align polarizations in the instrument and make the single-mode beam combiner close to polarization neutral. The single-mode fiber system exhibits very low birefringence (less than 23°), very low attenuation (3.6–7 dB km−1 across the K band), and optimized differential dispersion (less than 2.04 µrad cm2 at zero extension of the FDDLs). As a consequence, the typical fringe contrast losses due to the single-mode fibers are 6% to 10% in the lowest-resolution mode and 5% in the medium- and high-resolution modes of the instrument for a photometric throughput of the fiber chain of the order of 90%. There is no equivalent of this fiber system to route and modally filter beams with delay and polarization control in any other K-band beamcombiner.

Precise control of a strong X-Y coupling beam transportation for J-PARC muon g-2/EDM experiment

A strategy for designing a dedicated beam injection and storage scheme for the J-PARC Muon g-2/EDM experiment is described. To accomplish a three-dimensional beam injection into the MRI-type compact storage system, transverse beam phase spaces (X-Y coupling) and a pulsed kicker system are key to controlling the vertical motion inside the storage volume. Moreover, dedicated beam phase control through the beam channel of the storage magnet’s yoke is crucial. We introduce a five-dimensional phase-space correlation in addition to strong X-Y coupling to control the stored vertical beam size to a level as small as one-third that achievable by X-Y coupling alone.

Modification of Beam Transport Line Design for Simultaneous Top-up Injection to PF and PF-AR

The High Energy Accelerator Research Organization (KEK) has two light sources: Photon Factory (PF, 2.5 GeV) and Photon Factory Advanced Ring (PF-AR, 6.5 GeV). In 2017, the operation of a new beam transport line (BT) of PF-AR was started, and the simultaneous top-up injection for both PF and PF-AR was realized. These days, there have been strong demands for the reduction of the operating cost of accelerators, and its importance is greater in PF-AR with higher ring-energy of 6.5 GeV. In 2019, the 5 GeV operation was started in PF-AR. However, the new BT of PF-AR (ARBT) was designed for the energy of 6.5 GeV, then the simultaneous top-up injection is no longer available under the condition of 5 GeV operation of PF-AR and 2.5 GeV operation of PF. In order to mitigate this impact, the pseudo-top-top injection has been employed by fine-tuning the current of a common DC bending magnet placed at the intersection of ARBT and the BT of PF (PFBT) within a given time frame. However, this scheme reduces the flexibility of the operation schedules, and will not be able to respond adequately to low emittance optics of PF-AR that may bring the shorter beam lifetime. In order to realize true-top-up injection, a modification of BTs’ optics design was carried out to add new horizontal steering magnets and a flattened-duct. The beam commissioning succeeded and the application to the user-run will start in this May. This time, details of modification and its extended plan will be presented.

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.

A novel fiber-optic beam monitor

A novel beam monitor based on Ce-doped silica optical fibers is being presented. Four fibers are mounted on the outside of a beam transport pipe, at the location of a beam stop at a proton cyclotron. The secondary radiation caused by the proton beam interaction with the beam stop is measured by the optical fibers via Radiation-Induced Emission (RIE). The light signal in the individual fibers is correlated to the proton flux closest to the fiber and can therefore be used as a detector to monitor the position of the proton beam in the beam stop. Initial testing shows that monitoring of a 150 nA beam of 18 MeV protons into a beam dump is possible. The monitor can measure relative beam current and beam displacement in X and Y as a function of magnetic steering.

CSR-induced Projected Emittance Growth Study For The Beam Switchyard At The European XFEL

Minimizing projected emittance of high brightness electron beam is important for efficient overlap between electron beam and radiation pulse in an FEL facility. Coherent synchrotron radiation (CSR) emission in a single bending section in the beam transport system usually introduces different slice energy modulation hence different slice transverse kicks in the designed dispersion-free lattice, causing projected emittance growth. Here we present theoretical and simulation study of CSR effect on the projected emittance growth in the beam switchyard arc before SASE2 undulator beamline at the European XFEL. We analyze arc optics impact on CSR effect and discuss possible schemes for emittance degradation compensation.

Variably polarized superradiant THz source employing crossed undulator configuration

We propose a variably polarized high-power THz source based on a crossed undulator configuration optimized for superradiant emissions from a short-pulsed electron beam. It generates arbitrary polarization by superimposing orthogonal linear polarization from two planar undulators. The phase difference is adjusted by slightly varying the path length of the first radiation in the phase shifter located between the undulators, enabling continuous and fast polarization manipulation. To realize the crossed undulator scheme based on superradiance in the THz region, it is necessary to maintain an extremely short electron bunch length of the order of 100 fs or less throughout the two undulators. We have designed a prototype system with an R56-variable triple-bend phase shifter that enables isochronous electron beam transport. Through theoretical and numerical analyses, we found that although polarization has significant observation angle dependence, a degree of polarization higher than 0.9 and a significant radiation intensity can be obtained under an appropriate angular limitation. Depending on the electron beam current and the number of undulator periods, it is estimated that the effective intensity under an angular limitation for a degree of circular polarization of 0.9 can reach the order of 100 mW on average.

Resistive field generation in intense proton beam interaction with solid targets

The Brown–Preston–Singleton (BPS) stopping power model is added to our previously developed hybrid code to model ion beam–plasma interaction. Hybrid simulations show that both resistive field and ion scattering effects are important for proton beam transport in a solid target, in which they compete with each other. When the target is not completely ionized, the self-generated resistive field effect dominates over the ion scattering effect. However, when the target is completely ionized, this situation is reversed. Moreover, it is found that Ohmic heating is important for higher current densities and materials with high resistivity. The energy fraction deposited as Ohmic heating can be as high as 20%–30%. Typical ion divergences with half-angles of about 5°–10° will modify the proton energy deposition substantially and should be taken into account.

A torus source and its application for non-primary radiation evaluation

Objective. Non-primary radiation doses to normal tissues from proton therapy may be associated with an increased risk of secondary malignancies, particularly in long-term survivors. Thus, a systematic method to evaluate if the dose level of non-primary radiation meets the IEC standard requirements is needed. Approach. Different from the traditional photon radiation therapy system, proton therapy systems are composed of several subsystems in a thick bunker. These subsystems are all possible sources of non-primary radiation threatening the patient. As a case study, 7 sources in the P-Cure synchrotron-based proton therapy system are modeled in Monte Carlo (MC) code: tandem injector, injection, synchrotron ring, extraction, beam transport line, scanning nozzle and concrete reflection/scattering. To accurately evaluate the synchrotron beam loss and non-primary dose, a new model called the torus source model is developed. Its parametric equations define the position and direction of the off-orbit particle bombardment on the torus pipe shell in the Cartesian coordinate system. Non-primary doses are finally calculated by several FLUKA simulations. Main results. The ratios of summarized non-primary doses from different sources to the planned dose of 2 Gy are all much smaller than the IEC requirements in both the 15–50 cm and 50–200 cm regions. Thus, the P-Cure synchrotron-based proton therapy system is clean and patient-friendly, and there is no need an inner shielding concrete between the accelerator and patient. Significance. Non-primary radiation dose level is a very important indicator to evaluate the quality of a PT system. This manuscript provides a feasible MC procedure for synchrotron-based proton therapy with new beam loss model. Which could help people figure out precisely whether this level complies with the IEC standard before the system put into clinical treatment. What’ more, the torus source model could be widely used for bending magnets in gantries and synchrotrons to evaluate non-primary doses or other radiation doses.

Design, fabrication, and characterization of a drift tube linac type double gap re-buncher cavity.

The Low Energy High Intensity Proton Accelerator (LEHIPA) at Bhabha Atomic Research Centre, India, has recently been commissioned to the target energy of 20MeV by beam acceleration through a 352MHz radio frequency quadrupole (RFQ) and drift tube linac (DTL). The medium energy beam transport (MEBT) line of LEHIPA matches the 3MeV beam from RFQ to the DTL using four electromagnetic quadrupoles and a re-buncher cavity. In the beam transport lines of high frequency linacs, TM010 mode single gap buncher cavities are conventionally used, while a double gap re-buncher cavity was chosen for the short length LEHIPA MEBT based on beam dynamics simulations. The 352MHz double gap re-buncher cavity has been designed in a DTL type geometry with a cell length of βλ. This double gap cavity is found to be more power efficient with a higher shunt impedance and transit time factor than the conventional single gap buncher cavity. Electromagnetic design simulations, fabrication details, low power and high-power RF test results, and beam test results of the re-buncher cavity are presented in this paper.

Beam commissioning of the SARAF Injector and MEBT (protons)

The SARAF Phase II cryogenic linear accelerator (linac) is currently under construction and will be commissioned through collaborative efforts between SNRC and CEA. The linac will accelerate a 5 mA pulsed/continuous wave (CW) proton and deuteron beam, reaching energies of up to 35 MeV and 40 MeV, respectively. The linac injector consists of Phase I components, including an ECR ion source, a Low-Energy Beam Transport (LEBT) line, and a 4-rod Radio Frequency Quadruple (RFQ). Additionally, a new Medium Energy Beam Transport (MEBT) line has been installed and integrated into the injector infrastructure. The MEBT includes three rebuncher resonators, magneto-optical elements, and beam diagnostics. The MEBT beam diagnostics, as well as the Phase I D-plate installed downstream, were used for beam characterization. The downstream-installed Temporary Beam Line (TBL) and a prototype of the Gallium Indium Liquid Target (GALIT) were used for commissioning, operating at high beam duty cycles, including CW mode. The report describes the proton beam commissioning process of the injector prior to the delivery of the cryomodules, scheduled for the second half of 2023.

Design and experiments of a compact electrostatic low energy beam transport with a double Einzel-lens for a transportable neutron source.

A transportable, compact, accelerator-based neutron source is under development at Xian Jiaotong University. An electrostatic low energy beam transport (LEBT) structure with a double Einzel-lens setup was adopted due to its short length and low power consumption. It can transport a pulsed proton beam to the radio frequency quadrupole with a required beam current of 15 mA and an energy of 30keV. We performed detailed structure optimization and beam tracking to achieve beam matching and small emittance growth. In addition, the fast chopper, beam steering, and diagnostic devices are integrated into the LEBT. The fabrication and assembly of the proton injector have been completed, and beam commissioning was carried out to measure the beam current and Twiss parameters. The design strategy, beam simulation, and experimental results are presented and discussed in this paper.

Real-time beam transport simulator for wide energy-spread proton beam generated by laser acceleration

Real-time beam transport simulator for wide energy-spread proton beam generated by laser acceleration

Study on the production of hydrogen atomic beams by stripping negative hydrogen ions in a gas target

The neutralization process and beam quality of a hydrogen beam prepared by emitting 1–––10 MeV negative hydrogen ions to a hydrogen target were studied. The simulation of the neutralization of negative hydrogen ions followed the construction of the neutralization theoretical model and beam transport model. Finally, calculations and analyses were performed to determine the effects of various variableson the neutralization of negative hydrogen ions, such as the transport distance, energy, and target gas density. According to the findings, since negative hydrogen ions in the target gas are transmitted farther, neutralization efficiency first rises and then falls. To achieve the highest neutralization efficiency, the gas vessel must be thicker depending on the incoming negative hydrogen ions are more energetic. Conversely, the denser the target gas, the thinner the gas vessel must be. The maximal neutralization efficiency, however, is unaffected by the density of the target gas or the negative hydrogen ion energy.

Slit extraction and emittance results of a permanent magnet minimum-B quadrupole electron cyclotron resonance ion source

We present an experimental and simulation study of high charge state ion beams produced with a permanent magnet electron cyclotron resonance ion source (ECRIS) with minimum-B quadrupole magnetic field topology and slit extraction system. The unconventional topology generates fan-shaped plasma flux, favouring a rectangular aperture for beam extraction. We demonstrate successful low-energy transport of the slit beam, achieving up to 25 times higher beam currents compared to a round extraction aperture. Maximum beam intensities are 1.5 and 15 μA for argon 11＋ and 9＋, respectively. The results indicate microwave power-limited beam production. Emittance values are comparable to traditional ECRIS. Unlike in conventional ECRIS, the emittance increases monotonically with increasing charge state due to slit extraction, magnetic topology, and beam transport characteristics. The experimental transport efficiency for helium ranges from 32% to 50%, depending on the total beam current. The losses primarily occur due to the dipole magnet’s insufficient vertical aperture.

Topological Transitions and Surface Umklapp Scattering in Weakly Modulated Periodic Metasurfaces.

Controlling and manipulating surface waves is highly beneficial for imaging applications, nanophotonic device design, and light-matter interactions. While deep-subwavelength structuring of the metal-dielectric interface can influence surface waves by forming strong effective anisotropy, it disregards important structural degrees of freedom such as the interplay between corrugation periodicity and depth and its effect on the beam transport. Here, we unlock these degrees of freedom, introducing weakly modulated metasurfaces, structured metal-dielectric surfaces beyond effective medium. We utilize groove-structuring with varying depths and periodicities to demonstrate control over the transport of surface waves, dominated by the depth-period interplay. We show unique backward focusing of surface waves driven by an umklapp process-momentum relaxation empowered by the periodic nature of the structure and discover a yet unexplored, dual-stage topological transition. Our findings can be applied to any type of guided wave, introducing a simple and versatile approach for controlling wave propagation in artificial media.

Results of on-line orbit correction at the medium energy beam transport section of the RAON accelerator

Results of on-line orbit correction at the medium energy beam transport section of the RAON accelerator

Beam delivery characteristics of the Hitachi carbon ion scanning system at Osaka Heavy Ion Medical Accelerator in Kansai (HIMAK).

Using the pencil beam raster scanning method employed at most carbon beam treatment facilities, spots can be moved without interrupting the beam, allowing for the delivery of a dose between spots (move dose). This technique is also known as Dose-Driven-Continuous-Scanning (DDCS). To minimize its impact on HIMAK patient dosimetry, there's an upper limit to the move dose. Spots within a layer are grouped into sets, or "break points," allowing continuous irradiation. The beam is turned off when transitioning between sets or at the end of a treatment layer or spill. The control system beam-off is accomplished by turning off the RF Knockout (RFKO) extraction and after a brief delay the High Speed Steering Magnet (HSST) redirects the beam transport away from isocenter to a beam dump. The influence of the move dose and beam on/off control on the dose distribution and irradiation time was evaluated by measurements never before reported and modelled for Hitachi Carbon DDCS. We conducted fixed-point and scanning irradiation experiments at three different energies, both with and without breakpoints. For fixed-point irradiation, we utilized a 2D array detector and an oscilloscope to measure beam intensity over time. The oscilloscope data enabled us to confirm beam-off and beam-on timing due to breakpoints, as well as the relative timing of the RFKO signal, HSST signal, and dose monitor (DM) signals. From these measurements, we analyzed and modelled the temporal characteristics of the beam intensity. We also developed a model for the spot shape and amplitude at isocenter occurring after the beam-off signal which we called flap dose and its dependence on beam intensity. In the case of scanning irradiation, we measured move doses using the 2D array detector and compared these measurements with our model. We observed that the most dominant time variation of the beam intensity was at 1 kHz and its harmonic frequencies. Our findings revealed that the derived beam intensity cannot reach the preset beam intensity when each spot belongs to different breakpoints. The beam-off time due to breakpoints was approximately 100 ms, while the beam rise time and fall time (tdecay ) were remarkably fast, about 10 ms and 0.2 ms, respectively. Moreover, we measured the time lag (tdelay ) of approximately 0.2 ms between the RFKO and HSST signals. Since tdelay ≈ tdecay at HIMAK then the HSST is activated after the residual beam intensity, resulting in essentially zero flap dose at isocenter from the HSST. Our measurements of the move dose demonstrated excellent agreement with the modelled move dose. We conducted the first move dose measurement for a Hitachi Carbon synchrotron, and our findings, considering beam on/off control details, indicate that Hitachi's carbon synchrotron provides a stable beam at HIMAK. Our work suggests that measuring both move dose and flap dose should be part of the commissioning process and possibly using our model in the Treatment Planning System (TPS) for new facilities with treatment delivery control systems with higher beam intensities and faster beam-off control.

Study of thulium-167 cyclotron production: a potential medically-relevant radionuclide.

Introduction: Targeted Radionuclide Therapy is used for the treatment of tumors in nuclear medicine, while sparing healthy tissues. Its application to cancer treatment is expanding. In particular, Auger-electron emitters potentially exhibit high efficacy in treating either small metastases or single tumor cells due to their short range in tissue. The aim of this paper is to study the feasibility of a large-scale production of thulium-167, an Auger-electron emitter radionuclide, in view of eventual systematic preclinical studies. Methods: Proton-irradiated enriched erbium-167 and erbium-168 oxides were used to measure the production cross sections of thulium-165, thulium-166, thulium-167, and thulium-168 utilizing an 18-MeV medical cyclotron equipped with a Beam Transport Line (BTL) at the Bern medical cyclotron laboratory. The comparison between the experimental and the TENDL 2021 theoretical cross-section results were in good agreement. Additional experiments were performed to assess the production yields of thulium radioisotopes in the BTL. Thulium-167 production yield was also measured irradiating five different target materials (167 Er 2 O 3, 168 Er 2 O 3, nat Tm 2 O 3, nat Yb 2 O 3, 171 Yb 2 O 3) with proton beams up to 63MeV at the Injector II cyclotron of Paul Scherrer Institute. Results and Discussion: Our experiments showed that an 8-h irradiation of enriched ytterbium-171 oxide produced about 420 MBq of thulium-167 with a radionuclidic purity of 99.95% after 5 days of cooling time with a proton beam of about 53MeV. Larger activities of thulium-167 can be achieved using enriched erbium-168 oxide with a 23-MeV proton beam, obtaining about 1 GBq after 8-h irradiation with a radionuclidic purity of 99.5% 5 days post end of bombardment.