Continuous-wave Beam Research Articles

Positron annihilation lifetime spectroscopy at a superconducting electron accelerator

The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA. The electron beam is employed for production of several secondary beams including X-rays from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials science are performed in parallel. The adjustable repetition rate of the continuous-wave electron beams allows matching of the pulse separation to the positron lifetime in the sample under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to perform depth resolved defect spectroscopy and porosity studies especially for thin films.

Beam commissioning for a superconducting proton linac

To develop the next generation of safe and cleaner nuclear energy, the accelerator-driven subcritical (ADS) system emerges as one of the most attractive technologies. It will be able to transmute the long-lived transuranic radionuclides produced in the reactors of today's nuclear power plants into shorter-lived ones, and also it will provide positive energy output at the same time. The prototype of the Chinese ADS (C-ADS) proton accelerator comprises two injectors and a 1.5 GeV, 10 mA continuous wave (CW) superconducting main linac. The injector scheme II at the C-ADS demo facility inside the Institute of Modern Physics is a 10 MeV CW superconducting linac with a designed beam current of 10 mA, which includes an ECR ion source, a low-energy beam transport line, a 162.5 MHz radio frequency quadrupole accelerator, a medium-energy beam transport line, and a superconducting half wave resonator accelerator section. This demo facility has been successfully operating with an 11 mA, 2.7 MeV CW beam and a 3.9 mA, 4.3 MeV CW beam at different times and conditions since June 2014. The beam power has reached 28 kW, which is the highest record for the same type of linear accelerators. In this paper, the parameters of the test injector II and the progress of the beam commissioning are reported.

Temporal cloak with large fractional hiding window at telecommunication data rate

We design and experimentally investigate a temporal cloak scheme using ultrashort pulse compression and time-domain Fraunhofer diffraction. An input continuous-wave probe beam is compressed to ultrashort pulse train based on self-phase modulation effect and chirp compensation using single mode fiber. Accordingly, wide temporal gaps appear to act as the cloaking windows. The train of ultrashort pulses can be converted to continuous wave after experiencing enough dispersion, indicating that the temporal gaps are closed. Thus, any time events will be hidden in the temporal gaps from observers. In our study, we demonstrated a temporal cloak system, which is able to hide 88% of the whole time period and cloak pseudorandom digital data at a bitrate of 10 Gbit/s. The relationships of cloaking window fraction versus pump power and the condition of cloaking off are also investigated. These results present a new feasible way towards obtaining a high-fractional cloaking window at telecommunication data rate and hiding real-world messages.

Scintillation recording and playback in free-space optical links

The performance of a free-space optical (FSO) communication system is strongly affected by optical scintillation. Scintillation fades can cause errors when the power on a detector falls below its noise floor while surges can overload a detector. The very long time scale of scintillation compared to a typical bit in an FSO link means that error-correcting protocols designed for fiber optic links are inappropriate for FSO links. Comparing the performance effects of different components, such as photodetectors or protocols, such as forward error correction, in the field is difficult because conditions are constantly changing. On the other hand, laboratory-based turbulence simulators may not really simulate the effects of long-range propagation through the atmosphere. We have investigated a different approach. Scintillation has been measured during field tests using FSO terminals by sending a continuous wave beam through the atmosphere. A high dynamic range photodetector was digitized at a 5-KHz rate and files of the intensity variations were saved. Many hours of scintillation data under different environmental conditions and at different sites have been combined into a library of data. A fiber-optic-based scintillation playback system was then used in the laboratory to test modems and protocols with the recorded irradiance files. This enabled comparisons using the same atmospheric conditions allowing optimization of such parameters as detector dynamic range. It also allowed comparison and optimization of different error correcting protocols.

Diffraction of short-pulse Laguerre–Gaussian beams

Based on the numerical solution of the parabolic wave equation for the complex spectral amplitude of the wave field, the propagation of short-pulse vortex optical beams in a homogenous medium is studied by an example of a Laguerre–Gaussian femtosecond beam. It has been shown that the diffraction spreading of the Laguerre–Gaussian beam decreases with decreasing pulse duration as compared to continuous wave beams.

In-beam PET at clinical proton beams with pile-up rejection

Positron emission tomography (PET) is a means of imaging the β+-activity produced by the radiation field in ion beam therapy and therefore for treatment verification. Prompt γ-rays that are emitted during beam application challenge the detectors and electronics of PET systems, since those are designed for low and medium count rates. Typical PET detectors operated according to a modified Anger principle suffer from multiple events at high rates. Therefore, in-beam PET systems using such detectors rely on a synchronization of beam status and measurement to reject deteriorated data.In this work, a method for pile-up rejection is applied to conventional Anger logic block detectors. It allows for an in-beam data acquisition without further synchronization.Though cyclotrons produce a continuous wave beam, the radiation field shaping technique introduces breaks in the application. Time regimes mimicking synchrotrons as well as cyclotron based ones using double-scattering or pencil beam scanning field shaping at dose rates of 0.5, 1.0 and 2.0Gy/min were investigated. Two types of inhomogeneous phantoms were imaged. The first one simulates cavity structures, the other one mimics a static lung irradiation.It could be shown that, depending on the dose rate and the beam time structure, in-beam measurement including a few seconds decay time only, yield images which revealed all inhomogeneities in the phantoms. This technique can be the basis for the development of an in-beam PET system with traditional detectors and off-the-shelf electronics.

Strong scintillations of pulsed Laguerrian beams in a turbulent atmosphere.

Turbulent fluctuations of the energy density of broadband pulsed Laguerre-Gaussian beams are studied based on numerical solution of the parabolic wave equation for the complex spectral amplitude of the wave field by the split-step method. It is shown that in the regime of strong scintillations, the relative variance of energy density of the pulsed beams can take values smaller than unity, in contrast to the strong scintillation index of the continuous-wave beams, which tends to unity with increasing the turbulence strength. The level of residual spatial correlation of the energy density of pulsed beams exceeds that for the continuous-wave beams. It increases with shortening of the pulse duration and increasing of the refractive turbulence strength.

Magnet Design of an 800 MeV High Power Proton Cyclotron

The high-power proton beam has wide applications in neutron and neutrino physics, radioactive ion-beam production, ADS systems, proton radiography, and nuclear data measurement, etc. Cyclotrons are intrinsic in continuous-wave beam mode, which has the characteristics of high efficient power conversion rate, compact structure, and relatively low construction cost. A high-power 800-MeV proton cyclotron was proposed by CIAE, and the prestudy on the cyclotron was carried out since the year of 2009. The main magnet is among the most crucial and challenging subsystems. In contrast to low beam current cyclotron, the main magnet system of high beam current cyclotron needs to provide very strong vertical focusing force to avoid the beam blowup caused by dangerous resonances and space charge effects. In this paper, the 3-D modeling and optimizing work on this large-scale magnet system are presented in detail.

Efficient Second Harmonic Generation in 3D Nonlinear Optical-Lattice-Like Cladding Waveguide Splitters by Femtosecond Laser Inscription.

Integrated photonic devices with beam splitting function are intriguing for a broad range of photonic applications. Through optical-lattice-like cladding waveguide structures fabricated by direct femtosecond laser writing, the light propagation can be engineered via the track-confined refractive index profiles, achieving tailored output beam distributions. In this work, we report on the fabrication of 3D laser-written optical-lattice-like structures in a nonlinear KTP crystal to implement 1 × 4 beam splitting. Second harmonic generation (SHG) of green light through these nonlinear waveguide beam splitter structures provides the capability for the compact visible laser emitting devices. With Type II phase matching of the fundamental wavelength (@ 1064 nm) to second harmonic waves (@ 532 nm), the frequency doubling has been achieved through this three-dimensional beam splitter. Under 1064-nm continuous-wave fundamental-wavelength pump beam, guided-wave SHG at 532 nm are measured with the maximum power of 0.65 mW and 0.48 mW for waveguide splitters (0.67 mW and 0.51 mW for corresponding straight channel waveguides), corresponding to a SH conversion efficiency of approximately ~14.3%/W and 13.9%/W (11.2%/W, 11.3%/W for corresponding straight channel waveguides), respectively. This work paves a way to fabricate compact integrated nonlinear photonic devices in a single chip with beam dividing functions.

Design of a compact all-permanent magnet ECR ion source injector for ReA at the MSU NSCL

The design of a compact all-permanent magnet electron cyclotron resonance (ECR) ion source injector for the ReAccelerator Facility (ReA) at the Michigan State University (MSU) National Superconducting Cyclotron Laboratory (NSCL) is currently being carried out. The ECR ion source injector will complement the electron beam ion trap (EBIT) charge breeder as an off-line stable ion beam injector for the ReA linac. The objective of the ECR ion source injector is to provide continuous-wave beams of heavy ions from hydrogen to masses up to 136Xe within the ReA charge-to-mass ratio (Q/A) operational range from 0.2 to 0.5. The ECR ion source will be mounted on a high-voltage platform that can be adjusted to obtain the required 12keV/u injection energy into a room temperature radio-frequency quadrupole (RFQ) for further acceleration. The beam line consists of a 30kV tetrode extraction system, mass analyzing section, and optical matching section for injection into the existing ReA low energy beam transport (LEBT) line. The design of the ECR ion source and the associated beam line are discussed.

High-speed frequency-domain terahertz coherence tomography.

High-speed frequency-domain terahertz (THz) coherence tomography is demonstrated using frequency sweeping of continuous-wave THz radiation and beam steering. For axial scanning, THz frequency sweeping with a kHz sweep rate and a THz sweep range is executed using THz photomixing with an optical beat source consisting of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using coherent homodyne detection employing signal averaging for noise reduction and used as axial-scan data via fast Fourier transform. Axial-scan data are acquired while scanning a transverse range of 100 × 100 mm(2) by use of a THz beam scanner with moving neither sample nor THz transmitter/receiver unit. It takes 100 s to acquire axial-scan data for 100 × 100 points with 5 averaged traces at a sweep rate of 1 kHz. THz tomographic images of a glass fiber reinforced polymer sample with artificial internal defects are presented, acquired using the tomography system.

Finite element modeling of light propagation in turbid media under illumination of a continuous-wave beam.

Spatially resolved spectroscopy provides a means for measuring the optical properties of biological tissues, based on analytical solutions to diffusion approximation for semi-infinite media under the normal illumination of an infinitely small light beam. The method is, however, prone to error in measurement because the actual boundary condition and light beam often deviate from that used in deriving the analytical solutions. It is therefore important to quantify the effect of different boundary conditions and light beams on spatially resolved diffuse reflectance in order to improve the measurement accuracy of the technique. This research was aimed at using finite element method (FEM) to model light propagation in turbid media, subjected to normal illumination by a continuous-wave beam of infinitely small or finite size. Three types of boundary conditions [i.e., partial current (PCBC), extrapolated (EBC), and zero (ZBC)] were evaluated and compared against Monte Carlo (MC) simulations, since MC could provide accurate fluence rate and diffuse reflectance. The effect of beam size was also investigated. Overall results showed that FEM provided results as accurate as those of the analytical method when an appropriate boundary condition was applied. ZBC did not give satisfactory results in most cases. FEM-PCBC yielded a better fluence rate at the boundary than did FEM-EBC, while they were almost identical in predicting diffuse reflectance. Results further showed that FEM coupled with EBC effectively simulated spatially resolved diffuse reflectance under the illumination of a finite size beam. A large beam introduced more error, especially within the region of illumination. Research also confirmed an earlier finding that a light beam of less than 1 mm diameter should be used for estimation of optical parameters. FEM is effective for modeling light propagation in biological tissues and can be used for improving the optical property measurement by the spatially resolved technique.

Mid-wave infrared generation by difference frequency mixing of continuous wave lasers in orientation-patterned Gallium Phosphide

Frequency conversion of continuous wave beams was observed for the first time in orientation patterned gallium phosphide (OP-GaP) through difference frequency generation of 3400.5 nm light from mixing of 1064.6 nm and 1549.8 nm fiber laser beams. The dependence of the power of the generated beam on the polarization states of the two incident beams was studied theoretically and experimentally.

Nonlinear competition in nematicon propagation.

We investigate the role of competing nonlinear responses in the formation and propagation of bright spatial solitons. We use nematic liquid crystals (NLCs) exhibiting both thermo-optic and reorientational nonlinearities with continuous-wave beams. In a suitably prepared dye-doped sample and dual beam collinear geometry, thermal heating in the visible affects reorientational self-focusing in the near infrared, altering light propagation and self-trapping.

Intracardiac Doppler Echocardiography for Monitoring of Pulmonary Artery Pressures in High-Risk Patients Undergoing Transcatheter Aortic Valve Replacement

Uncontrolled pulmonary hypertension may cause worse outcomes after transcatheter aortic valve replacement (TAVR), while hemodynamic monitoring is desirable for risk control. Pulmonary artery pressure (PAP) readings obtained by intracardiac Doppler echocardiography were evaluated. In 114 patients with symptomatic aortic stenosis and median Society of Thoracic Surgeons scores of 10.5% (interquartile range, 7.7%-15.0%), transfemoral and transapical TAVR was guided by intracardiac Doppler echocardiography. The continuous-wave Doppler beam interrogated the jet of tricuspid regurgitation from the "home view" position. Systolic PAP (PAPs) was estimated as the sum of the pressure gradient derived from the maximum transtricuspid regurgitation jet velocity and the central venous pressure. Mean PAP (PAPm) was calculated by the mean gradient method (1) and the Chemla formula (2). Measurements were obtained immediately before and after TAVR. Pre- and postinterventional readings showed marginal pressure underestimation in comparison with measurements derived from right-heart catheterization: PAPs, -2.7 (95% CI, -3.3 to 2.1) and -1.4 (95% CI, -1.9 to -0.9); PAPm by the mean gradient method, -1.9 (95% CI, -2.2 to -1.6) and -0.1 (95% CI, -0.4 to 0.2). Agreement (95% limits) for PAPs was -8.6 to 3.2 and -6.8 to 4.0; agreement for PAPm by the mean gradient method was -5.4 to 1.6 and -3.4 to 3.2. The repeatability coefficient (95% limits of agreement) for PAPs was excellent: 3.4 (-4.2 to 2.5) and 5.5 (-5.3 to 5.8); repeatability for PAPm was higher by the mean gradient method than by the Chemla method. In ≥ 85% of patients with pulmonary hypertension, PAPm improved after valve deployment. Intracardiac Doppler echocardiography-derived monitoring of PAP by the mean gradient method is accurate and well applicable to high-risk TAVR candidates for intraprocedural risk control.

Improvements of PKU PMECRIS for continuous hundred hours CW proton beam operation.

In order to improve the source stability, a long term continuous wave (CW) proton beam experiment has been carried out with Peking University compact permanent magnet 2.45 GHz ECR ion source (PKU PMECRIS). Before such an experiment a lot of improvements and modifications were completed on the source body, the Faraday cup and the PKU ion source test bench. At the beginning of 2015, a continuous operation of PKU PMECRIS for 306 h with more than 50 mA CW beam was carried out after success of many short term tests. No plasma generator failure or high voltage breakdown was observed during that running period and the proton source reliability is near 100%. Total beam availability, which is defined as 35-keV beam-on time divided by elapsed time, was higher than 99% [S. X. Peng et al., Chin. Phys. B 24(7), 075203 (2015)]. A re-inspection was performed after another additional 100 h operation (counting time) and no obvious sign of component failure was observed. Counting the previous source testing time together, this PMECRs longevity is now demonstrated to be greater than 460 h. This paper is mainly concentrated on the improvements for this long term experiment.

Status of intense permanent magnet proton source for China-accelerator driven sub-critical system Linac.

Two compact intense 2.45 GHz permanent magnet proton sources and their corresponding low energy beam transport (LEBT) system were developed successfully for China accelerator driven sub-critical system in 2014. Both the proton sources operate at 35 kV potential. The beams extracted from the ion source are transported by the LEBT, which is composed of two identical solenoids, to the 2.1 MeV Radio-Frequency Quadrupole (RFQ). In order to ensure the safety of the superconducting cavities during commissioning, an electrostatic-chopper has been designed and installed in the LEBT line that can chop the continuous wave beam into a pulsed one. The minimum width of the pulse is less than 10 μs and the fall/rise time of the chopper is about 20 ns. The performance of the proton source and the LEBT, such as beam current, beam profile, emittance and the impact to RFQ injection will be presented.

A study on co-axial water-jet assisted fiber laser grooving of silicon

Potential of fiber laser grooving in silicon has been investigated with continuous wave beam and modulated beam of micro-second durations. Though silicon shows considerably high transparency for the utilized laser wavelength (1.07μm), due to the increase of absorption coefficient with temperature its micro-machining is feasible with good precision by laser. A co-axial water-jet has been employed for ejecting out the molten silicon effectively and simultaneously for rapid cooling. This in turn, overcomes the limitation of micro-machining of silicon with laser pulses of relatively longer durations and alleviates the oxidation problem also. It has been observed that with increasing water-jet speed at constant laser parameters, spatter and recast are reduced. Grooves of controlled depth in 14–520μm range, free from micro-cracks and thermal damage could be obtained by controlling the laser and process parameters. Compared to argon-jet assisted laser grooving this technique produced grooves of relatively better quality in terms of lesser micro-cracks, spatter and thermal damage.

Demonstration of a high-intensity neutron source based on a liquid-lithium target for Accelerator based Boron Neutron Capture Therapy

A free surface liquid-lithium jet target is operating routinely at Soreq Applied Research Accelerator Facility (SARAF), bombarded with a ~1.91MeV, ~1.2mA continuous-wave narrow proton beam. The experiments demonstrate the liquid lithium target (LiLiT) capability to constitute an intense source of epithermal neutrons, for Accelerator based Boron Neutron Capture Therapy (BNCT). The target dissipates extremely high ion beam power densities (>3kW/cm2, >0.5MW/cm3) for long periods of time, while maintaining stable conditions and localized residual activity. LiLiT generates ~3×1010n/s, which is more than one order of magnitude larger than conventional 7Li(p,n)-based near threshold neutron sources. A shield and moderator assembly for BNCT, with LiLiT irradiated with protons at 1.91MeV, was designed based on Monte Carlo (MCNP) simulations of BNCT-doses produced in a phantom. According to these simulations it was found that a ~15mA near threshold proton current will apply the therapeutic doses in ~1h treatment duration. According to our present results, such high current beams can be dissipated in a liquid-lithium target, hence the target design is readily applicable for accelerator-based BNCT.

Design of the new couplers for C-ADS RFQ**Supported by Strategic Priority Research Program of Chinese Academy of Sciences (XDA03020500)

A new special coupler with a kind of bowl-shaped ceramic window for a proton linear accelerator named the Chinese Accelerator Driven System (C-ADS) at the Institute of Modern Physics (IMP) has been simulated and constructed and a continuous wave (CW) beam commissioning through a four-meter long radio frequency quadruple (RFQ) was completed by the end of July 2014. In the experiments of conditioning and beam, some problems were promoted gradually such as sparking and thermal issues. Finally, two new couplers were passed with almost 110 kW CW power and 120 kW pulsed mode, respectively. The 10 mA intensity beam experiments have now been completed, and the couplers during the operation had no thermal or electro-magnetic problems. The detailed design and results are presented in the paper.