Showing papers on "Flattop published in 2021"

Interference effects in nonlinear Compton scattering due to pulse envelope

Abstract: Nonlinear Compton scattering is calculated for the collision of an electron with a plane-wave pulse. A midinfrared (IR) peak arises in the photon spectrum due to long-range interference associated with the pulse envelope. The case of a flattop pulse is studied as a toy model for pulse envelope effects and reduced to two final-state momentum integrations; the case of a sine-squared pulse is studied numerically. A perturbative expansion in charge-field coupling reveals that already at intermediate intensities, many orders are required to correctly capture the structure of the mid-IR peak. By regularizing the classical result, it is shown that the mid-IR peak is due to plane-wave ponderomotive effects from the pulse envelope. Finally, it is shown that the mid-IR peak can be isolated using energy, angle and polarization filters.

Focus shaping of high numerical aperture lens using physics-assisted artificial neural networks

Abstract: We present a physics-assisted artificial neural network (PhyANN) scheme to efficiently achieve focus shaping of high numerical aperture lens using a diffractive optical element (DOE) divided into a series of annular regions with fixed widths. Unlike the conventional ANN, the PhyANN does not require the training using labeled data, and instead output the transmission coefficients of each annular region of the DOE by fitting weights of networks to minimize the delicately designed loss function in term of focus profiles. Several focus shapes including sub-diffraction spot, flattop spot, optical needle, and multi-focus region are successfully obtained. For instance, we achieve an optical needle with 10λ depth of focus, 0.41λ lateral resolution beyond diffraction limit and high flatness of almost the same intensity distribution. Compared to typical particle swarm optimization algorithm, the PhyANN has an advantage in DOE design that generates three-dimensional focus profile. Further, the hyperparameters of the proposed PhyANN scheme are also discussed. It is expected that the obtained results benefit various applications including super-resolution imaging, optical trapping, optical lithography and so on.

Focusing flattop beam shaping with complex modulation holography

Abstract: A complex modulation algorithm for focusing beam shaping with a phase-only spatial light modulator is designed. This method modulates the amplitude and phase of a collimated beam synchronously, and the modulated beam passing through an objective lens will generate a shaped focusing beam. The characteristic of the complex modulation was studied, while the Mixed-Region Amplitude Freedom (MRAF) method was selected as a comparative reference in this research. In the theoretical simulation, the complex modulation shows excellent performance with a roughness of 0.32% RMS and 0.54% MAX. Otherwise, to further verify this complex modulation algorithm, an experiment was implemented to generate a square-shaped focusing flattop beam. The complex modulation generated a flattop beam with a roughness of 3.1% RMS and 6.1% MAX, better than the MRAF method. This result also proves that this complex modulation has better robustness.

A Pulsed Power Supply Based on an Optimized SFPFN Scheme Producing Large Currents With a Flat Top on a Heavily Inductive Load

Abstract: A power supply able to generate a pulsed large current (∼26 kA) with a flattop period > 10 ms for a heavily inductive load was designed based on the sequentially fired pulse forming network (SFPFN) scheme with high-energy-transfer efficiency. To overcome the high voltage stress on switching devices due to the SFPFN scheme, a novel electrical circuit topology was developed. In particular, the current changing rate ( di/dt ) on switching devices is limited by new designs of crowbar branch and pulse-forming unit (PFU). The assembled power supply is shown to provide an output current of a flattop of 26 kA and 12 ms on a load of 40 μ H and 4.24 mΩ with the predicted improvement in energy transfer efficiency.

Beam shaping with high energy utilization and uniformity using gradientorthogonal gratings

Abstract: A flattop beam is useful in ultrafast laser processing. A laser beam shaping method for high energy utilization and uniformity is presented using a complex hologram displayed on a spatial light modulator. The hologram consists of a geometric mask, an external blazed grating, and internal gradient orthogonal gratings. The gradient orthogonal gratings can change the incident light energy distribution and obtain flattop beams with high energy utilization. Experimental results show that the presented method can obtain an arbitrary geometric shape with a steep edge and high uniformity. Meanwhile, the bigger the geometric mask size, the higher the energy utilization will be, and it is up to 78.70%.

Requirements for an Inductive Voltage Adder as Driver for a Kicker Magnet with Short Circuit Termination

Abstract: At CERN pulse generators based on Thyratron switches and SF6 gas filled pulse forming lines, used for driving kicker magnets, are to be replaced with semiconductor technology. Preliminary investigations show the inductive voltage adder is suitable as a pulse generator for this application. To increase the magnetic field without raising the system voltage, a short-circuit termination is often applied to a kicker magnet. Because of the electrical length of a transmission line magnet, wave propagation needs to be considered. To allow for the wavefront reflected from the short-circuit termination back to the generator, a novel approach for an inductive adder architecture has been investigated. It is based on a modified generator interface, circulating the current back into the load, until the stored energy is absorbed at the end of the pulse. This approach allows for a smaller magnetic core size compared to a conventional design with a matched load. Moreover, it enables more energy-efficient operation involving smaller storage capacitors. This paper summarizes the conceptual design features and furthermore gives an overview of the parameter space for possible applications at CERN. INTRODUCTION Currently, kicker magnets at CERN are predominantly driven by pulse generators based on thyratron switches and pulse forming lines (PFL) or pulse forming networks (PFN). A pulse generator based on a PFL or a PFN represents a matched source with an impedance matching the characteristic impedance of the kicker magnet. The voltage divider formed by the impedance of a matched source and the matched load impedance causes a reduction of the driving voltage by a factor of two and hence, requires the driving voltage to be twice the voltage applied to the kicker magnet. The insulation of some of the cables used as pulse forming lines comprises SF6 gas which is known to be harmful to the environment [1], and, hence, should be replaced in the near future. Thyratron switches sometimes exhibit spontaneous turn on, which can lead to miskicks and damage to accelerator components, moreover, they are becoming increasingly difficult to source [2]. Some kicker magnets are specified to be terminated in a short-circuit. Compared to a termination with a matched resistor, this leads to a doubling of the current flowing through the kicker magnet and thereby the magnetic field for a given system impedance and magnet length. Hence, the advantages are a doubling of the kick strength in relation to the voltage seen by the kicker magnet, as well as savings on space along the beam line. In the frame of a feasibility study the replacement of the existing PFL generators by an appropriate voltage source with low inner impedance, while keeping the design of the short circuit kicker magnets, is currently under investigation. MATCHED SOURCE VS. LOW IMPEDANCE SOURCE Figure 1: Simplified circuit: matched source (Z Z ) and low impedance voltage source (Z 0). Figure 1 shows a transmission-line kicker magnet connected to either a matched source or a low impedance voltage source by a cable with the characteristic impedance Z . The transmission-line kicker magnet is the type of kicker magnet generally used at CERN [2]. It consists of inductive segments formed by parallel conductors and ferrite cores, interleaved with capacitance to ground to resemble the equivalent circuit of a transmission line with a specified characteristic impedance Z . The two kicker systems mentioned above exhibit considerable differences regarding energy flow. With the matched source, energy reflected at the short circuit termination is continually absorbed by the inner impedance of the source, and simultaneously resupplied by the internal voltage source V . The voltage source with low inner impedance initially feeds energy into the kicker magnet and the connecting cable. The energy is stored inductively and kept constant during the pulse flattop. Finally, the energy is extracted by the source at the end of the pulse. This approach has the advantage, that only the energy stored in the inductance has to be supplied by the source. Moreover, it opens the possibility to later recuperate this energy. WAVE PROPAGATION Figure 2 shows the simplified schematic of the idealized kicker system. The pulse generator is represented by a low impedance pulsed voltage source. For circuit simulation the kicker magnet and the connecting cable are modelled as ideal lossless transmission lines with the characteristic impedance Z and the single transit times τ and τ , respectively. Figure 2: Schematic of the idealized kicker system. 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-WEPAB351 MC7: Accelerator Technology T16 Pulsed Power Technology WEPAB351 3521 C on te nt fr om th is w or k m ay be us ed un de rt he te rm s of th e C C B Y 3. 0 lic en ce (© 20 21 ). A ny di st ri bu tio n of th is w or k m us tm ai nt ai n at tr ib ut io n to th e au th or (s ), tit le of th e w or k, pu bl is he r, an d D O I

High-Power Test of a Highly Over-Coupled X-Band RF Gun Driven by Short RF Pulses

Abstract: Beam brightness, a key figure of merit of RF photocathode guns, can be improved by increasing the cathode surface field which suppresses emittance growth from space charge. The surface field in normal-conducting structures is mainly limited by RF breakdown and it has been experimentally discovered that RF breakdown rate exponentially depends on RF pulse length. A highly over-coupled 1.5-cell X-band photocathode gun has been developed to be powered by 9 ns RF pulses with 3 ns rising time, 3 ns flat-top, and 3 ns falling time generated by an X-band metallic power extractor. In the recent experiment at Argonne Wakefield Accelerator facility, cathode surface field up to ∼350 MV/m with a low breakdown rate has been obtained under ∼250 MW input power. Strong beam loading from dark current was observed during RF conditioning and quickly recovered to a negligible level after the gun reached the maximum gradient. Detailed high-power test results and data analysis will be reported in this manuscript.

Characterization of Low Emittance Electron Beams Generated by Transverse Laser Beam Shaping

Abstract: Linac based X-ray free electron lasers demand a high beam quality from the electron source, therefore RF photoinjectors are used to generate the electron bunches for state-of-the-art beam brightness. One important figure of merit for these injectors is the transverse emittance of the generated electron beam, which can be minimized by shaping the photocathode laser pulses. Best performance can be achieved with ellipsoidal laser pulses, but 3D shaping is technically challenging. Typically, a quasi-uniform transverse laser profile is truncated from the Gaussian profile generated by the laser with an aperture to reduce the transverse nonlinear space charge forces. This is investigated in detail by optimizing the laser transverse profile at the Photoinjector Test facility at DESY in Zeuthen (PITZ), where photoinjector R&D is conducted for the European XFEL and FLASH free electron lasers at DESY in Hamburg. In this contribution we present experimental results at high acceleration gradients (up to 60 MV/m) for both 250 pC and 500 pC. For a bunch charge of 500 pC an emittance reduction of about 30% compared to the commonly used transverse flat-top laser distribution was achieved.

Demonstration of a general scaling law for far-field propagation

Abstract: This paper conducts experiments that demonstrate the utility of a general scaling law (GSL) for far-field propagation. In practice, the GSL accurately predicts the diffraction-limited peak irradiance in a far-field plane, regardless of the beam shape in a near-field plane. Within the experimental setup, we use a reflective, phase-only spatial light modulator to generate various beam shapes from expanded and collimated laser-source illumination, including both flattop and Gaussian beams with obscurations, in addition to phased arrays with these beam shapes. We then focus the resulting near-field source plane to a far-field target plane and measure the peak target irradiance to compare to the associated GSL prediction. Overall, the results show excellent agreement with less than 1% error for all test cases. Such experiments present a convenient and relatively inexpensive approach to demonstrating laser-system architectures (of varying complexity) that involve far-field propagation.

Inductive Adder Prototype for FCC-hh Injection Kicker System

Abstract: The future circular collider (FCC) requires a highly reliable injection kicker system. Present day kicker systems often rely on thyratron-based pulse generators and a pulse forming network or line: the thyratron is susceptible to selftriggering. Hence, an alternative pulse generator topology, based on fast semiconductor switches, is considered for the FCC. One possibility is an inductive adder (IA). A prototype IA has been designed and built: the main challenges are the fast rise time, high output current, low system impedance and a 2.3 μs pulse duration combined with low droop. This paper presents the results of measurements on the prototype IA where the rated output current and output voltage were achieved separately. Suggested improvements to the IA hardware are identified and proposals are presented that could help improve the kicker system performance.

Characterization of the slow extraction frequency response

Abstract: The main physics program of the CERN Super Proton Synchrotron (SPS) is dedicated to the fixed target physics experiments hosted in the North experimental Area (NA). Protons are delivered to the NA via third-integer resonant slow extraction over an almost 5 s flattop. In order to maximize the usable intensity delivered to the experiments, the flux of extracted particles should be kept as constant as possible. This is a very general requirement for fixed target experiments served by synchrotrons. Power supply ripples are a well-known issue in resonant slow extraction, affecting the quality of the spill. A long-standing effort is ongoing at CERN to characterize the SPS slow extraction frequency response to its main power supplies. In this paper, beam dynamics simulations are employed to understand and characterize the process, combined with dedicated beam based measurements.

O+E Band BDFA with Flattop 116 nm Gain Bandwidth Pumped with 250 mW at 1256 nm

Abstract: We present highly efficient bismuth-doped fiber amplifier covering almost all O- and E-bands. Using 250 mW single wavelength pumping at 1256 nm and low OH bismuth-doped fiber, we managed to achieve 26dB peak gain with -3dB bandwidth of 116 nm.

Utilization of adaptive flattop time with large area silicon drift detectors

Abstract: Energy-dispersive detection of X-rays using silicon drift detectors has a wide range of applications, especially for non-destructive material analysis. As X-ray spectra are acquired by signal processing of detector output signals, a key element is a shaping filter used for determination of X-ray energy values within digital signal processing. A flattop region in this filter prevents degradation of spectra by ballistic deficit effects. Detectors with large active areas demand high flattop times due to slow maximum signal rise times, causing pile-up effects and loss of count rate capability. In this work an approach using an energy filter with variable flattop time is presented, adapting the flattop time on a pulse-to-pulse basis to signal rise time. Implementation in hardware and experimental investigation is done using a 150 mm2 active area silicon drift detector. Spectroscopic performance is investigated using a 55Fe source operating at 1 × 106 counts/s. Peaking times of 50 ns and 100 ns are considered. In two series of measurements, influence of ballistic deficit is investigated for static flattop times between 150 ns and 800 ns, and for adaptive flattop time with 150 ns to 450 ns additional flattop time. Full width at half maximum, position, symmetry, and area of Mn-K α peak are evaluated and compared. Results at similar influence of ballistic deficit show increased signal throughput for adaptive flattop time by 17 % using the 50 ns peaking time, and by 15 % using the 100 ns peaking time. Comparison at similar signal throughput, on the other hand, shows superior spectra quality achieved by adaptive flattop time, indicating a significantly lower impact of ballistic deficit.

Photoinjector Drive Laser Temporal Shaping for Shanghai Soft X-Ray Free Electron Laser

Abstract: The initial Shanghai soft X ray free electron laser (SXFEL) designed shape of the photocathode driver laser is flattop produced by α-BBO stacking. The advantage of this design is attractive to produce electron bunches with low initial emittance and high uniformity along the bunch length. However, some unavoidable modulations are generated along the laser pulse which trigger bunch modulation generated at the source, due to the fast response time (tens of femtosecond) of copper cathode. In order to eliminate the modulations a temporal Gaussian driver laser was designed and tested. Measurement results show the electron bunch longitudinal modulations were removed. In this paper, we present two kinds of driver laser pulse temporal shaping methods based on α-BBO stacking and UV grating pair shaping. Moreover, the corresponding electron bunch temporal profile is also presented.

Quadrupole Focusing Lenses for Heavy Ion Linac

Abstract: Simulation results of pulsed current electromagnet quadrupoles with integral of the magnetic field gradient up to 7 T are presented. Magnets for the DTL and MEBT focusing channels are designing for the heavy ion linac in Institute for Theoretical and Experimental Physics (ITEP– NRC “Kurchatov Institute”). Appropriate conditions which promise getting the magnetic lens parameters required at restrictions on the overall length 130 mm as well as on the beam aperture ≥45 mm are defined. It is shown that the channel acceptance to beam emittance ratio desired not less than 3 can be provided by conventional low-carbon steel up to magnetic aperture of 50 mm while beyond this size permendur is out of competition. Some aspects of the pulsed power supply system are considered and main parameters of the pulse current generator (PCG) are given.

Non-Adiabatic Longitudinal Bunch Manipulation at Flattop of the J-PARC MR

Abstract: The J-PARC MR delivers the high intensity proton beams for the neutrino experiment. Eight bunches of high peak current are extracted by the extraction kickers, therefore the neutrino beam has the similar time structure. The new Intermediate Water Cherenkov Detector (IWCD) is a key detector for the future neutrino experiment and a low peak time structure is desired by the IWCD. Thus, we consider longitudinal manipulation at flattop of the MR for reducing the peak current. The manipulation requires the longer repetition period to extend the flattop. This reduces the output beam power. The manipulation should be quickly done to minimize the loss of the beam power, while the beam gap must be kept for the rise time of the extraction kicker. We propose a non-adiabatic bunch manipulation using the multiharmonic rf voltage. By using the neighbor harmonic of the accelerating harmonic, the first and eighth bunches can be decelerated and accelerated, respectively. After a certain period, the rf phase is flipped to π for debunching. Thanks to the initial deceleration and acceleration, the beam gap for the kickers can be maintained long enough. We present the concept and the longitudinal simulation result.