Beam Feedback System Research Articles

Stochastic cooling notch filter developments for the high-precision spectrometer ring in the high intensity heavy-ion accelerator facility project.

Stochastic cooling of the high-precision spectrometer ring (SRing) at the High Intensity Heavy-Ion Accelerator Facility (HIAF) project in China, which is used mainly for experiments with radioactive fragment beams, is applied to speed up the cooling process of a stored ion beam. In this article, a new coaxial-type notch filter with an amplitude equalizer in the long branch and an optical-type notch filter with phase-stabilized optical fiber are discussed and evaluated for the SRing stochastic cooling system. Both prototypes of coaxial and optical notch filters are fabricated and tested. The minimum notch depth of coaxial and optical notch filters reaches to 26 and 40 dB, respectively. The performance of both coaxial notch filter and optical fiber notch filter is presented in this work. These developments will be used not only for the longitudinal stochastic cooling system but also have potential for the beam feedback system.

SwissFEL double bunch operation

SwissFEL has a unique capability, among the normal conducting linac-based light sources, to simultaneously serve two separate undulator lines (Aramis and Athos) up to the machine repetition rate of 100 Hz using the double bunch operation mode. It increases twice the experiments throughput of the facility with modest additional investment. Two electron bunches spaced 28 ns apart are extracted from the cathode by two laser pulses with individually controlled repetition rates. The bunches are accelerated up to about 3 GeV in the main linac using the same rf macropulse. After separation, one bunch serves the Athos soft x-ray beamline and the other is further accelerated to serve the hard x-ray beamline – Aramis. A fast and high-stability beam kicker separates the two bunches without disturbing the electron beam and consequently the x-ray lasing. The timing and control system sets hybrid machine modes utilizing independent operation of the two undulator lines with individually programmed repetition rates. Beam diagnostics and feedback systems have to operate with two closely spaced bunches where the two beams share the same machine path. The low-level rf system manipulates the rf amplitude and phase within a fraction of the rf macropulse to provide decoupling of the acceleration parameters of the first and the second bunch. This manuscript presents measurements that show that the bunch separation does not degrade FEL lasing stability.13 MoreReceived 7 July 2022Accepted 4 November 2022DOI:https://doi.org/10.1103/PhysRevAccelBeams.25.120701Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAccelerator/storage ring control systemsBeam diagnosticsBeam injection, extraction & transportBeam optics correction schemesBeam optics transportBeam processesBeam techniquesRadio frequency power sourcesAccelerators & Beams

Development of a fourfold dielectric-filled reentrant cavity as a beam position monitor (BPM) in a proton therapy facility

At the Paul Scherrer Institute (PSI), the superconducting cyclotron “COMET” delivers a 250 MeV proton beam for radiation therapy in pulses of 1ns at the cyclotron-RF frequency of 72.85 MHz. Accurate measurement of the beam position at proton beam currents of 0.1–10 nA in the beam transport line downstream of the degrader is of crucial importance for the treatment safety and quality, beam alignment and feedback systems. This is essential for efficient operation and beam delivery. These measurements are usually performed with intercepting monitors such as ionization chambers (ICs). In this paper, we present a novel non-intercepting position sensitive cavity resonator. The resonant monitor, tuned to the second harmonic of the cyclotron's RF, is based on the detection of the transverse magnetic dipole mode of the EM field generated by the beam. This mode is only excited for off-center beam positions and is measured with the help of four floating cavities within a common grounded cylinder. This paper discusses the BPM fundamental characteristics, design optimization and the underlying parametric investigations involving the contribution of the different modes and crosstalk. We estimate the expected signals from the prototype BPM for position offsets from simulations and compare them with test-bench measurements and beam measurements with the prototype and the improvised BPM design. We conclude by summarizing the achieved position sensitivity, precision, and measurement bandwidth.

Trifold Frequency Multiplier by using Triode for the Feedback System of BEPC II

In BEPC II(Upgrade of Beijing Electron-Position Collider), multi-bunches and high current operation mode is usually used. Due to the influence of high frequency cavity structure or resistance impedance, the beam will be unstable. If the beam is unstable, the luminosity and brightness of the accelerator will be decreased. In order to improve the beam current and brightness of accelerator and the collider luminosity, the beam feedback system is needed to suppress the instability.
 The trifold frequency multiplier by using triode is simple to operate, and it has good performance. In the experiments, the 500MHz signal was passed through the Triode Frequency Multiplier to get the 1.5GHz signal, and its output amplitude stability is 1.7mV, and its synchronization stability is 3.46ps compared with the 500MHz input signal.

Reduction of beam induced RF-heating in the horizontal stripline kicker at the TPS

In preparation for 500 mA operation at the Taiwan Photon Source (TPS), we redesigned the horizontal stripline kicker for the beam feedback system to gain a smaller loss factor with higher shunt impedance. We introduced ground fenders (see figure 1) to this new design which resulted in the reduction of the loss factor and substantial increase of the kicker shunt impedance. The transverse profile of the kicker electrodes was matched to the racetrack beam pipe in the straight sections to minimize broadband impedance. The ground fenders can reduce the leakage of image currents through the gaps between the two strip line electrodes and also help to achieve a better impedance matching for the TEM modes in the transmission lines formed by the stripline electrodes and beam pipe in the kicker. The RF design and analysis of trapped resonant modes in the kicker were simulated by the 3-D electromagnetic code GdfidL [1]. Results of the RF design and analysis of trapped resonant modes will be discussed together with analytical estimates of coupled bunch instabilities at a beam current of 500 mA.

Apparatus for crystal extraction system at the U-70 synchrotron of IHEP

Studies on the steering of particle beams using channeling in aligned bent crystals have been performed at IHEP for many years. Different types of extraction schemes with bent crystals and technical equipment have been developed for experiments to test new crystal devices and to study beam collimation processes. A special apparatus has been designed to control the crystal extraction system. In this article, we describe the crystal devices and in-vacuum goniometers, the beam diagnostic tools (intensity and profile measurements), and the beam feedback system.

Suppression of transverse beam instabilities by stripline kickers at TPS

Collective beam instabilities could limit the accelerator performance if proper countermeasures are not in place. Active beam feedback systems are commonly used to suppress beam instabilities. The resistive wall impedance including phase-II insertion devices at TPS are calculated with analytic formulas. The growth rate of transverse coupled bunch instabilities due to wall impedance is estimated by theory. The RF properties of existing stripline kickers in TPS are analysed with a 3-D electromagnetic simulation code GdfidL. Based on the above analysis, the requirements for a beam feedback system are calculated and the results are reported.

Control of Intra-Bunch Vertical Motion in the SPS with GHz Bandwidth Feedback

A GHz bandwidth vertical beam feedback system has been in development at the CERN SPS for control of unstable vertical beam motion in single bunch and bunch train configurations. We present measurements and recent studies of stable and unstable motion for intensities up to 2 × 1011 p/bunch[1]. The system has been operated at 3.2 GS/s with 16 samples across a 5-ns RF bucket (4.2 ns 3σ bunch at injection) and experimental results confirm damping of intra-bunch instabilities in Q20, Q22 and Q26 optics configurations. Instabilities with growth rates of 1/200 turns are well-controlled from injection, consistent with the achievable gains for the 2 installed stripline kickers with 1 kW broadband power.Measurements from multiple studies in single-bunch and bunch train configurations show achieved damping rates, control of multiple intra-bunch modes, behavior of the system at injection (including interaction with the existing vertical damper) and final damped noise floor. The work is motivated by anticipated intensity increases from the LIU and HL-LHC upgrade programs [2], and has included the development of a new 1 GHz bandwidth slotline kicker structure and associated amplifier system.

Tuning of 3-tap bandpass filter during acceleration for longitudinal beam stabilization at FAIR

During acceleration in the heavy-ion synchrotrons SIS18/SIS100 at GSI/FAIR longitudinal beam oscillations are expected to occur. To reduce longitudinal emittance blowup, dedicated LLRF beam feedback systems are planned. To date, damping of longitudinal beam oscillations has been demonstrated in SIS18 machine experiments with a 3-tap filter controller (e.g. [1]), which is robust in regard to control parameters and also to noise. On acceleration ramps the control parameters have to be adjusted to the varying synchrotron frequency. Previous results from beam experiments at GSI indicate that a proportional tuning rule for one parameter and an inversely proportional tuning rule for a second parameter is feasible, but the obtained damping rate may not be optimal for all synchrotron frequencies during the ramp. In this work, macro-particle simulations are performed to evaluate, whether it is sufficient to adjust the control parameters proportionally (inversely proportionally) to the change in the linear synchrotron frequency, or if it is necessary to take more parameters, such as bunch-length and synchronous phase, into account to achieve stability and a considerable high damping rate for excited longitudinal dipole beam oscillations. This is done for single- and dual-harmonic acceleration ramps.

Tuning of 3-tap Bandpass Filter During Acceleration for Longitudinal Beam Stabilization at FAIR

During acceleration in the heavy-ion synchrotrons SIS18/SIS100 at GSI/FAIR longitudinal beam oscillations are expected to occur. To reduce longitudinal emittance blowup, dedicated LLRF beam feedback systems are planned. To date, damping of longitudinal beam oscillations has been demonstrated in SIS18 machine experiments with a 3-tap filter controller (e.g. [1]), which is robust in regard to control parameters and also to noise. On acceleration ramps the control parameters have to be adjusted to the varying synchrotron frequency. Previous results from beam experiments at GSI indicate that a proportional tuning rule for one parameter and an inversely proportional tuning rule for a second parameter is feasible, but the obtained damping rate may not be optimal for all synchrotron frequencies during the ramp. In this work, macro-particle simulations are performed to evaluate, whether it is sufficient to adjust the control parameters proportionally (inversely proportionally) to the change in the linear synchrotron frequency, or if it is necessary to take more parameters, such as bunch-length and synchronous phase, into account to achieve stability and a considerable high damping rate for excited longitudinal dipole beam oscillations. This is done for single- and dual-harmonic acceleration ramps.

Coherent instabilities in the booster and collider of the heavy ion accelerator complex NICA

In the first part some knowledge about the coherent instabilities in cyclic accelerators and storage rings necessary for the analysis of the collective effects in the heavy ion collider NICA is given. The second part discusses the possibilities for arising of coherent instabilities in the booster and in the collider of the NICA complex. Both coupling impedances and instability thresholds and growth rates have been estimated for single and coupled bunches. Parameters of the beam feedback system for damping of the instabilities have been analyzed. The investigation has been performed at the Veksler and Baldin Laboratory of High Energy Physics, JINR.

Transverse beam feedback system for PLS storage ring

As the stored beam current increases over 240 mA, transverse coupled-beam instability limits higher beam current in Pohang Light Source. A bunch by bunch transverse feedback system has been developed to cure these beam instabilities. It consists of beam oscillation detectors, betatron phase adjuster, power amplifiers and a stripline kicker. Design of each circuit and its functions are described with simple trigonometric equations. The result of the beam test has shown more than 30 dB damping of the beam oscillation in the full bandwidth of the system.

Performance of Alcator C-Mod core Thomson scattering system

Design of the Alcator C-Mod Thomson scattering (TS) diagnostic is discussed and the results of the measurements are presented. The TS system has six spatial channels with observation volumes evenly distributed between the midplane and the edge of the plasma. Each channel is capable of measuring the electron density in the range Ne=5×1019–5×1021 m−3 and temperature from Te=200 eV to 10 keV. A 30 Hz, 1.5 J per pulse Nd-YAG laser is employed allowing the measurements of evolution of Te and Ne profiles during plasma shot. A laser beam position control and feedback system provides for the beam alignment stability and reliable electron density measurements. Examples of the core density and temperature profiles measured at different stages of the plasma evolution are discussed.

RF acceleration system at cooler-synchrotron TARN II

An ion cooler-synchrotron TARN II has been constructed at INS. The RF acceleration system in this ring has a specification of wide frequency range, from 0.62 to 7.50 MHz, and of high sensitivity of beam monitoring devices for RF frequency control. With use of this RF acceleration system, beam acceleration has been successfully achieved for the light ion beams. In the present paper, the designed RF parameters including the beam feedback system, and the details of hardware for the RF system, are described as well as the experimental results of beam acceleration.

The TRISTAN timing system

The TRISTAN accelerator complex comprises four accelerators: a 200 MeV electron linac for positron production, a 2.5 GeV linac, an 8 GeV accumulation ring (AR) and a 30 GeV main ring (MR). The TRISTAN timing system is divided into fast and slow timing systems. The fast timing system supplies timing signals (fast timing) for devices whose operation is synchronized with bunched beams from either the linac or the AR. These signals are also used in various beam monitors and beam feedback systems. The slow timing system generates trigger signals (slow timing) in order to achieve synchronization between the magnetic field and the rf accelerating voltage of the AR or MR. These triggers are also used for the automatic operation of machines. The TRISTAN timing system fulfills the following features with the required flexibility and extensibility while in the operation mode: 1. (1) the linac gun trigger signals and the AR revolution clock are synchronized within ∼ 100 ps in timing accuracy, and a short pulse (∼ 1.5 ns) from the linac is injected and accumulated into an arbitrarily selected bucket of AR for a long time; 2. (2) bucket matching between the AR and MR is achieved within ± 6 ps in timing accuracy and a single bunched beam from the AR is injected into an arbitrarily selected bucket of the MR; 3. (3) the slow timing system manages the operation mode of the AR and MR with both flexibility and extensibility; 4. (4) the synchronization signals are transmitted through coaxial cables over a circumference of 3 km from the main control room.

Simulation of instabilities in the presence of beam feedback

The effect of longitudinal and transverse instabilities in electron storage rings is simulated by tracking many superparticles for many turns through a model of a machine lattice. This lattice model is defined by a series of machine elements such as RF stations (including longitudinal and transverse wake fields), beam pick-ups, feedback kicker magnets, etc. The machine elements may be interconnected in any specified way so as to produce for example feedback on the longitudinal or transverse beam motion. Each superparticle is treated in six-dimensional phase space and the effects of quantum excitation and radiation damping are included. Insofar as possible the program has been structured to allow study of all known single-beam effects (such as synchro-betatron resonances, transverse mode coupling etc.) in the presence or the absence of some form of beam feedback. The primary goal of the program was to study the effect of a reactive beam feedback system on the threshold for transverse mode coupling.

Operation of the PEP Transverse Beam Feedback

The PEP Storage Ring has been equipped with a wide band beam feedback system capable of damping the vertical and horizontal motion of six bunches. The oscillation detection is done at a symmetry point on the Storage Ring and feedback is applied at the same location one orbital period later. The signal is synchronously gated and the system appears as twelve independent feedback loops, operating on the two coordinates of each of the six bunches. Two beam deflection electrodes are driven each by a low-Q push-pull amplifier which is tuned at the 72nd harmonic of the revolution frequency and suppressed-carrier modulation is generated by a sequence of the detected bunch oscillations. The design parameters are reviewed as well as the salient features of the hardware, and the impact of this system on the machine operation is evaluated in the light of experimental results.