Harmonic RF System Research Articles

Matched phase sweep method for the optimization of bunching factor in dual rf systems of high-intensity hadron synchrotrons

The scheme of multiturn painting beam injection in a dual harmonic rf system is widely employed in high-intensity proton or heavy ion synchrotrons to alleviate space charge effects. With a momentum offset of the injected beams, a large bunching factor can be achieved at the end of multiturn painting injection process. However, the momentum offset can increase the instantaneous beam peak current during the first 1/4 synchrotron period, causing a reduced bunching factor during the beam injection process. To address this issue, a matched phase sweep method is developed. By matching the synchrotron oscillation frequency and beam injection period, an optimal curve for phase sweeping can be directly obtained. The effectiveness of the method has been checked via simulations and machine study, which are in good agreement. The matched phase sweep method is employed for the increase of the beam power in the upcoming upgrade project of the China Spallation Neutron Source and can also be generally applied for the operation in the presence of dual harmonic rf system in high-intensity hadron synchrotrons. Published by the American Physical Society 2024

Development of single mode cavity at 1.5 GHz for the third harmonic RF-system in PETRA IV

The PETRA IV storage ring currently under development at DESY will require a third harmonic 1.5 GHz RF-system to prevent negative effects on both, lifetime and emittance, caused by Touschek effect and Intrabeam scattering. These cavities lengthen the bunches and thereby reduce their charge density. For this 3rd harmonic system, a one-cell single-mode cavity with a simple mechanical and electrical structure is under design that should also reduce Higher Order Modes (HOMs) to a quality factor less than 100. Therefore, the well-known approach of the Choke Mode Cavity was chosen, that use a radial line damper to attenuate the HOMs and a radial choke that traps the acceleration mode. The general behaviour of the choke mode structure was simulated, discussed and optimized for the requirements of a one-cell cavity with high effective shunt impedance, high-quality factor and simple manufacturing.

Development of dual-harmonic RF system for CSNS-II

The upgrade of the China Spallation Neutron Source (CSNS-II) encompasses the development of a dual harmonic RF system for the Rapid Cycling Synchrotron (RCS). The objective of this system is to achieve a maximum second harmonic voltage of 100 kV. To meet this requirement, a high gradient cavity is being used in place of the traditional ferrite loaded cavity. Magnetic alloy (MA) loaded cavities, which can attain very high field gradients, have demonstrated their suitability for high-intensity proton synchrotrons. As a result, designing an RF system with MA-loaded cavities has emerged as a primary focus. Over the past decade, substantial advancements have been made in the development of MA-loaded cavities at CSNS. This paper provides an overview of the RF system that incorporates the MA-loaded cavity and presents the high-power test and beam test results of the system.

Measurements of longitudinal Loss of Landau damping in the CERN Proton Synchrotron

Landau damping represents the most efficient stabilization mechanism in hadron synchrotron accelerators to mitigate coherent beam instabilities. Recent studies allowed expanding the novel analytical criteria of loss of Landau damping (LLD) to the double harmonic RF system case above transition energy, providing an analytical estimate of the longitudinal stability. The threshold has a strong dependence on the voltage ratio between the harmonic and the main RF systems. Based on that, measurements of single bunch oscillations after a rigid-dipole perturbation have been performed in the CERN Proton Synchrotron (PS). Several configurations have been tested thanks to the multi-harmonic RF systems available in the PS. Higher-harmonic RF systems at 20 MHz and 40 MHz, both in phase (bunch shortening mode) and in counter-phase (bunch lengthening mode) with respect to the principal one at 10 MHz, have been measured.

Multifold bunch splitting in a high-intensity medium-energy proton synchrotron

Bunch splitting is an RF manipulation method of changing the bunch structure, bunch numbers and bunch intensity in a high-intensity synchrotron that serve as the injector for a particle collider. An efficient way to realize bunch splitting is to use the combination of different harmonic RF systems, such as the two-fold bunch splitting of a bunch with a combination of fundamental harmonic and higher harmonic RF systems. The two-fold bunch splitting and three-fold bunch splitting methods have been experimentally verified and successfully applied in the CERN PS. In this paper, a generalized multifold bunch splitting method is given. The five-fold bunch splitting method using specially designed multi-harmonic RF systems was studied and tentatively applied to the medium-stage synchrotron (MSS), the third accelerator of the injector chain of the Super Proton-Proton Collider (SPPC), to mitigate the pileup effects and collective instabilities of a single bunch in the SPPC. The results show that the five-fold bunch splitting is feasible and both the bunch population distribution and longitudinal emittance growth after the splitting are acceptable, e.g., a few percent in the population deviation and less than 10% in the total emittance growth. Intensity-related effects and beam feedback issues during the splitting process have also been addressed.

Construction and First Test Results of the Barrier and Harmonic RF Systems for the NICA Collider

Construction and First Test Results of the Barrier and Harmonic RF Systems for the NICA Collider

Optimization of design parameters for SPPC longitudinal dynamics

As the second stage of the CEPC-SPPC project, SPPC (Super Proton-Proton Collider) aims at exploring new physics beyond the Standard Model. The key design goal for the SPPC accelerator complex is to reach 75 TeV in center of mass energy with a circumference of 100 km for the collider and an injector chain of four accelerators in cascade to support the collider. As an important part of the SPPC conceptual study, the longitudinal beam dynamics was studied systematically, which includes the dynamics in the collider and its complex injector chain. First, the bunch filling scheme of the SPPC complex was designed on the basis of various constraints such as the technical challenges of the kicker magnets, limited extraction energy per injection and so on. Next, the study on the longitudinal dynamics in the collider focused on the RF scheme to meet the requirements for luminosity and mitigate relevant instabilities. A higher harmonic RF system (800 MHz) together with the basic RF system (400 MHz) to form a dual-harmonic RF system was employed to mitigate collective instabilities and increase the luminosity by producing shorter bunches. In addition, the longitudinal matchings between the bunches in the different accelerator stages were studied, with special attention to the space charge effects and the beam loading effect in the two lower energy rings (p-RCS and MSS), which result in an optimization of the RF schemes. A set of self-consistent beam and RF parameters for the SPPC complex was obtained. The collider and the three proton synchrotrons of the injector chain have unprecedented features, thus this study demonstrates how a future proton-proton collider complex looks like.

Mitigation of intensity limitation in the CERN SPS using a double RF system

The Super Proton Synchrotron (SPS) at CERN is the injector of the Large Hadron Collider (LHC). Multi-bunch instabilities limit the intensity of the beam that can be accelerated to 450 GeV in the SPS and transferred to the LHC. Without mitigation measures, the threshold of bunch intensity of the longitudinal instability is three times below the nominal bunch intensity of the LHC beam. The High Luminosity LHC project (HL-LHC), which requires a doubling of the nominal bunch intensity, relies on improvement of beam stability in the SPS. A fourth harmonic RF system allows, presently, to stabilize the beam up to nominal LHC intensity. It increases the synchrotron frequency spread inside the bunch, providing more efficient Landau damping of beam instability. However, nonlinearities of the synchrotron frequency distribution inside the bunch pose a limitation on bunch length. This paper explores possible intensity increase in the SPS by studying the effect on beam stability of the voltage ratio between two RF systems. The results are substantiated by beam measurements and particle-tracking simulations. An optimized voltage program of the second RF system during the cycle has been tested in operation and beam stability and the quality have been successfully improved.

Theoretical study of a dual harmonic system and its application to the CSNS/RCS**Supported by National Natural Science Foundation of China (11175193)

Dual harmonic systems have been widely used in high intensity proton synchrotrons to suppress the space charge effect, as well as reduce the beam loss. To investigate the longitudinal beam dynamics in a dual rf system, the potential well, the sub-buckets in the bunch and the multi-solutions of the phase equation are studied theoretically in this paper. Based on these theoretical studies, optimization of bunching factor and rf voltage waveform are made for the dual harmonic rf system in the upgrade phase of the China Spallation Neutron Source Rapid Cycling Synchrotron (CSNS/RCS). In the optimization process, the simulation with space charge effect is done using a newly developed code, C-SCSIM.

A code for the optimization of RF voltage waveform and longitudinal beam dynamics simulation in an RCS

In a Rapid Cycling Synchrotron (RCS), the dipole and quadrupole fields oscillate sinusoidally with a high repetition rate. High RF voltage is required to match the rapid change of the dipole field, and the choices of the RF voltage and synchronous phase waveform are important issues in the design of an RCS. The longitudinal beam dynamics simulation plays a key role in optimizing the RF voltage waveform. A code has been developed for both RF voltage waveform optimization and longitudinal beam dynamics simulation in an RCS. The code can be applied to both fundamental and dual harmonic RF systems, with dipole field oscillating as a sine wave or slightly deviated sine wave. In the simulation, the space charge effect is included, and the influence of the higher order modes of RF cavity on the beam can also be simulated. The code was applied to the RCS of China Spallation Neutron Source (CSNS), for optimizing the RF voltage waveform of dual harmonic RF system and beam dynamics simulation study.

Low-charge, hard x-ray free electron laser driven with anX-band injector and accelerator

After the successful operation of the Free Electron Laser in Hamburg (FLASH) and the Linac Coherent Light Source (LCLS), soft and hard x-ray free electron lasers (FELs) are being built, designed, or proposed at many accelerator laboratories. Acceleration employing lower frequency rf cavities, ranging from $L$-band to $C$-band, is usually adopted in these designs. In the first stage bunch compression, higher-frequency harmonic rf system is employed to linearize the beam's longitudinal phase space, which is nonlinearly chirped during the lower frequency rf acceleration process. In this paper, a hard x-ray FEL design using an all $X$-band accelerator at 11.424 GHz (from photocathode rf gun to linac end) is presented, without the assistance of any harmonic rf linearization. It achieves LCLS-like performance at low charge using $X$-band linac drivers, which is more versatile, efficient, and compact than ones using $S$-band or $C$-band rf technology. It employs initially 42 microns long (rms), low-charge (10 pC) electron bunches from an $X$-band photoinjector. An overall bunch compression ratio of roughly 100 times is proposed in a two stage bunch compressor system. The start-to-end macroparticle 3D simulation employing several computer codes is presented in this paper, where space charge, wakefields, and incoherent and coherent synchrotron radiation effects are included. Employing an undulator with a short period of 1.5 cm, a Genesis FEL simulation shows successful lasing at a wavelength of 0.15 nm with a pulse length of 2 fs and a power saturation length as short as 20 meters, which is equivalent to LCLS low-charge mode. Its overall length of both accelerators and undulators is 180 meters (much shorter than the effective LCLS overall length of 1230 meters, including an accelerator length of 1100 meters and an undulator length of 130 meters), which makes it possible to be built in places where only limited space is available.

Superconducting Radio-Frequency Systems for High-β Particle Accelerators

This article addresses the physics and engineering of the superconducting radio-frequency systems for high-β particle accelerators. I consider different geometries for cavities, discuss criteria for optimization, and options for higher-order mode damping. In reviewing recent progress in the field, SRF systems are classified according to the functions they perform. These systems are fundamental RF accelerating systems, deflecting/crab cavities, harmonic RF systems, and SRF photoemission electron guns.

Collective effects for long bunches in dual harmonic RF systems**Supported by National Natural Science Foundation of China (10075065)

The storage of long bunches for large time intervals needs flattened stationary buckets with a large bucket height. Collective effects from the space charge and resistive impedance are studied by looking at the incoherent particle motion for the matched and mismatched bunches. Increasing the RF amplitude with particle number provides r.m.s wise matching for modest intensities. The incoherent motion of large amplitude particles depends on the details of the RF system. The resulting debunching process is a combination of the too small full RF acceptance together with the mismatch, enhanced by the collective effects. Irregular single particle motion is not associated with the coherent dipole instability. For the stationary phase space distribution of the Hofmann-Pedersen approach and for the dual harmonic RF system, stability limits are presented, which are too low if using realistic input distributions. For single and dual harmonic RF system with d=0.31, the tracking results are shown for intensities, by a factor of 3 above the threshold values. Small resistive impedances lead to coherent oscillations around the equilibrium phase value, as energy loss by resistive impedance is compensated by the energy gain of the RF system.

Emittance-dominated long bunches in dual harmonic RF system**Supported by National Natural Science Foundation of China (10075065)

The storage of long bunches for long time intervals needs flattened stationary buckets with a large bucket height. The longitudinal motion of the initially mismatched beam has been studied for both the single and dual harmonic RF systems. The RF amplitude is determined to be r.m.s wise matched. The bucket height of the single harmonic system is too small even for shorter bunch with only 20% increased energy spread. The Halo formation and even debunching can be seen after a few synchrotron periods for single particles with large amplitude. In the case of small energy spread for a cooled beam, Coulomb interaction cannot be ignored. The external voltage has to be increased to keep the r.m.s bunch length unchanged. The new voltage ratio R(N) simplifies physics for the emittance-dominated bunches with modest particle number N. For the single harmonic system, substantial amount of debunching occurs without increasing the external voltage, but very little if the RF amplitude is doubled. Results from the ORBIT tracking code are presented for the 1 GeV bunch in the HESR synchrotron, part of the GSI FAIR project.

Transient beam loading effects in harmonic rf systems for light sources

Harmonic cavities have been used in storage rings to lengthen bunches and increase beam lifetimes dominated by Touschek scattering. Transient beam loading in the harmonic cavities generated by asymmetries in the fill pattern causes significant variation of the bunch synchronous phase and bunch length along the bunch train when the longitudinal restoring force has been reduced. This results in a significant reduction in the mean bunch lengthening and potential lifetime increase. We describe how beam current modulations give rise to transient effects much larger than expected from the linear model of the beam cavity interaction. We also develop a tracking simulation to predict results and apply this simulation to an analysis of the beam loading transients for the case of passive and active normal and superconducting third harmonic rf systems using Advanced Light Source parameters.

Commissioning of a higher harmonic RF system for the Advanced Light Source

We report on the commissioning of a higher harmonic RF system designed to improve the Touschek lifetime of the Advanced Light Source. In our best results, we have achieved over a factor of two increase in the beam lifetime. Transient beam loading of the harmonic cavities by unequal fill patterns presents the greatest limitations on lifetime improvement. We also describe several interesting effects of the harmonic cavities on the operation of the longitudinal and transverse multibunch feedback systems.

Commissioning of the New High Intensity 72 MeV Injector II for the SIN Ring Cyclotron

Commissioning of the SIN Injector II cyclotron started in spring 1984 and by spring 1985 a prpton beam current of 0.55 mA was extracted. The performance of the principal components of the overall injector can be summarized as follows: (1) The nominal ion source extraction voltage is 60 kV and the maximum current from the 860 keV Cockcroft-Walton pre-injector is presently about 10 mA DC; (2) a phase acceptance up to 40 deg can be achieved without a buncher, but making use of a third harmonic flattopping RF-system; (3) the 100 turns in the cyclotron are completely separated leading to an extraction efficiency of more than 99.9% Initial tests in combination with our 600 MeV ring cyclotron have resulted in a 0.18 mA beam on target. The majority of future operation of the 600 MeV cyclotron is scheduled to be with Injector II.- The next steps toward higher intensity are improvements of the ion source proton efficiency and the addition of a buncher in the injection line. The ultimate intensity limit is thought to be in the mA region due to an instability from longitudinal space charge forces.

A Second Harmonic (6-16 MHz) RF System with Feedback-Reduced Gap Impedance for Accelerating Flat-Topped Bunches in the CERN PS Booster

Since all experimental and theoretical evidence indicates that the beam intensity in the PSB is at present limited by the Laslett Q shift after trapping, a second harmonic accelerating system is being added with the aim of producing flat-topped bunches. Calculations indicate that the resulting improvement in bunching factor should permit an intensity increase of 25 to 40 %. Additionally, the bucket area is increased by 25 to 45 % depending on intensity.

The Experimental Study of a Higher Harmonic rf System in PETRA

At the natural bunchlength the single bunch currents in PETRA are limited by satellite resonances and a new vertical instability, which causes a vertical blow-up of a oarticle bunch if the single bunch current reaches the threshold value of that instability. The "PETRA instability" was explained in terms of head-tail mode coupling and according to this model the vertical blow-up was cured by bunchlengthening changing the longitudinal damping partition. The disadvantage of this kind of bunchlengthening is the increase of energy spread leading to a reduction of quantum life time during energy ramminq. The satellite resonances are avoided by a careful steering of the longitudinal and transverse tunes. A more effective method of bunch lengthening is the use of a higher harmonic rf system. Such a system was installed in PETRA. Wiith the higher harmonic rf system the threshold current of the vertical "PETRA instability" could be increased from 3 mA (natural bunchlength) to 15 mA (with higher harmonic rf system). Besides bunchlengthening the higher harmonic system leads to a considerable reduiction of satellite effects in the range of normal PETRA tunes.

PIA, the Positron Intensity Accumulator for the PETRA Injection

The storage ring PETRA is designed to store 4.1012 electrons and positrons in 2xfour 500 MHz rf buckets. Since the intensity of the positron linac is about 2.107 particles per 500 MHz bucket, 2.105 bunches are required for injection and accumulation in PETRA. In order to get reasonably short filling times the storage ring DORIS presently is acting as an intermediate bunch accumulation facility. This scheme has been successfully in operation since September 1978. Now the increased demand for elementary particle physics and for synchrotron radiation experiments at DORIS has motivated the construction of the small 450 MeV intermediate storage ring PIA, located between the positron linac and DESY. The circumference is 28,8 m. About 1 . 109 positrons will be injected at a rate of 50 Hz. A first harmonic cavity (10,4 MHz) accumulates positrons in a 0,8 m long bunch which is further compressed to 0,25 m length by a 12th harmonic rf system (125 MHz) for injection into the DESY 500 MHz system. After acceleration to 6,5 GeV in DESY single positron bunches are injected into PETRA.