Main Linac Research Articles

The main linac cavity for Cornell's energy recovery linac: Cavity design through horizontal cryomodule prototype test

Future particle accelerators will require continuous wave operation of SRF cavities capable of supporting high beam currents. An example of this is the Energy Recovery Linac (ERL) at Cornell University, a next generation light source designed to run high currents (100mA) with a high bunch repetition rate (1.3GHz). Obtaining the beam emittance necessary to meet design specification requires strong damping of higher-order modes that can lead to beam breakup. We discuss the optimization and verification of the accelerating cavity. Next we show that an ERL constructed from the optimized cavity geometry – including realistic shape errors – can support beam currents in excess of 300mA while still maintaining beam stability. A niobium prototype 7-cell cavity was fabricated and tested in a horizontal cryomodule. We show that the prototype cavity exceeds quality factor and gradient specifications of 2×1010 at 16.2MV/m at 1.8K by 50%, reaching Q=(3.0±0.3)×1010. The prototype cavity also satisfies all design constraints and has a higher order mode spectrum consistent with the optimized shape geometry. At 1.6K, the cavity set a record for quality factor of a multicell cavity installed in a horizontal cryomodule reaching Q=(6.1±0.6)×1010.

Choke-mode damped structure design for the Compact Linear Collider main linac

Choke-mode damped structures are being studied as an alternative design to waveguide damped structures for the main linac of the Compact Linear Collider (CLIC). Choke-mode structures have the potential for lower pulsed temperature rise and simpler and less expensive fabrication. An equivalent circuit model based on transmission line theory for higher-order-mode damping is presented. Using this model, a new choke geometry is proposed and the wakefield performance is verified using Gdfidl. This structure has a comparable wakefield damping effect to the baseline design which uses waveguide damping. A prototype structure with the same iris dimensions and accelerating gradient as the nominal CLIC design, but with the new choke geometry, has been designed for high-power tests.

Estimation of emittance growth with simple orbit correction in long linacs

Estimating emittance growth is important for evaluation of performance of linear accelerators, especially where stable and low emittance beam is required, such as linear colliders. Usually, estimation of emittance growth is performed using tracking simulations, including the Monte Carlo method, which tend to take a long time. We have developed a much faster and simpler method of quantitative estimation. Formulas of orbit and emittance due to random misalignment without corrections were reported in our previous paper [K. Kubo,Phys. Rev. ST Accel. Beams 14, 014401 (2011)]. Here, formulas of emittance growth after a simple orbit correction are reported. This method is valid for very long linacs with many components, where statistical treatment is justified. It is shown that the results from the method agree well with tracking simulations for the International Linear Collider main linac.

Ground motion optimized orbit feedback design for the future linear collider

The future linear collider has strong stability requirements on the position of the beam along the accelerator and at the interaction point (IP). The beam position will be sensitive to dynamic imperfections in particular ground motion. A number of mitigation techniques have been proposed to be deployed in parallel: active and passive quadrupole stabilization and positioning as well as orbit and IP feedback. This paper presents a novel design of the orbit controller in the main linac and beam delivery system. One global feedback controller is proposed based on an SVD-controller (Singular Value Decomposition) that decouples the large multi-input multi-output system into many independent single-input single-output systems. A semi-automatic procedure is proposed for the controller design of the independent systems by exploiting numerical models of ground motion and measurement noise to minimize a target parameter, e.g. luminosity loss. The novel design for the orbit controller is studied for the case of the Compact Linear Collider (CLIC) in integrated simulations, which include all proposed mitigation methods. The impact of the ground motion on the luminosity performance is examined in detail. It is shown that with the proposed orbit controller the tight luminosity budget for ground motion effects is fulfilled and accordingly, an essential feasibility issue of CLIC has been addressed. The orbit controller design is robust and allows for a relaxed BPM resolution, while still maintaining a strong ground motion suppression performance compared to traditional methods. We believe that the described method could easily be applied to other accelerators and light sources.

Optimisation of the new low surface field accelerating structure for the ILC

The main superconducting radio frequency (SRF) linacs of the international linear collider (ILC) operate at a frequency of 1.3GHz with a π phase advance per cell in the standing wave mode. An option being considered to reduce the overall footprint and project cost is to enhance the cavity gradient. The present baseline design for the main linacs of ILC demands the cavities to be able to reach a gradient of 35MV/m—although during commissioning and operation the gradient will be 31.5MV/m. This research concerns itself with the new cavity design with a view to reaching higher gradients. This design is focussed on minimising the surface electromagnetic fields and maximising the bandwidth of the accelerating mode. This new shape, which is referred to as the New Low Surface Field (NLSF) design. A design of a complete nine-cell cavity, including power couplers and higher order mode damping couplers is presented.

E_{cm}=500 GeV CLIC \gamma\gamma collider based on its drive Beam FEL

CLIC is a linear e^+e^- collider project which employs a drive beam to accelerate the main beam. Various portions of the drive beam provide RF power for corresponding units of the main linac through energy extracting RF structures. CLIC can operate over a wide range of center-of-mass energies, from 150 GeV to 3 TeV. In addition to e^+e^- collider, \gamma\gamma and \gamma e colliders based on CLIC can be realized. In order to generate high energy photons, a Free electron laser (FEL) produced from the CLIC drive beam linac can be used. In this paper we show that the CLIC drive beam parameters satisfy the requirements of the FEL mode of operation.

Transient beam-loading model and compensation in Compact Linear Collider main linac

A new model to compensate for the transient beam loading in the CLIC main linac is developed. It takes into account the CLIC specific power generation scheme and the exact 3D geometry of the accelerating structure including couplers. A new method of calculating unloaded and loaded voltages during the transient is proposed and a dedicated optimization scheme of the rf pulse to compensate the transient beam-loading effect is presented. It is demonstrated that the 0.03% limit on the rms relative bunch-to-bunch energy spread in the main beam after acceleration can be reached. The optimization technique has been used to increase the rf to beam efficiency while preserving the CLIC requirements and to compensate for the energy spread caused by the Balakin-Novokhatski-Smirnov damping and transient process in the subharmonic buncher. Effects of charge jitters in the drive and main beams are studied. It is shown that within the 0.1% CLIC specification limit on the rms spread in beams charge the energy spread is not significantly affected.

BXERL photo-injector based on a 217 MHz normal conducting RF gun

The Beijing X-ray Energy Recovery Linac (BXERL) test facility is proposed in Institute of High Physics (IHEP). In this proposal, the main linac requires the injector to provide an electron beam with 5 MeV energy and 10 mA average current. An injector based on DC gun technology is the first candidate electron source for BXERL. However, the field emission in the DC gun cavity makes it much more difficult to increase the high voltage to more than 500 kV. Another technology based on a 217 MHz normal conducting RF gun is proposed as the backup injector for this test facility. We have designed this RF gun with 2D SUPERFISH code and 3D MICROWAVE STUDIO code. In this paper, we present the optimized design of the gun cavity, the gun RF parameters and the set-up of the whole injector system. The detailed beam dynamics have been done and the simulation results show that the injector can generate electron bunches with RMS normalized emittance 1.0 πmm·mrad, bunch length 0.77 mm, beam energy 5.0 MeV and energy spread 0.60%.

Demonstration of the high RF power production feasibility in the CLIC power extraction and transfer structure [PETS

A fundamental element of the CLIC concept is two-beam acceleration, where RF power is extracted from a high current, low energy drive beam in order to accelerate the low current main beam to high energy. The CLIC Power Extraction and Transfer Structure (PETS) is a passive microwave device in which bunches of the drive beam interact with the constant impedance of the periodically loaded waveguide and excite preferentially the synchronous mode. The RF power produced is collected downstream of the structure by means of the RF power extractor; it is delivered to the main linac using the waveguide network connecting the PETS to the main CLIC accelerating structures. The PETS should produce 135 MW at 240 ns RF pulses at a very low breakdown rate: BDR < 10{sup -7}/pulse/m. Over 2010, a thorough high RF power testing program was conducted in order to investigate the ultimate performance and the limiting factors for the PETS operation. The testing program is described and the results are presented.

Wakefield and surface electromagnetic field optimisation of manifold damped accelerating structures for CLIC

The main travelling wave linacs of the compact linear collider (CLIC) operate at a frequency of 11.9942 GHz with a phase advance per cell of 2 π /3. In order to minimise the overall footprint of the accelerator, large accelerating gradients are sought. The present baseline design for the main linacs of CLIC demands an average electric field of 100 MV/m. To achieve this in practical cavities entails the dual challenges of minimising the potential for electrical breakdown and ensuring the beam excited wakefield is sufficiently suppressed. We present a design to meet both of these conditions, together with a description of the structure, CLIC_DDS_A, expressively designed to experimentally test the ability of the structure to cope with high powers.

Effects of machine errors on the ILC main linac

For a practical linac, the beam property is affected seriously by any machine imperfections. In this paper, the effects of several main errors in the ILC main linac, such as quadrupole misalignment, magnet strength error and cavity misalignment, were studied by a theoretical method. The tolerance for each error was also obtained. Comparison with the numerical simulation result is made and the agreement is quite good.

Analytical solutions for transient and steady state beam loading in arbitrary traveling wave accelerating structures

Analytical solutions are derived for both transient and steady state gradient distributions in the traveling wave (TW) accelerating structures with arbitrary variation of parameters over the structure length. The results of the unloaded and beam loaded cases are presented. Finally, the exact analytical shape of the rf pulse waveform was found in order to apply the transient beam loading compensation scheme during the structure filling time. The obtained theoretical formulas were cross-checked by direct numerical simulations on the CLIC main linac accelerating structure and demonstrated a good agreement. The proposed methods provide a fast and reliable tool for the initial stage of the TW structure analysis.

Single-bunch emittance dilution in the perfect ILC main linac

In the ILC (International Linear Collider) main linac, low emittance preservation is the most important issue for beam dynamics study. As the main sources of emittance dilution, the dispersive and wakefield effects were studied in this paper. The theoretical calculations and numerical simulations of these two effects on single-bunch emittance dilution, without any misalignment errors, are presented in detail.

Estimation of orbit change and emittance growth due to random misalignment in long linacs

In linear accelerators, the transverse beam orbit is induced by tilts of accelerating cavities (deviation of accelerating field direction from designed beam direction) and transverse offset of quadrupole magnets. Estimating induced orbit and emittance growth due to such random errors is important for evaluation of performance of linacs, especially where stable and low emittance beam is required, such as linear colliders. Usually, such estimations are performed using tracking simulations, including the Monte Carlo method, which tends to take a long time. Here, a much faster and simpler method of quantitative estimation of beam orbit and emittance growth is reported. This method is valid for very long linacs with many components, where statistical treatment of errors is justified. It is shown that the results from the method agree well with tracking simulations for the ILC (International Linear Collider) main linac.

Design Studies for a High Resolution Cold Cavity Beam Position Monitor

In order to preserve a low emittance beam along the ~11 km long main linacs of the International Linear Collider (ILC), precise monitoring and control of the beam orbit is mandatory. A resolution <; 1 μm is required for the beam position monitors (BPM), which are located inside the cryomodules, i.e. operated at cryogenic temperatures. By using the electromagnetic field simulation programs (CST Studio Suite), we have designed a high resolution cold cavity BPM, operating in the L-Band at a 1.3 GHz dipole-mode frequency. This enables the measurement of the beam position with a Project-X like beam structure, as well as with ILC beam parameters at the superconducting RF test accelerator in the Fermilab New Muon Lab (NML) building. The results of the design studies predicts a resolution potential of a few hundred nanometers with a signal decay time of 50 ns.

Measuring the Magnetic Center Behavior and Field Quality of an ILC Superconducting Combined Quadrupole-Dipole Prototype

The main linacs of the proposed International Linear Collider (ILC) consist of superconducting cavities operated at 2 K. The accelerating cavities are contained in a contiguous series of cryogenic modules that also house the main linac quadrupoles, thus the quadrupoles also need to be superconducting. In an early ILC design, these magnets are about 0.6 m long, have cos (2?) coils, and operate at constant field gradients up to 60 T/m. In order to preserve the small beam emittances in the ILC linacs, the e+ and e- beams need to traverse the quadrupoles near their magnetic centers. A quadrupole shunting technique is used to measure the quadrupole alignment with the beams; this process requires the magnetic centers move by no more than about 5 micrometers when their strength is changed. To determine if such tight stability is achievable in a superconducting quadrupole, we at SLAC measured the magnetic center motions in a prototype ILC quadrupole built at CIEMAT in Spain. A rotating coil technique was used with a better than 0.1 micrometer precision in the relative field center position, and less than a 2 micrometer systematic error over 30 minutes. This paper describes the warm-bore cryomodule that houses the quadrupole in its Helium vessel, the magnetic center measurement system, the measured center data and strength and harmonics magnetic data.

Magnetic Shielding Effect at Cryogenic Temperature Evaluated for the STF Cryomodule

The Superconducting RF Test Facility (STF) is a project at KEK to build and operate a test linac with high-gradient superconducting cavities. Two types of high gradient accelerating cavities, with design accelerating gradients of 35 MV/m and 45 MV/m, will be tested along with other essential technologies needed for the International Linear Collider main linac systems at the STF. In order to achieve the design accelerating gradients, the ambient magnetic fields inside the cavities must be well shielded, to a level of mG . The permeability of the shielding material shows a strong temperature dependence, which often results in a degradation of the shielding effect in a cryomodule when cooled down to the operating temperature. The magnetic field inside the magnetic shield for the STF cavities is measured along the STF beam axis at cryogenic temperature and compared with measurements at room temperature. The measurements at both temperatures are compared with simulation where the permeability curves of the shielding material obtained at room temperature, liquid nitrogen temperature and liquid helium temperature are used. The shielding effect dependence on temperature of the magnetic shield inside the STF cryomodule is presented along with a comparison between measurements and simulation.

Status of IFMIF EVEDA, One Year After Its Start-Up

The Engineering Design and Engineering Validation Activities (EVEDA) of IFMIF, the International Fusion Materials Irradiation Facility, are started as one of three projects of the Broader Approach Agreement as the collaborative works between Japan and Europe, in June 2007.The main objective of the project is deliver the detailed, complete, and fully integrated engineering design the IFMIF. The designs of key subsystems are validated by executing prototyping or mockup studies. The main outcomes one year after the start of the project are: the design of the prototype accelerator of low-energy part up to 9 MeV with 125 mA continuous wave deuteron beam was updated and optimized to employ the superconducting resonators as the main linac; the purification methods for controlling the erosion/corrosion and radioactive products in the flowing lithium used as the neutron producing target material were examined under the laboratory-scale; and the concept of the irradiation test modules was elaborated further by conducting thermo-mechanical and hydraulic analyses.

Superconducting RF Cavity Development for the International Linear Collider

The International Linear Collider is planned as the next energy-frontier electron-positron accelerator. The main linacs of the collider are based on superconducting radio-frequency cavity technology, and will accelerate electron and positron beams up to 250 + 250 GeV at the center-of-mass energy. Based on the Reference Design Report issued in 2007, the ILC Global Design Effort has moved into the Technical Design phase. This paper describes the status of the design, R&D efforts, and plans of the superconducting RF cavity for the ILC.

Magnetization Effects on the Superconducting Combined Magnet Prototype for XFEL

The superconducting combined magnets for the main linac are part of the Spanish contribution to XFEL. Each magnet consists of a super-ferric quadrupole for focusing and two dipoles (horizontal and vertical) for steering, glued on the beam tube. The magnets will be operated in a superfluid helium bath. The aperture is 78 mm. The quadrupole gradient is 35 T/m whereas each dipole field is about 0.04 T. This paper reports about the magnetic measurements made on the first prototype. Measured field quality matches calculated values, both at room and cold conditions. Magnetization has been also measured in all the coils, with single or combined powering. Asymmetric and strongly non-linear transfer functions have been observed when quadrupole and dipoles were powered simultaneously. On the other hand, detailed computations were made with ROXIE to understand that issue. Results matched measurements when only one set of coils was powered, but not when two of them were energized. It is likely that the effect of the transport current or the coil-ends-which are not modeled by ROXIE-could explain the difference.