RF Focusing Research Articles

NEW CONCEPT OF THE MAIN PART OF MULTICHARGED IONS LINEAR ACCELERATOR ON THE COMBINED RF FOCUSING BASIS

A new concept of the main part of multicharged ions linear accelerator (MILAC) NSC KIPT on the combined RF focusing (CRFF) basis is proposed. In CRFF acceleration and focusing of the charged particles occurs at the expense of the same accelerating field. Absence of an external focusing field sources a design and operation of the accelerator main part considerably simplifies. The construction principle of accelerating&focusing channels with CRFF is considered. The mathematical modeling findings of a beam dynamics for particles with mass-to-charge ratio of A/q=5 in the energies range of 1…9.2 МeV/u are resulted. Realization of this focusing method on a basis of an interdigital H type accelerating structure is proposed.

COMPARATIVE ANALYSIS OF ALTERNATING-PHASE AND COMBINED RF FOCUSING ON THE EXAMPLE OF THE He+ LINEAR ACCELERATOR

Considered are the peculiarities of focusing by RF field on the example of a helium ion linac with the output energy of 4 MeV of two types: first is the active accelerator based on the modified alternating-phase focusing (MAPF) and second is a proposed accelerator with the combined RF focusing (CRFF) technique used. By means of computer simulation, it is shown that the calculated parameters of the CRFF-based structure offer advantages over the similar parameters of the MAPF-based accelerator. For instance, at low injection current (less than 5 mA) about 60% of charged particles are captured into acceleration process in the CRFF-based structure, while for the MAPF-based one the number is about 42%; the maximum current of accelerated helium ions is 49 and 12 mA, respectively. Owing to the investigations conducted, the conclusion about the prospects of the usage of the CRFF in the future modernization of the helium ion accelerator is presented.

THE PROSPECTS FOR SMALL-GAP RESONATORS BASED ON COMBINED RF FOCUSING FOR USE IN PROTON LINACS

For high-current ion linacs operating over the medium energy range, the use of an accelerating structure that implements combined rf focusing (CRFF) is suggested. Considered is the focusing provided by short IH-, CH-, and two-gap spoke cavities. The absence of external focusing elements and the structural simplicity of the proposed resonators makes them promising for use in accelerators operating in continuous or low-duty mode, which are built on superconducting cavities.

Evaluation of a New DC-Ion Funnel Drift Tube for Use in Proton Transfer Reaction Mass Spectrometry.

We have developed and characterized a novel drift tube called the direct current-ion funnel (DC-ion funnel) drift tube, consisting of 20 traditional ring electrodes and 5 new DC-focusing electrodes (DC-FEs) for use in proton transfer reaction mass spectrometry (PTR-MS). Ion trajectory simulations demonstrate the ion focusing effect of the DC-FE and DC-ion funnel drift tube. Further comparative experiments show that the PTR-MS with the novel DC-ion funnel drift tube has a higher sensitivity (3.8-7.3 times for the volatile organic compounds considered in this work) than the PTR-MS with a traditional drift tube. Different from conventional radiofrequency (rf) focusing methods, the DC-ion funnel drift tube can realize ion focusing with only a DC electric field and no additional rf power supply, which makes it especially suitable for instruments requiring miniaturization and low power consumption to improve detection sensitivity. In addition, the DC-ion funnel drift tube can easily be coupled to other types of mass spectrometers to increase their detection sensitivity.

Emittance compensation schemes for a superconducting rf injector

Contemporary particle injector technologies provide different advantages depending on the chosen design. In the case of copper rf injectors these is primarily the high accelerating field, enabling the generation of high charge bunches with very low emittance. However, the cost of that is a comparably low repetition rate. DC guns, on the other hand, can provide higher repetition rates and consequently increased beam currents at lower beam quality, i.e., increased emittance. The concept of a superconducting rf injector offers the opportunity to combine the advantages of both these concepts. However, it demands special concepts for emittance compensation, as the common approach with overlapping magnetic fields during the rf acceleration interferes with the limitations of superconductivity. The ELBE SRF Gun project is one of the most advanced in this field. Gun II, the second SRF injector at the Electron Linear accelerator with high Brilliance and low Emittance (ELBE), introduces new features for emittance compensation which were studied in detail over the last years. One of these methods is the integration of a superconducting solenoid into the cryostat. Another method uses rf focusing by retracting the photocathode's tip from the last cell of the resonator. This paper discusses both of these schemes by briefly outlining their setups, discussing results of numerical simulations of their impact, and presenting results of initial experimental beam measurements with Gun II.

Comment on “Collision monochromatization ine+e−colliders”

Bogomyagkov and Levichev [Phys. Rev. Accel. Beams 20, 051001 (2017)PRABCJ2469-988810.1103/PhysRevAccelBeams.20.051001] have recently reported on monochromatization in collision schemes with crossing angle. From their results, in particular, it may seem that: (1) horizontal dispersion at the IP can provide monochromatization factor Λ≫1 while retaining Piwinski angle ϕ>1, (2) production rate in such a scheme for FCC-ee at 62.5 GeV can be larger than that in the nominal crab waist collision, and (3) strong rf focusing can be used for monochromatization purposes. We demonstrate here that the first two statements are not correct, and the last one is very doubtful.

Results of using the axisymmetric RF focusing by means of field spatial harmonics at 7 MeV proton linac

For several decades, axially symmetric channels with RF focusing by means of nonsynchronous spatial harmonics of the accelerating field are offered as an attractive alternative to proven and reliable RFQ linacs. In a number of works an effectiveness of channels with axially symmetric RF focusing by means of the nonsynchronous harmonics of the field was demonstrated in the proton energy range up to 2 MeV. An effectiveness of discussed channels for protons at energies up to 7 MeV is considered in this paper. Numerical simulation results of proton self-consistent dynamics in a channel with axisymmetric RF focusing are presented and discussed in this article.

Investigation of the hybrid electron linac with negative group velocity

Hybrid accelerator, incorporating travelling wave (TW) and standing wave (SW) structures is proposed and discussed. Accelerator can sum up an RF focusing in the SW buncher and lower losses in the TW accelerating structure walls. Moreover, the structure without dumping load is proposed. Input power and beam loading are chosen to minimize power reflection from buncher maintain travelling wave regime in the accelerating structure while beam loading is on. In this case SW buncher operates as a dumping load, so all input power either goes to beam or dissipates in the wall losses, it increases structure efficiency. RF characteristics and beam dynamics simulations have been performed.

THE DESIGN OF A COMPACT RFQ NEUTRON GENERATOR

The output and target lifetime of a conventional electrostatic neutron generator are limited by the voltage stand-off capability and the acceleration of molecular species from the ion source. As an alternative, we suggest that the deuterium beam achievable from a compact high intensity ECR source can be injected directly into a compact RFQ to produce a more efficient compact neutron production system. Only the d+ ions are accelerated by the RFQ, which can also produce much higher output energies than electrostatic systems, resulting in a higher neutron output with a longer target lifetime. The direct injection of the beam makes the system more compact than the multielement, electrostatic systems typically used for extraction of the beam and subsequent transport and matching into the RFQ. We have designed and optimized a combined extraction/matching system for a compact high current deuterium ECR ion source injected into a high frequency RFQ structure, allowing a beam of about 12 mA of d+ ions to be injected at a modest ion source voltage of 25 kV. The end wall of the RFQ resonator serves as the ground electrode for the ion source, resembling DPI (direct plasma injection). For this design, we used the features of the code IGUN to take into account the electrostatic field between the ion source and the RFQ end wall, the stray magnetic field of the ECR source, the defocusing space charge of the low energy deuteron beam, and the rf focusing in the fringe field between the RFQ vanes and the RFQ flange.

Tapered channel for six-dimensional muon cooling towards micron-scale emittances

A high-luminosity muon collider requires a significant reduction of the six-dimensional emittance prior to acceleration. Obtaining the desired final emittances requires transporting the muon beam through long sections of a beam channel containing rf cavities, absorbers, and focusing solenoids. Here we propose a new scheme to improve the performance of the channel, consequently increasing the number of transmitted muons and the lattice cooling efficiency. The key idea of our scheme is to tune progressively the main lattice parameters, such as the cell dimensions, rf frequency, and coil strengths, while always keeping the beam emittance significantly above the equilibrium value. We adopt this approach for a new cooling lattice design for a muon collider, and examine its performance numerically. We show that with tapering the cooling rate is not only higher than conventional designs, but also maintains its performance through the channel, resulting in a notable 6D emittance decrease by 3 orders of magnitude. We also review important lattice parameters, such as the required focusing fields, absorber length, cavity frequency, and voltage.

Transverse RF focusing in bunching cells for standing-wave linac

Electron RF linacs for X-ray imaging applications require a small beam radius to achieve fine spatial resolution. By adopting transverse RF focusing in an accelerator, one can achieve a compact system without external magnets. A small beam radius can be obtained by transverse RF focusing with a high accelerating gradient, but the beam energy is usually limited by the available power of RF sources. In contrast, RF phase focusing (transverse RF focusing which depends on the beam phase) is enhanced by modifying the accelerating structure. Since RF phase focusing is effective while the beam velocity is low, the RF phase focusing is enhanced in the built-in bunching section which is used in compact accelerators. This paper reports two approaches to enhance the RF phase focusing. First, the phase velocities of bunching cells are optimized for maximum beam-focusing while preserving beam-bunching. Second, the first bunching cell is formed with a longitudinally asymmetric geometry to suppress the defocusing effect on a portion of the beam slices. In simulations using the PARMELA code, the beam radius was reduced by 50% compared to that produced by on-crest acceleration with longitudinally symmetric cells.

Applications of cavity transverse modes in accelerators

Different applications of TE modes in accelerator physics are discussed. In this discussion, the Fourier transform of the squared axial component of magnetic field, $B_{z}{}^{2}(z)$, plays an important role. If it turns out to be zero, the rf field-particle energy transfer is negligible and the focal length of the TE mode lens is phase independent. Such rf lens focuses continuous beams just as a solenoid. In order to compensate spherical aberrations and emittance growth caused by field nonlinearities of a focusing solenoid, the rf focusing is used also even if the beam space charge is taken into account. If the transform has its maximum, excitations of TE mode resonances and electron beam self-focusing are possible. Furthermore, the rf field of the TE mode can be used to expand the radial acceptance of a FEL for the THz region.

Analysis of slice transverse emittance evolution in a photocathode RF gun

The slice transverse emittance of an electron beam is of critical significance for an X-ray FEL. In a photocathode RF gun, the slice transverse emittance is not only determined by the emission process, but also influenced strongly by the nonlinear space charge effect. In this paper, we study the slice transverse emittance evolution in a photocathode RF gun using a simple model that includes effects of RF acceleration, focusing, and space charge force. The results are compared with IMPACT-T space charge simulations and may be used to understand the development of the slice emittance in an RF gun.

Beam dynamics in a high brightness linac for short wavelength SASE-FEL experiments

Short wavelength SASE-FEL requires generation and transport up to the undulator entrance of low emittance (∼1–2 mm mrad) high peak current (∼1–3 kA) electron beams with energy higher than 1 GeV. The propagation of such a high brightness beam takes place in a transition regime from space charge to emittance dominated dynamics, as the beam energy increases. In addition, in downstream magnetic compressor devices where the peak current increases up to kA range, the transition may occur again. Under these conditions, the electron beam has to be properly matched to the linac accelerating sections in order to keep under control emittance oscillations driven by residual space charge effects. Generalized invariant envelope matching conditions are discussed in this paper, showing that an equilibrium between RF focusing forces and space charge/emittance defocusing forces can be attained without any additional external focusing along the linac (no quadrupoles), thus reducing alignment problems and mitigating emittance dilutions due to misalignments and beam parameter jitters.

Hamiltonian analysis of transverse beam dynamics in high-brightness photoinjectors

A general Hamiltonian suitable for perturbative analysis of rapidly accelerating beams is derived from first principles. With the proper choice of coordinates, the resulting Hamiltonian has a simple and familiar form, yet is able to take into account the rapid acceleration, rf focusing, magnetic focusing, and average space-charge forces in rf photoinjectors. From the linear Hamiltonian, the beam-envelope evolution is solved and analyzed, which better illuminates the invariant-envelope solution as well as the theory of emittance compensation. The third-order nonlinear Hamiltonian is derived and analyzed to some extent. To make the analysis systematic and self-contained, alternative derivations are given for the smoothed ponderomotive rf focusing and the transfer matrix of a rf cavity.

Longitudinal beam dynamics simulation in electron rings in strong rf focusing regime

Obtaining very short bunches in an electron storage ring is one of the frontiers of the accelerator physics. The strong rf focusing (SRFF) is a way to have short bunches at a given position in the ring, thanks to the principle of the bunch length modulation. Until now, the bunch length modulation has been studied only in the limit of zero current; in this paper we present the results of a simulation code suitable to study the effects of coherent synchrotron radiation and vacuum chamber wakefields on the single bunch longitudinal dynamics in the SRFF regime . The code has been applied to three different lattices that can be realized in the Frascati ${\mathrm{e}}^{+}/{\mathrm{e}}^{\ensuremath{-}}$ collider DA\ensuremath{\Phi}NE for a possible experiment on bunch length modulation.

Manipulation of image intensity distribution at 7.0 T: Passive RF shimming and focusing with dielectric materials

To investigate the effects of high dielectric material padding on RF field distribution in the human head at 7.0 T, and demonstrate the feasibility and effectiveness of RF passive shimming and focusing with such an approach. The intensity distribution changes of gradient-recalled-echo (GRE) and spin-echo (SE) images of a human head acquired with water pads (dielectric constant = 78) placed in specified configurations around the head at 7.0 T were evaluated and compared with computer simulation results using the finite difference time domain (FDTD) method. The contributions to the B(1) field distribution change from the displacement current and conductive current of a given configuration of dielectric padding were determined with computer simulations. MR image intensity distribution in the human head with an RF coil at 7.0 T can be changed drastically by placing water pads around the head. Computer simulations reveal that the high permittivity of water pads results in a strong displacement current that enhances image intensity in the nearby region and alters the intensity distribution of the entire brain. The image intensity distribution in the human head at ultra-high field strengths can be effectively manipulated with high permittivity padding. Utilizing this effect, the B(1) field inside the human head of a given RF coil can be adjusted to reduce the B(1) field inhomogeneity artifact associated with the wave behavior (RF passive shimming) or to locally enhance the signal-to-noise ratio (SNR) in targeted regions of interest (ROIs; RF field focusing).

Slot-finger superconducting structure with rf focusing

The linear accelerator based on a superconducting structure with a high accelerating gradient accordingly has a strong rf defocusing factor. Usually, a quadrupole or solenoid focusing system is used in such accelerators. Both of these systems complicate the linear accelerator. The quadrupoles are located outside the cavity and a transition between the cold and the warm systems is required. Therefore an additional drift space between cryostats is needed, which can cause parametric resonance in the longitudinal plane in the energy range of $(3--20)\text{ }\text{ }\mathrm{MeV}$. In the system with the solenoid the high magnetic field can affect the superconductivity. We consider the novel superconducting H resonator based on the TE211 mode with a slot and rf finger, providing the high-intensity beam focusing in the large range of low energy of $(3--50)\text{ }\text{ }\mathrm{MeV}$. Above 50 MeV we suggest using the slot structure with the external quadrupoles.

3D MDEFT imaging of the human brain at 4.7 T with reduced sensitivity to radiofrequency inhomogeneity

A modification to the 3D modified driven equilibrium Fourier transform (MDEFT) imaging technique is proposed that reduces its sensitivity to RF inhomogeneity. This is especially important at high field strengths where RF focusing effects exacerbate B(1) inhomogeneity, causing significant signal nonuniformity in the images. The adiabatic inversion pulse used during the preparation period of the MDEFT sequence is replaced by a hard (nonadiabatic) pulse with a nominal flip angle of 130 degrees. The spatial inhomogeneity of the hard pulse preparation compensates for the inhomogeneity of the excitation pulses. Uniform signal intensity is obtained for a wide range of B(1) amplitudes and the high CNR characteristic of MDEFT is retained. The new approach was validated by numerical simulations and successfully applied to human brain imaging at 4.7 T, resulting in high-quality T(1)-weighted images of the whole human brain at high field strength with uniform signal intensity and contrast, despite the presence of significant RF inhomogeneity.

Cumulative beam breakup in linear accelerators with time-dependent parameters

A formalism presented in a previous paper for the analysis of cumulative beam breakup (BBU) with arbitrary time dependence of the beam current and with misalignment of the cavities and focusing elements [J. R. Delayen, Phys. Rev. ST Accel. Beams 6, 084402 (2003)] is extended to include time dependence of the focusing and coupling between the beam and the dipole modes. Such time dependence, which could result from an energy chirp imposed on the beam or from rf focusing, is known to be effective in reducing BBU-induced instabilities and emittance growth. The analytical results are presented and applied to practical accelerator configurations and compared to numerical simulations.