Oliver Boine-Frankenheim

Analytic Modeling, Simulation and Interpretation of Broadband Beam Coupling Impedance Bench Measurements

Abstract First, a generalized theoretical approach towards beam coupling impedances and stretched-wire measurements is introduced. Applied to a circular symmetric setup, this approach allows to compare beam and wire impedances. The conversion formulas for TEM scattering parameters from measurements to impedances are thoroughly analyzed and compared to the analytical beam impedance solution. A proof of validity for the distributed impedance formula is given. The interaction of the beam or the TEM wave with dispersive material such as ferrite is discussed. The dependence of the obtained beam impedance on the relativistic velocity β is investigated and found as material property dependent. Second, numerical simulations of wakefields and scattering parameters are compared. The applicability of scattering parameter conversion formulas for finite device length is investigated. Laboratory measurement results for a circularly symmetric test setup, i.e. a ferrite ring, are shown and compared to analytic and numeric models. The optimization of the measurement process and error reduction strategies are discussed.

Interaction of heavy ion beams with dense plasmas

The main objective of the experimental plasma physics activities at the Gesellschaft fur Schwerionenforschung (GSI) is the interaction processes of heavy ions with dense ionized matter. Gas discharge plasma targets were used for energy loss and charge state measurements in a regime of electron density and temperature up to 10 19 cm -3 and 20 eV, respectively. Progress has been achieved in the understanding of charge-exchange processes in fully ionized hydrogen plasma. An improved model taking excitation-autoionization processes into account has removed most of the discrepancies of previous theoretical descriptions. Furthermore, it was found that the energy loss of the ion beam serves as an excellent diagnostic tool for measuring the electron density in partially ionized plasmas such as argon. The experience with these methods will be used in the future to diagnose dense laser produced plasmas. A setup with a 100 J/5 GW Nd:glass laser, currently under construction, will provide access to density range up to 10 21 cm -3 and temperatures of more than 100 eV. To reach electron densities near solid-state density (10 23 cm -3 ), heavy ion heated frozen rare gas crystals were used. The first hydrodynamic motion of ion heated solid material was observed. Vacuum-ultraviolet (VUV) spectroscopy was applied to diagnose these strongly coupled nonideal plasmas.

A theory of the beam loss-induced vacuum instability applied to the heavy-ion synchrotron SIS18

A theoretical model to describe the vacuum instability induced by the lost particles in heavy-ion accelerators is proposed and applied to the U+28 beam lifetime measurements in the SIS18 at GSI, where the instability is of concern for high-intensity operation. The method allows to derive values of the desorption yield, the charge-exchange cross-section and the total pumping speed from the measurements of the beam lifetime I(t). The obtained desorption yields are comparable with those found in the other heavy-ion machines. Directions of possible cures are discussed.

Beam Instabilities in Hadron Synchrotrons

Beam instabilities cover a wide range of effects in particle accelerators and they have been the subjects of intense research for several decades. As the machines performance was pushed new mechanisms were revealed and nowadays the challenge consists in studying the interplays between all these intricate phenomena, as it is very often not possible to treat the different effects separately. The aim of this paper is to review the main mechanisms, discussing in particular the recent developments of beam instability theories and simulations.

Space-Charge Structural Instabilities and Resonances in High-Intensity Beams.

The existence of a structural resonance stop band caused by space charge in high-current beams, where the resonance frequency is associated with 90° phase advance per focusing period, is well known and alternatively referred to in the literature as envelope instability or as fourth-order resonance. We show, however, that this stop band is actually a coincidence of a structural fourth-order resonance and the much stronger envelope instability as competing mechanisms--depending on the time scale and initial matching. A similar complexity of behavior--dependent on the distribution function--is also found between a third-order instability and a sixth-order resonance in a 60° stop band. We claim that these findings are a generic property of high-intensity beams in periodic focusing; they also allow a reinterpretation of the 90° linear accelerator stop band previously observed experimentally at the UNILAC accelerator.

Interpretation of transverse tune spectra in a heavy-ion synchrotron at high intensities

Two different tune measurement systems have been installed in the GSI heavy-ion synchrotron SIS-18. Tune spectra are obtained with high accuracy using these fast and sensitive systems. Besides the machine tune, the spectra contain information about the intensity dependent coherent tune shift and the incoherent space charge tune shift. The space charge tune shift is derived from a fit of the observed shifted positions of the synchrotron satellites to an analytic expression for the head-tail eigenmodes with space charge. Furthermore, the chromaticity is extracted from the measured head-tail mode structure. The results of the measurements provide experimental evidence of the importance of space charge effects and head-tail modes for the interpretation of transverse beam signals at high intensity.

Optimum laser parameters for 1D radiation pressure acceleration

Laser ion acceleration (Wilks et al ., 2001; Passoni et al ., 2010) has become an interesting field of research in the past years. Several experiments, such as LIGHT (Schollmeier et al ., 2008; Bagnoud et al ., 2010; Busold et al ., 2013; 2014 a ; 2014 b ) are performed worldwide. High intense, pulsed laser beams are used to generate and accelerate a plasma. For higher laser intensities (>10 21 W cm −1 ), simulations (Esirkepov et al ., 2004; Macchi et al ., 2005; 2009; 2010; Robinson et al ., 2008; Rykovanov et al ., 2008; Henig et al ., 2009; Schlegel et al ., 2009; Shoucri et al ., 2011; 2013; 2014; Kar et al ., 2012; Korzhimanov et al ., 2012; Shoucri, 2012) have revealed a new acceleration mechanism: The Radiation Pressure Acceleration. The entire foil target is accelerated by the radiation pressure of the laser pulse. Ideally, a sharp peak spectrum is generated, with energies up to GeV and nearly solid body density. This work faces on a detailed analysis of the acceleration mechanism in order to develop the optimum laser- and target parameters for the process. The analysis is supported by one-dimensional PIC simulations, using the commercial code VSim

Beam losses in heavy ion drivers

While beam loss issues have hardly been considered in detail for heavy ion fusion scenarios, recent heavy ion machine developments in different labs (European Organization for Nuclear Research(CERN), Gesellschaft fur Schwerionenforschung (GSI), Institute for Theoretical and Experimental Physics (ITEP), Relativistic Heavy-Ion Collider (RHIC)) have shown the great importance of beam current limitations due to ion losses. Two aspects of beam losses in heavy ion accelerators are theoretically considered: (1) secondary neutron production due to lost ions, and (2) vacuum pressure instability due to charge exchange losses, Calculations are compared and found to be in good agreement with measured data. The application to a Heavy-Ion Driven Inertial Fusion (HIDIF) scenario is discussed.

Beam dynamics analysis of dielectric laser acceleration using a fast 6D tracking scheme

A six-dimensional symplectic tracking approach exploiting the periodicity properties of Dielectric Laser Acceleration (DLA) gratings is presented. The longitudinal kick is obtained from the spatial Fourier harmonics of the laser field within the structure, and the transverse kicks are obtained using the Panofsky-Wenzel theorem. Additionally to the usual, strictly longitudinally periodic gratings, our approach is also applicable to periodicity chirped (sub-relativistic) and tilted (deflection) gratings. In the limit of small kicks and short periods we obtain the 6D Hamiltonian, which allows, for example, to obtain matched beam distributions in DLAs. The scheme is applied to beam and grating parameters similar to recently performed experiments. The paper concludes with an outlook to laser based focusing schemes, which are promising to overcome fundamental interaction length limitations, in order to build an entire microchip-sized laser driven accelerator.

Transverse decoherence and coherent spectra in long bunches with space charge

The transverse bunch spectrum and the transverse decoherence/recoherence following an initial bunch offset are important phenomena in synchrotrons and storage rings, and are widely used for beam and lattice measurements. Incoherent shifts of the particles betatron frequency and of the synchrotron frequency modify the transverse spectrum and the bunch decoherence. In this study we analyze the effects of transverse space charge and of the rf nonlinearity on the decoherence signals. The transverse bunch decoherence and the resulting coherent spectra are measured in the SIS18 synchrotron at GSI Darmstadt for different bunch parameters. The frequencies of the bunch head-tail modes provide a direct measure for the self-field space charge tune shift. Particle tracking simulations together with an analytical model are used to describe the modifications in the decoherence signals and in the coherent spectra due to space charge and the rf bucket nonlinearity. Transverse coherent oscillations of bunches induced by a fast kicker magnet are routinely used in synchrotrons or storage rings to measure, for example, the tune or other ring parameters, see e.g. (1). The transverse offset of a bunch, averaged over the bunch length, can be recorded every single turn. The spectrum is then concentrated around the base-band Q f0 f 0 , where Q f0 is the fractional part of the betatron tune Q0 and f0 is the revolution frequency. This diagnostics is usually used for time- resolved and very accurate measurements of the tune Qf0. Transverse bunch decoherence is a process of a turn-to- turn reduction of the total bunch offset signal after an initial bunch displacement. In a linear focusing lattice the bunch decoherence is a manifestation of the lattice chro- maticitywhere the synchrotron dynamics also plays an important role, causing the signal recoherence exactly after the synchrotron period. Other damping mechanisms, as due to lattice nonlinearities, additionally damp the trans- verse oscillations. Transverse decoherence is often used as a machine diagnostics tool. Undesired transverse bunch oscillations can also appear after the bunch-to-bucket transfer between synchrotrons. In order to use transverse decoherence as a diagnostics tool for intense bunches of arbitrary length and also to control undesired oscilla- tions of such bunches, it is important to understand the decoherence in the presence of transverse space charge and nonlinear synchrotron oscillations. We demonstrate that the decoherence signal can be explained in terms of the transverse head-tail bunch mode spectrum. For finite chromaticity also the k> 0 head-tail modes contribute to the bunch coherent spectrum. The shift of the head-tail mode frequencies due to space charge and wall currents can be well explained in terms of the analytical expressions for an airbag bunch distribution (2,3). The head-tail mode frequencies are also modified by changes in the individual particle synchrotron frequency. In long bunches, one has to account for the spread of the synchrotron frequencies. Both transverse space charge and nonlinear synchrotron oscillations are important to under- stand the decoherence signals and transverse spectra. We demonstrate that, once the spectrum and decoherence mod- ifications are understood, they can be used to extract useful information about the bunches. In this paper we describe measurements of transverse bunch spectra and decoherence signals obtained in the heavy-ion synchrotron SIS18 at GSI Darmstadt. The ob- served modification of the head-tail spectrum and of the decoherence signal caused by transverse space charge and nonlinear synchrotron oscillations are explained in terms of our theoretical approach. This approach is based on an expansion of an analytical theory for head-tail modes in combination with particle tracking simulations. In Sec. II we use theoretical and numerical approaches to analyze the effects of space charge and nonlinear syn- chrotron motion on the transverse spectra and on the bunch decoherence signal. We show that a simple model for the head-tail mode frequencies with fitting parameters can be Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- bution of this work must maintain attribution to the author(s) and the published articles title, journal citation, and DOI. PHYSICAL REVIEW SPECIAL TOPICS - ACCELERATORS AND BEAMS 15, 114201 (2012)