Fermilab, Batavia, USA
SLAC, Menlo Park, California, USA
CERN, Geneva, Switzerland
Radio Frequency noise induced bunch lengthening can strongly affect the Large Hadron Collider performance through luminosity reduction, particle loss, and other effects. Models and theoretical formalisms demonstrating the dependence of the LHC longitudinal bunch length on the RF station noise spectral content have been presented*,**. Initial measurements validated these studies and determined the performance limiting RF components. For the existing LHC LLRF implementation the bunch length increases with a rate of 1 mm/hr, which is higher than the intrabeam scattering diffusion and leads to a 27% bunch length increase over a 20 hour store. This work presents measurements from the LHC that better quantify the relationship between the RF noise and longitudinal emittance blowup. Noise was injected at specific frequency bands and with varying amplitudes at the LHC accelerating cavities. The experiments presented in this paper confirmed the predicted effects on the LHC bunch length due to both the noise around the synchrotron frequency resonance and the noise in other frequency bands aliased down to the synchrotron frequency by the periodic beam sampling of the accelerating voltage.
*T. Mastorides et.al., "RF system models for the LHC with Application to Longitudinal Dynamics,"
**T. Mastorides et.al., "RF Noise Effects on Large Hadron Collider Beam Diffusion"
Fermilab, Batavia, USA
SLAC, Menlo Park, California, USA
CERN, Geneva, Switzerland
LBNL, Berkeley, California, USA
The feedback control of intrabunch instabilities driven by E-Clouds or strong head-tail coupling (TMCI) requires sufficient bandwidth to sense the vertical position and drive multiple sections of a nanosecond scale bunch. These requirements impose challenges and limits in the design and implementation of the feedback system. This paper presents models for the feedback subsystems: receiver, processing channel, amplifier and kicker, that take into account their frequency response and limits. These models are included in multiparticle simulation codes (WARP/CMAD/Head-Tail) and reduced mathematical models of the bunch dynamics to evaluate the impact of subsystem limitations in the bunch stabilization and emittance improvement. With this realistic model of the hardware, it is possible to analyze and design the feedback system. This research is crucial to evaluate the performance boundary of the feedback control system due to cost and technological limitations. These models define the impact of spurious perturbations, noise and parameter variations or mismatching in the performance of the feedback system. The models are validated with simulation codes and measurements of lab prototypes.
JLAB, Newport News, Virginia, USA
University of Connecticut, Storrs, Connecticut, USA
MPI-P, München, Germany
USTC/NSRL, Hefei, Anhui, People's Republic of China
BNL, Upton, Long Island, New York, USA
Stony Brook University, Stony Brook, USA
USC, Los Angeles, California, USA
CLASSE, Ithaca, New York, USA
LBNL, Berkeley, California, USA
CalPoly, San Luis Obispo, California, USA
Measurements of coherent tune shifts at the Cornell Electron Storage Ring Test Accelerator (CesrTA) have been made for electron and positron beams under a wide variety of beam energies, bunch charge, and bunch train configurations. Comparing the observed tunes with the predictions of several electron cloud simulation programs allows the evaluation of important parameters in these models. These simulations will be used to predict the behavior of the electron cloud in damping rings for future linear colliders. We outline recent improvements to the analysis techniques that should improve the fidelity of the modeling.
Cornell University, Ithaca, New York, USA
CLASSE, Ithaca, New York, USA
SLAC, Menlo Park, California, USA
We present results of a series of studies obtained using the simulation code CMAD to study how electron clouds affect the dynamics of positron beams in CesrTA. The study complements ongoing experiments dedicated for studying the same phenomena. The simulation involves tracking positrons through the CesrTA lattice and simultaneously computing the force exerted due to space charge of the electrons on each of the tracked positrons. The electrons themselves are allowed to evolve under the influence of the positrons. Several results bear a close resemblance to what has been observed experimentally.
Euclid TechLabs, LLC, Solon, Ohio, USA
LETI, Saint-Petersburg, Russia
ANL, Argonne, USA
Beam breakup (BBU) effects resulting from parasitic wakefields limit considerably the intensity of the drive beam that can be transported through a dielectric accelerating structure and hence the accelerating field that can be achieved. We have been developing techniques to control BBU effects using a quadrupole channel or solenoid surrounding the wakefield device. We report here on the status of simulations and experiments on BBU and its mitigation, emphasizing an experiment at the Argonne Wakefield Accelerator facility using a 26 GHz dielectric wakefield device fitted with a solenoid to control BBU. We present calculations based on a particle-Green’s function beam dynamics code (BBU-3000) that we are developing. The code allows rapid, efficient simulation of BBU effects in advanced linear accelerators.
Fermilab, Batavia, USA
Tech-X, Boulder, Colorado, USA
Fermilab, Batavia, USA
The brightness of the antiproton beam in Fermilab’s 8 GeV Recycler ring is limited by a transverse instability. This instability has occurred during the extraction process to the Tevatron for large stacks of antiprotons even with dampers in operation. This paper describes observed features of the instability, introduces the threshold phase density to characterize the beam stability, and finds the results to be in agreement with a resistive wall instability model. Effective exclusion of the longitudinal tails from Landau damping by decreasing the depth of the RF potential well is observed to lower the threshold density by up to a factor of two.
Fermilab, Batavia, USA
* A. Burov, “Dancing Bunches as van Kampen Modes”, this conference.
PPPL, Princeton, New Jersey, USA
SLAC, Menlo Park, California, USA
CLS, Saskatoon, Saskatchewan, Canada
Theory predicts that CSR in storage rings should appear in whispering gallery modes *, which are resonances of the entire vacuum chamber and are characterized by their high frequencies and concentration of the field near the outer wall of the chamber. The theory assumes that the chamber is a smooth circular torus. We observe that a power spectrum from the NSLS-VUV ring **, which has a vacuum chamber in bends like that of the model, shows a series of sharp peaks with frequencies close to those of the theory. Sharp peaks are also seen in highly resolved spectra at the Canadian Light Source***, and those are invariant in position under large changes in the machine setup (energy, fill pattern, bunch length, CSR bursting or steady, optical beam line, etc.). Invariance of the spectrum suggests that it is due to resonances like whispering gallery modes, but they must be strongly perturbed from the circular case because of large outer wall excursions at the two IR ports.
* R. L. Warnock and P. Morton, Part. Accel. 25, 113 (1990).
** G. L. Carr et al., Proc. PAC 2001, Chicago.
*** T. May et al., IEEE 33rd Intl. Conf. IR Millimeter and THz Waves, 2008.
Tech-X, Boulder, Colorado, USA
Low-frequency oscillations (LFOs) have been observed in a high average power gyrotron and the trapped electron population contributing to the oscillation has been measured. As high average power gyrotrons are the most promising millimeter wave source for thermonuclear fusion research, it is important to get a better understanding of this parasitic phenomenon to avoid any deterioration of the electron beam quality thus reducing the gyrotron efficiency. 2D Particle-in-cell (PIC) simulations quasi-statically model the development of oscillations of the space charge in the adiabatic trap, but the physics of the electron dynamics in the adiabatic trap is only partially understood. Therefore, understanding of the LFOs remains incomplete and a full picture of this parasitic phenomenon has not been seen yet. In this work, we use a 3D conformal finite-difference time-domain (CFDTD) PIC method to accurately and efficiently study the LFOs in a high average power gyrotron. Complicated structures, such as a magnetron injection gun, can be well described. Employing a highly parallelized computation, the model can be simulated in time domain more realistically.
University of Michigan, Spin Physics Center, Ann Arbor, MI, USA
Universität Bonn, Helmholtz-Institut für Strahlen- und Kernphysik, Bonn, Germany
JLAB, Newport News, Virginia, USA
Spin resonances can cause partial or full depolarization or spin-flip of a polarized beam. We studied 1st-, 2nd- and 3rd-order spin resonances with a 2.1 GeV/c vertically polarized proton beam stored in the COSY Cooler Synchrotron. We observed almost full spin-flip when crossing the 1st-order G*gamma=8−nuy vertical-betatron-tune spin resonance and partial depolarization near some 2nd- and 3rd-order resonances. We observed almost full depolarization near the 1st-order G*gamma=8−nux horizontal spin resonance and partial depolarization near some 2nd- and 3rd-order resonances. Moreover, we found that a 2nd-order nux resonance seems about as strong as some 3rd-order nux resonances, while some 3rd-order nuy resonances seem much stronger than a 2nd-order nuy resonance. It was thought that, for flat accelerators, vertical spin resonances are stronger than horizontal, and lower order resonances are stronger than higher order ones. The data suggest that many higher-order spin resonances, both horizontal and vertical, must be overcome to accelerate polarized protons to high energies; the data may help RHIC to better overcome its snake resonances between 100 and 250 GeV/c.
USC, Los Angeles, California, USA
BNL, Upton, Long Island, New York, USA
LANL, Los Alamos, New Mexico, USA
IPFN, Lisbon, Portugal
UCLA, Los Angeles, California, USA
Current Filamentation Instability, CFI, is of central importance for the propagation of relativistic electron beams in plasmas. CFI could play an important role in the generation of magnetic fields and radiation in the after-glow of gamma ray bursts and also in energy transport for the fast-igniter inertial confinement fusion concept. Simulations were conducted with the particle-in-cell code QuickPIC* for e- beam and plasma parameters at the Brookhaven National Laboratory – Accelerator Test Facility, BNL-ATF. Results show that for a 2cm plasma the instability reaches near saturation. An experimental program was proposed and accepted at the BNL-ATF and an experiment is currently underway. There are three components to the experimental program: 1) imaging of the beam density/filaments at the exit from the plasma, 2) measurement and imaging of the transverse plasma density gradient and measurement of the magnetic field and 3) identifying the radiation spectrum of the instability. Preliminary results from phase one will be presented along with the progress and diagnostic design for the following phases of the experiment.
* C. Huang et. al. Journal of Computational Physics 217, 2(2006)