Fermilab, Batavia
Funding: FRA
Fermilab’s High Intensity Neutrino Source (HINS), the 325 MHz low energy section of Project X consists of an RFQ, 18 copper cavities and a section of superconducting spoke resonator cavities, all driven by a single 2.5 MW klystron. Each cavity has a high power ferrite vector modulator which provides individual RF power control. This paper proposes a scheme that optimizes RF drive and vector modulator control. The different gradients, acceleration phase angles, unloaded Q’s and beam loading are taken into account to optimize the cavities detuning angles, forward power, and loaded Q’s. This scheme makes an efficient use of the klystron’s high bandwidth ability to modulate the forward power, hence minimizing the burden on the high power vector modulator during the RF pulse. The proposed method is explained in details, a parameter sensitivity analysis is performed and the impact on the total power consumption for the RF station is calculated.
ASCo, Clayton, Victoria
RF cavities are routinely detuned slightly from resonance to maintain Robinson stability of the beam as beam loading increases. Detuning the cavities results in a reduction of the overall energy efficiency of the RF and can waste many MW hours of energy per year. It is therefore desirable to only detune as much as required by the beam loading to maintain stability. A new system for monitoring the load impedance of the Storage Ring RF cavities has been developed at the Australian Synchrotron. The system utilises the Analogue devices AD8302 chip to monitor the load impedance of the Cavities and allow for more efficient detuning of the system. An overview and commissioning results of this system will be presented.
Fermilab, Batavia
DESY, Hamburg
ANL, Argonne
KEK, Ibaraki
Funding: *Work supported by Fermi Research Alliance, LLC. under ContractNo. DE-AC02-07CH11359 with the United States Department of Energy.
In September 2008 the DESY-FLASH accelerator was run with up to 550, 3 nano-coulomb bunches at 5 Hz repetition rate. This test is part of a longer term study aimed at validating ILC parameters by operation as close as possible to ILC beam currents and RF gradients. The present paper reports on the analysis that has been done in order to understand the RF control system performance during this test. Actual klystron power requirements and beam stability are evaluated with heavy beam loading conditions. Results include suggested improvements for upcoming tests in 2009
KEK, Ibaraki
JAEA/J-PARC, Tokai-Mura, Naka-Gun, Ibaraki-Ken
NETS, Fuchu-shi
Upgrade of J-PARC linac has been planed using 972 MHz rf system. The rf field regulation is required to be less than ±1% in amplitude and ±1deg. in phase. The basic digital llrf concept is same as the present 324 MHz llrf system using a compact PCI crate. The main alterations are rf and clock generator (RF&CLK), mixer and IQ modulator (IQ&Mixer) and digital llrf algorithm. Since the typical decay time is faster (due to higher operational frequency than present 324 MHz cavity), chopped beam compensation is one of the main concerns. Performance of the digital feedback system using a cavity simulator is summarized.
NSRRC, Hsinchu
The direct RF feedback has been adopted in storage ring to reduce the beam loading effect for maximizing the stored beam current. Its performance in reducing beam loading is determined by the operational parameters, including the feedback gain, RF phase shift and the loop delay time. This paper presents a mathematical method, based on the Pedersen model, to study the effects of the direct RF feedback on beam loading. Through an example, the influences of different operational parameters on the performance of the direct RF feedback is analyzed by examining the characteristic equation of the feedback loop. The Nyquist criterion is applied for the determination of system stability.
TU Darmstadt, RTR, Darmstadt
GSI, Darmstadt
Funding: This work was partly supported by Deutsche Telekom Stiftung.
The effects of heavy beam loading on the RF control loops of a double-harmonic cavity system are examined. This cavity system that will be realized at the GSI Helmholtzzentrum für Schwerionenforschung in the scope of the SIS18 upgrade program consists of a main broadband cavity and a second harmonic narrowband cavity. The cavities comprise both an amplitude and a phase feedback loop. In addition, the narrowband cavity includes a feedback loop which controls its resonance frequency to follow the main RF frequency. After modelling the cavity system and the feedback loops, an analytic controller design is presented. In addition, longitudinal beam dynamics are added to the cavity model to allow a detailed simulation of the cavity-beam interaction. Realistic simulation results are given for an acceleration cycle of heavy-ions to demonstrate the performance of the RF control loops.
KEK, Ibaraki
KEKB is a multi-bunch, high-current electron-positron collider. Newly installed crab cavities realized an effective head-on collision, while maintaining finite-angle crossing orbits. Bunches form a single train followed by a beam abort gap. We observed a beam phase advancing along a train due to transient beam loading. Since there is a difference in the beam phase between the two beams, a longitudinal displacement of the collision vertex is expected under the crabbing collision. Estimated variations agree with those detected by the Belle*. A displacement in the horizontal beam position was observed in correspondence with the variations in the beam phase. We found that the horizontal displacement was caused by a transverse kick of the crab cavities to phase-shifted bunches. Moreover, a rapid phase advancing was observed at the leading part in a train in the LER. We suspect that some longitudinal wakes with low Q values in accelerator components might contribute to the rapid change in the beam phase.
*H. Kichimi et al., to be published.
DESY, Hamburg
ANL, Argonne
Fermilab, Batavia
KEK, Ibaraki
Funding: Work at Argonne supported by U.S. Department of Energy, Office of Science, office of Basic Energy, Sciences, under Contract No. DE-AC02-06CH11357
XFEL and ILC both intend to accelerate long beam pulses of a few thousand bunches and high average current. It is expected that the superconducting accelerating cavities will eventually be operated close to their respective gradient limits as they are pushed to higher energies. In addition, a relative energy stability of <10-4 must be maintained across all bunches. These parameters will ultimately push the limits of several sub systems including the low-level rf control, which must properly compensate for the heavy beam loading while avoiding problems from running the cavities close to their quench limits. An international collaboration led by DESY has begun a program of study to demonstrate such ILC-like conditions at FLASH, which serves as a prototype for both XFEL and ILC. The objective is to achieve reliable operation with pulses of 2400 3-nC bunches spaced by 330 ns (a current of 9 mA) while meeting the required energy stability and while operating accelerating cavities close to their quench limits. Other goals include measurement of cryoload from HOM heating and evaluation of rf power overhead for the ILC. The paper will describe the program and report recent results.
LBNL, Berkeley, California
University of Nevada, Reno, Reno, Nevada
Funding: Supported by the Office of High Energy Physics of the U.S. DOE under Contract No. DE-AC02-05CH11231, and DARPA.
Laser wakefield acceleration experiments were carried out by using various hydrogen-filled capillary discharge waveguides. Self-trapping of electrons showed strong correlation with the delay between the onset of the discharge current and arrival of the laser pulse (discharge delay). By de-tuning discharge delay from optimum guiding performance, self-trapping was found to be stabilized. Several possible scenarios for the enhanced trapping will be discussed along with spectroscopy of the transmitted laser light and the discharge recombination light.
Tech-X, Boulder, Colorado
CIPS, Boulder, Colorado
LBNL, Berkeley, California
University of Nevada, Reno, Reno, Nevada
Funding: Supported by the US DOE Office of Science, Office of High Energy Physics under grant No. DE-FC02-07ER41499; used NERSC resources under grant DE-AC02-05CH11231.
Laser wakefield accelerators (LWFA) have accelerated ~100 pC electron bunches to GeV energies over cm scale distances, via self-trapping from the plasma. Self-trapping cannot be tolerated in staged LWFA modules for high-energy physics applications. The ~1% energy spread of self-trapped electron bunches is too large for light source applications. Both difficulties could be resolved via external injection of a low-emittance electron bunch into a quasilinear LWFA, for which the dimensionless laser amplitude is less than two. However, significant beam charge will result in nonlinear beam loading effects, which will make it challenging to preserve the low emittance. The cold, relativistic fluid model of the parallel VORPAL framework* will be used to simulate the laser-driven electron wake, in the presence of an idealized electron beam. Profiles of the electron beam density, laser pulse envelope and plasma channel will be varied to find a nonlinear beam loading configuration that approximately flattens the electric fields across the beam. Hybrid fluid-PIC simulations will be used to measure the self-consistent emittance growth of the beam.
* C. Nieter and J.R Cary, J. Comp. Phys. 196 (2004), p. 448.
LBNL, Berkeley, California
Funding: Supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
Uneven beam fill patterns in storage rings, such as gaps in the fill patterns, leads to periodic, or transient loading of the modes of the RF cavities. We show that an analogous effect can occur in the loading of a dipole cavity mode when the beam passes off the electrical center of the cavity mode. Although this effect is small, it results in a variation of the transverse offset of the beam along the bunch train. For ultralow emittance beams, such as optimized third generation light sources and damping rings, this effect results in a larger projected emittance of the beam compared with the single bunch emittance. The effect is particularly strong for the case when a strong dipole mode has been purposely added to the ring, such as a deflecting, or ‘‘crab'' cavity. We derive an approximate analytic solution for the variation of the beam-induced deflecting voltage along the bunch train. We also show via a tracking simulation the combined effect of the periodic loading of the fundamental and dipole modes.
SLAC, Menlo Park, California
A proposed high-brightness synchrotron light source (PEP-X) is under design at SLAC. The 4.5-GeV PEP-X storage ring has four theoretical minimum emittance (TME) cells to achieve the very low emittance and two double-bend achromat (DBA) cells to provide spaces for IDs. Damping wigglers will be installed in zero-dispersion straights to reduce the emittance below 0.1 nm. Ion induced beam instability is one critical issue due to its ultra small emittance. Third harmonic cavity can be used to lengthen the bunch in order to improve the beam life time. Bunch-train filling pattern is proposed to mitigate both the fast ion instability and beam loading effect. This paper investigates the fast ion instability and beam loading for different beam filling patterns.
THALES, Colombes
CELLS-ALBA Synchrotron, Cerdanyola del Vallès
EuroMev, Buc
A turn key 100 MeV linac was provided by THALES Communications in order to inject electrons into the booster synchrotron of ALBA*. The linac was commissioned in October 2008. This paper will remind the main features of the linac** and will give results obtained during the commissioning tests. The energy and emittance measurements have been done on the transfer line concieved and realized by CELLS. Specified and measured beam parameters will be compared to show the performance of the entire system.
* D. Einfeld "Progress of ALBA", EPAC08, Genoa, Italy, June 2008.
** A. Falone et Alt, "Status of the 100 MeV preinjector for the ALBA synchrotron", EPAC08, Genoa, Italy, June 2008.