LANL, Los Alamos, New Mexico, USA
Beam bunching optimization at low energy (750keV) before injecting into a DTL (100MeV) is essential for beam transport, emittance reduction, and focusing on to a target. The LANSCE simultaneously utilizes H+ and H− beam (with a timing variation) for many important national security sciences. In addition to quadrupole, several bunchers are utilized in the transport. A technique with pre-bunching at lower frequency and main bunching at higher frequency is utilized for beam injection into the linac. The buncher parameters (voltage and frequency) are well established for operations. However, there is the possibility that the parameters vary with time due to electrical malfunction or adverse tuning during a beam development activity. Some effort is needed to correct the parameters as a non-optimized pre-bunching setup can alter the beam phase space and the nominal beam intensity at a desired location. Here, we examine emittance and phase space distribution variation for H− beam due to variation of the low frequency (16 MHz) buncher voltage, which typically operates at 25 kV peak. Beam phase dynamics with buncher voltage variation is also examined using the beam transport code Parmila.
LA-UR-16-23822
LANSCE: Los Alamos Neutron Science Center
DTL: Drift Tube Linac
ISU, Pocatello, Idaho, USA
Fermilab, Batavia, Illinois, USA
Northern Illinois University, DeKalb, Illinois, USA
Fermilab, Batavia, Illinois, USA
Northern Illinois Univerity, DeKalb, Illinois, USA
Masking a dispersive beamline such as a dogleg or a chicane [1, 2] is a simple way to shape a beam in the longitudinal and transverse space. This technique is often employed to generate arbitrary bunch profiles for beam/laser-driven accelerators and FEL undulators or even to reduce a background noise from dark currents in electron linacs. We have been investigating a beam-modulation of a slit-masked chicane, which was deployed for crystal-channeling experiments at the injector beamline of the Fermilab Accelerator Science and Technology (FAST) facility. With a nominal beam of 3 ps bunch length, Elegant simulations showed that a slit-mask with slit period 900 um and aperture width 300 um induces a modulation with bunch-to-bunch space of about 187 um (0.25 nC), 270 um (1 nC) and 325 um (3.2 nC) with 3 ~ 6% correlated energy spread: An initial energy modulation pattern has been observed in the electron spectrometer downstream of the masked chicane using a micropulse charge of 260 pC and 40 micropulses. Investigations of the beam longitudinal modulation are planned with a Martin-Puplett interferometer and a synchro-scan streak camera at a station between the chicane and spectrometer.
[1] D.C.Nguyen, B.E.Carlsten, NIMA 375, 597 (1996)
[2] P.Muggli, V.Yakimeno, M.Babzien, et al., PRL 101, 054801 (2008)
Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
JLab, Newport News, Virginia, USA
For a high-brightness electron beam with low energy and high bunch charge traversing a recirculation beamline, coherent synchrotron radiation and space charge effect may result in the microbunching instability (MBI). Both tracking simulation and Vlasov analysis for an early design of Circulator Cooler Ring* for the Jefferson Lab Electron Ion Collider reveal significant MBI. It is envisioned these could be substantially suppressed by using a magnetized beam. In this work, we extend the existing Vlasov analysis, originally developed for a non-magnetized beam, to the description of transport of a magnetized beam including relevant collective effects. The new formulation will be further employed to confirm prediction of microbunching suppression for a magnetized beam transport in a recirculating machine design.
*Ya. Derbenev and Y. Zhang, COOL'09 (FRM2MCCO01)
LBNL, Berkeley, California, USA
SLAC, Menlo Park, California, USA
LBNL, Berkeley, California, USA
SLAC, Menlo Park, California, USA
LCLS-II is a high-repetition rate (1 MHz) Free Electron Laser (FEL) X-ray light source now under construction at SLAC National Accelerator Laboratory. During transport to the FEL undulators, the electron beam is subject to a space charge-driven microbunching instability that can degrade the electron beam quality and lower the FEL performance if left uncontrolled. The present LCLS-II design is well-optimized to control the growth of this instability out of the electron beam shot noise. However, the instability may also be seeded by irregularities in the beam current profile at the cathode (due to non-uniformities in the temporal profile of the photogun drive laser pulse). In this paper, we describe the sensitivity of the microbunching instability to small-amplitude temporal modulations on the emitted beam current profile at the cathode, using high-resolution simulations of the LCLS-II beam delivery system.
Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
JLab, Newport News, Virginia, USA
Microbunching instability (MBI) has been a challenging issue in high-brightness electron beam transport for modern accelerators. The existing Vlasov analysis of MBI is based on single-pass configuration*. For multi-pass recirculation or a long beamline, the intuitive argument of quantifying MBI, by successive multiplication of MBI gains, was found to underestimate the effect**. More thorough analyses based on concatenation of gain matrices aimed to combine both density and energy modulations for a general beamline**. Yet, quantification still focuses on characterizing longitudinal phase space; microbunching residing in (x,z) or (x',z) was observed in particle tracking simulation. Inclusion of such cross-plane microbunching structures in Vlasov analysis shall be a crucial step to systematically characterize MBI for a beamline complex in terms of concatenating individual beamline segments. We derived a semi-analytical formulation to include the microbunching structures in longitudinal and transverse phase spaces. Having numerically implemented the generalized formulae, an example lattice*** is studied and reasonable agreement achieved when compared with particle tracking simulation.
* Heifets et al., PRSTAB 5, 064401 (2002), Huang and Kim, PRSTAB 5, 074401 (2002), and Vneturini, PRSTAB 10, 104401 (2007)
** Tsai et al., IPAC'16 (TUPOR020)
*** Di Mitri, PRSTAB 17, 074401 (2014)