Projects (past)

RF CAVITY BREAKDOWN IN MAGNETIC FIELDS

Field emisison can be a serious problem for the operation of rf cavities (a). In the presence of B-fields the electron trajectories can be focused into particular spots and create more damage. As a result the maximum achievable gradient becomes correlated to the B-field. Both data and our model show this behavior (b). One solution is to apply magnetic solution (c). Another solution is to use more robust materials such as Al or Be. A modular cavity was designed to test this hypothesis (d).

RF cavities are commonly used to accelerate beams. However, when they are exposed to external magnetic fields they can loose part of their strength. Experimental data showed a steady reduction of the maximum achievable when the field was increased from 0-4 T. In this project, we detailed a model to describe the operation of rf cavities in magnetic fields. We found that the cause of damage in the cavity was due the impact of field-emitted electrons focused by the magnetic field of the cavity surfaces. Such electrons would induce local cyclic heating of the cavity body. Eventually, this can lead to breakdown. Our predictions matched well the experimental data. A possible solution could be magnetic insulation: Design a cavity wherein its surfaces overlap with the magnetic field lines. Another solution is to use more robust materials to construct cavities such as Al or Be. In fact, a later test with a Be modular cavity found a factor of two better performance exactly as predicted by our model.

Related publications:

RF breakdown with external magnetic fields in 201 and 805 MHz cavities, R. B. Palmer, R. C. Fernow, J. C. Gallardo, D. Stratakis, and D. Li, Phys. Rev. ST - Accel. Beams 12, 031002 (2009)
Effects of external magnetic fields on the operation of high-gradient accelerating structures, D. Stratakis, J. C. Gallardo, and R. B. Palmer, Nucl. Instr. and Meth. A 620, 147 (2010).
Magnetically insulated high-gradient accelerating structures for muon accelerators, D. Stratakis, J. C. Gallardo, and R. B. Palmer, Journal of Physics G: Nuclear and Particle Physics 37, 105011 (2010).
Numerical study of a magnetically insulated front-end channel for a Neutrino Factory, D. Stratakis, R. C. Fernow, J. C. Gallardo, R. B. Palmer, and D. V. Neuffer, Phys. Rev. ST - Accel. Beams 14, 011001 (2011).

GENERALIZED PHASE-SPACE TOMOGRAPHY FOR INTENSE BEAMS

The partial merger of five beamlets (left) in configuration space and x-x' phase space: Experimental measurement and tomographic phase-space reconstruction (middle); simulation with WARP code; (right).

In this project a Tomographic diagnostic for phase-space mapping of intense particle beam was studied. The diagnostic was extended to beams with space-charge by assuming linear forces and was implemented using either solenoidal or quadrupole focusing lattices. The technique was benchmarked against self-consistent simulation and against a direct experimental sampling of phase-space using a pinhole scan. It was demonstrated that tomography can work for time-resolved phase-space mapping and slice emittance measurement. The technique was applied to a series of proof-of-principle tests conducted at the University of Maryland Electron Ring. Please see Reference 5 for more details.

Related publications:

Tomography as a diagnostic tool for phase space mapping of intense particle beams, D. Stratakis, R. A. Kishek, H. Li, S. Bernal, M. Walter, B. Quinn, M. Reiser, and P.G. O'Shea, Phys. Rev. ST - Accel. Beams 9, 112801 (2006).
Tomographic phase-space mapping of intense particle beams using solenoids, D. Stratakis, K. Tian, R.A. Kishek, I. Haber, M. Reiser, and P.G. O'Shea, Physics of Plasmas (Letters) 14, 120703 (2007).
Experimental and numerical study of phase mixing of an intense particle beam, D. Stratakis, R.A. Kishek, I. Haber, S. Bernal, M. Reiser, and P.G. O'Shea, Phys. Rev. ST - Accel. Beams 12, 064201 (2009).
Experimental verification of tomographic phase-space imaging for beams with space-charge using a pinhole scan, D. Stratakis, R. A. Kishek, I. Haber, R. B. Fiorito, M. Reiser, and P. G. O'Shea, Journal of Applied Physics 107, 104905 (2010).
Generalized phase-space tomography for intense beams, D. Stratakis, R.A. Kishek, S. Bernal, R. B. Fiorito, I. Haber, M. Reiser, P.G. O'Shea, K. Tian, J. C. T. Thangaraj, Physics of Plasmas 17, 056701 (2010).

DIELECTRIC WAKEFIELD ACCELERATION

Conceptual drawing of DWA (left); measured acceleration from Ref. 6 (right). An energy gain (loss) of about 150 keV is observed for the high energy tail (low energy core) of the beam.

In this project we demonstrated first evidence of wakefield acceleration of a relativistic electron beam in a dielectric-lined slab-symmetric structure. The high energy tail of a 60 MeV electron beam was accelerated by 150 keV in a 2 cm-long, slab-symmetric SiO2 waveguide, with the acceleration or deceleration clearly visible due to the use of a beam with a bifurcated longitudinal distribution that serves to approximate a driver-witness beam pair. This split-bunch distribution is verified by longitudinal reconstruction analysis of the emitted coherent transition radiation. The dielectric waveguide structure is further characterized by spectral analysis of the emitted coherent Cherenkov radiation at THz frequencies, from a single electron bunch, and from a relativistic bunch train with spacing selectively tuned to the second longitudinal mode (TM02). Start-to-end simulation results reproduced aspects of the electron beam bifurcation dynamics, emitted THz radiation properties, and the observation of acceleration in the dielectric-lined, slab-symmetric waveguide

Related publications:

Dielectric wake eld acceleration of a relativistic electron beam in slab-symmetric dielectric lined waveguide, A. Andonian, D. Stratakis, M. Babzien, S. Barber, M. Fedurin, E. Hemsing, K. Kusche, P. Muggli, B. O'Shea, X. Wei, O. Williams, V. Yakimenko and J. B. Rosenzweig, Physical Review Letters 108, 244801 (2012).

MUON 6D COOLING CHANNELS FOR MUON ACCELERATORS

When the beam passes through material it losses energy via ionization. By using a strong focusing field and by placing an rf cavity next to the absorbed the transverse emittance can decrease (cooling). Several cooling designs have been explored : The most common ones are helical cooling channels (right-top) or rectilinear channels (right-bottom).

A muon collider would require a reduction of the six-dimensional emittance of the captured muon beam by 5 or 6 orders of magnitude. In this project, we examined a rectilinear and helical cooling lattice that should meet this requirement. First, we prepared a conceptual design of our proposed schemes. Then, we established the theoretical framework to predict and evaluate the performance of ionization cooling channels. We then applied it to our specific cases and found excellent agreement. At the last stage, we carried out a end-to-end simulation of 6D cooling for a muon collider scenario and showed a notable reduction of the 6D emittance by at least 5 orders of magnitude. This was validated with two independent codes: ICOOL and G4beamline. Since these channels would require magnetic fields up to several Tesla we also worked closely with magnet experts and estimated the forces and conductor materials needed to build such channels. Finally, in collaboration with scientists at Lawrence Berkeley National Laboratory we examined the influence of space-charge field and showed that they can trigger emittance growth and beam loss if not we controlled.

Related publications:

Tapered channel for 6-dimensional muon cooling towards micron scale emittances [Editors' Suggestion], D. Stratakis, R. C. Fernow, J. S. Berg and R. B. Palmer, Phys. Rev. ST - Accel. Beams 16, 091001 (2013).
Compact muon production and collection scheme for high-energy physics experiments [Editors' Highlights], D. Stratakis and D. V. Neuffer, Journal of Physics G: Nuclear and Particle Physics 41, 125002 (2014).
Conceptual design and modeling of particle matter interaction systems for muon based applications, D. Stratakis, H. K. Sayed, C. T. Rogers, A. Alekou and J. Pasternak, Phys. Rev. ST - Accel. Beams 17, 071001 (2014).
Infuence of space-charge fields on the cooling process of muon beams [Editors' Suggestion], D. Stratakis, R. B. Palmer and D. P. Grote, Phys. Rev. ST - Accel. Beams 18, 044201 (2015).
Rectilinear 6D cooling channel for a Muon Collider: A numerical and theoretical study [Editors' Suggestion], D. Stratakis and R. B. Palmer, Phys. Rev. ST - Accel. Beams 18, 031003 (2015).

GENERATION OF ULTRA SHORT "MAGNETIZED" BUNCHES FOR ADVANCED ACCELERATORS

Generation of electron beams with high phase-space density, short bunch length and high peak current is an essential requirement for future linear colliders and bright electron beam sources. Unfortunately, such bunches cannot be produced directly from the source since forces from the mutual repulsion of electrons would destroy the brilliance of the beam within a short distance. In this project we detailed a beam dynamics study of an innovative two-stage compression scheme that can generate ultra-short bunches without degrading the beam quality. In the first stage, the beam is compressed with an advanced velocity bunching technique in which the longitudinal phase space is rotated so that electrons on the bunch tail become faster than electrons in the bunch head. In the second stage, the beam is further compressed with a conventional magnetic chicane. With the aid of numerical simulations we show that our two-staged scheme is capable to increase the current of a 50 pC bunch to a notable factor of 100 while the emittance growth can be suppressed to 1% with appropriate tailoring of the initial beam distribution.

References:

A hybrid approach for generating ultra-short bunches for advanced accelerator applications, D. Stratakis, Nucl. Instr. and Meth. A 821, 1 (2016).
Emittance preservation during bunch compression with a magnetized beam, D. Stratakis, Nucl. Instr. and Meth. A 811, 6 (2016).

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