IPAC 2012 New Orleans, USA

FACET related talks :
Monday 21st May
MOOAB01: A Proton-driven Plasma Wakefield Accelerator Experiment with CERN SPS Bunches – P.Muggli (MPI)
Existing relativistic proton (p+) bunches carry large amounts of energy (kJ) and are therefore attractive as drivers for plasma-based particle accelerators, such as the plasma wakefield accelerator or PWFA. However, short (~ps) p+ bunches capable of driving large amplitude (~GV/m) wakefields are not available today. It was recently proposed to use long (~300ps) p+ bunches self-modulated at the plasma wavelength by a transverse two-stream instability in a high-density (~10**14-10**15/cc) plasma to resonantly drive wakefields. Based on this idea and on the long term prospect for short p+ bunches a p+-driven PWFA experimental program was proposed to study the acceleration of electrons to the TeV energy range. Initial experiments will use the 450GeV, 1-3¢1011 p+ bunches from the CERN SPS and plasmas 5-10m in length. The wakefields will be sampled by an externally injected, low energy (10-20MeV) electron bunch that will gain energy in the GeV range. The experimental plan, as well as the expected results will be presented.

MOOAB02: First Results from the Electron Hose Instability Studies in FACET – E. Adli (University of Oslo) W. An, C. Joshi, K.A. Marsh, W.B. Mori (UCLA) S. Corde, R.J. England, J.T. Frederico, S.J. Gessner,M.J. Hogan, S.Z. Li, M.D. Litos, Z.Wu (SLAC) P.Muggli (MPI)
We present the first results from experimental studies of the electron hose instability in the plasma wakefield acceleration experiments at FACET. Theory and PIC simulations of an electron beam as it travels through a plasma indicate that hosing may lead to a significant distortion of the transverse phase space. The FACET dump line is equipped with a Cherenkov light based spectrometer which can resolve transverse motion as a function of beam energy. We compare the predictions from simulations and theory to the experimental results obtained.

MOOAB03: FACET First Beam Commissioning – G. Yocky (SLAC), C.I. Clarke, W.S. Colocho, F.-J. Decker, M.J. Hogan, N. Lipkowitz, J. Nelson, P.M. Schuh, J. Sheppard, H. Smith, T.J. Smith, M. Stanek, J.L. Turner, S.P.Weathersby, U.Wienands, M.Woodley (SLAC)
The FACET (Facility for Advanced aCcelerator Experimental Tests) facility at SLAC has been under construction since summer 2010. Its goal is to produce ultrashort and transversely small bunches of very high intensity (20kA peak current) to facilitate advanced acceleration experiments like PWFA and DLA. In June of 2011 the first electron beam was brought into the newly constructed bunch-compression chicane. Commissioning work included restarting the linac and damping ring, verifying hardware, establishing a good beam trajectory, verifying the optics of the chicane, commissioning diagnostic devices for transverse and longitudinal bunch size, and tuning up the beam size and bunch compression. Running a high-intensity beam through the linac without BNS damping and with large energy spread is a significant challenge. Optical aberrations as well as wakefields conspire to increase beam emittance and the bunch compression is quite sensitive to details of the beam energy and orbit, not unlike what will be encountered in a linear-collider final-focusing system. In this paper we outline the steps we took while commissioning as well as the challenges encountered and how they were overcome.

All Posters with "FACET" in the Title or Abstract: 

MOPPC073: Improvements in the PLACET Tracking Code – A. Latina (CERN), E. Adli, D. Schulte, J. Snuverink (CERN) B. Dalena (CEA/IRFU)
The tracking code PLACET simulates beam transport and orbit corrections in linear accelerators. It incorporates single- and multi-bunch effects, static and dynamic imperfections. It has an interface based on both Tcl/Tk and Octave to provide maximum flexibility and easy programming of complex scenarios. Recently, new functionality has been added to expand its simulation and tuning capabilities, such as: tools to perform beam-based alignment of non-linear optical systems, possibility to track through the interaction region in presence of external magnetic fields (detector solenoid), higher order imperfections in magnets, better tools for integrated feedback loops. Moreover, self-contained frameworks have been created to ease the simulation of CLIC Drive Beam, CLIC Main Beam, and other existing electron machines such as CTF3 and FACET.

MOPPR086: Non-destructive Energy Spectrum Measurements at FACET – S.J. Gessner (SLAC), E. Adli,
S. Corde, R.J. England, J.T. Frederico, M.J. Hogan, S.Z. Li, M.D. Litos, D.R.Walz, Z.Wu (SLAC)
Synchrotron radiation produced in a wiggler magnet located in a region of high dispersion is used to determine the energy spectrum of the beam with minimal disruption of the beam itself. This technique is used at the FACET facility at SLAC to provide non-invasive single shot measurements of the pulse-by-pulse energy spectrum of the beam delivered to experiments. The theoretical and experimental aspects of the measurement will be described. The possibility of using the measured energy spectrum in combination with accelerator modeling to recover the longitudinal phase space and the corresponding femtosecond current profile will be discussed.

TUEPPB09: First Measurements of the FACET Coherent Terahertz Radiation Source – Z. Wu (SLAC), A.S. Fisher, M.J. Hogan, H. Loos (SLAC)
The Facility for Accelerator science and Experimental Tests (FACET) at SLAC provides a high peak current, sub-ps bunched beam that is ideal for THz photon generation via coherent transition radiation. This paper presents preliminary characterization of the THz pulses generated by FACET electron beam. A one-micron thick Ti foil has been inserted into the beam path and the radiated photons collected. Michelson spectroscopy yields frequency content spanning from 0.25 THz to 2.3 THz and peaked at around 0.5 THz. Multiple scans at different bunch compression show a monotonic increase of the peak radiation frequency as the electron bunch gets shorter. Using the Kramers-Kronig relation, the temporal profile of the THz pulse is reconstructed from the power spectrum indicating a »4 picosecond main pulse followed by a long oscillating tail due to the water absorption lines and detector response. Knife-edge scans measure a 4.4 mm x 4.8 mm transverse spot size at the focal point of the THz optical path. The total collected energy per pulse is 0.69 mJ measured by a Joulemeter. Fitting this total energy to the spatiotemporal profile of the THz pulse yields peak e-field amplitude of 1.5MV/cm.

TUPPC051: FACET Tolerances for Static and Dynamic Misalignment – J.T. Frederico (SLAC),M.J. Hogan, T.O. Raubenheimer (SLAC)
The Facility for Advanced Accelerator and Experimental Tests (FACET) at the SLAC National Accelerator Laboratory is designed to deliver a beam with a transverse spot size on the order of 10 micron x 10 micron in a new beamline constructed at the two kilometer point of the SLAC linac. Commissioning the beamline requires mitigating alignment errors and their effects, which can be significant and result in spot sizes orders of magnitude larger. Sextupole and quadrupole alignment errors in particular can introduce errors in focusing, steering, and dispersion which can result in spot size growth, beta mismatch, and waist movement. Alignment errors due to static misalignments, mechanical jitter, energy jitter, and other physical processes can be analyzed to determine the level of accuracy and precision that the beamline requires. It is important to recognize these effects and their tolerances in order to deliver a beam as designed.

TUPPC052: Longitudinal Beam Tuning at FACET – N. Lipkowitz (SLAC), F.-J. Decker, J. Sheppard, U.Wienands, M.Woodley, G. Yocky (SLAC)
Commissioning of the Facility for Advanced acCelerator Experimental Tests (FACET) at SLAC began in July 2011. In order to achieve the high charge density required for users such as the plasma wakefield acceleration experiment, the electron bunch must be compressed longitudinally from ~6 mm down to 20 microns. This compression scheme is carried out in three stages and requires careful tuning, as the final achievable bunch length is highly sensitive to errors in each consecutive stage. In this paper, we give an overview of the longitudinal dynamics at FACET, including beam measurements taken during commissioning, tuning techniques developed to minimize the bunch length, optimization of the new “W” chicane at the end of the linac, and comparison with particle tracking simulations. In addition, we present additional diagnostics and improved tuning techniques, and their expected effect on performance for the upcoming 2012 user run.

TUPPR029 Performance of Linear Collider Beam-Based Alignment Algorithms at FACET – A. Latina (CERN), D. Schulte (CERN) E. Adli (University of Oslo)
The performance of future linear colliders will depend critically on beam-based alignment (BBA) and feedback systems, which will play a crucial role both in the linear and in the non-linear systems of such machines, e.g., the linac and the final-focus. Due to its characteristics, FACET is an ideal test-bench for BBA algorithms and linear collider beam-dynamics in general. We present the results of extensive computer simulations and their experimental verification.

WEPPP004 A Reciprocity Principle for Wakefields in a Two-Channel Coaxial Dielectric Structure – G.V. Sotnikov (NSC/KIPT) J.L. Hirshfield, T.C. Marshall, G.V. Sotnikov S.V. Shchelkunov (Yale University, Beam Physics Laboratory)
The reciprocity principle is often used in applications of classical electromagnetism. We have employed this principle for testing wakefields set up by an electron bunch in a two-channel coaxial dielectric structure (CDWA). For numerical studies we take a ~ 1-THz fused silica structure which we plan to test at FACET/SLAC; it has dimensions: outer shell, OD=800 micron, ID=500 micron; inner shell OD=181 micron, ID=50 micron. The structure is energized by a 23-GeV, 3-nC bunch having axial RMS size=25 micron. FACET has no drive bunch of annular shape as required for a CDWA; nevertheless, our analytical studies and simulations prove that for the axial wakefield, an annular drive bunch can be replaced by a pencil-like bunch of the same charge traveling in the annular vacuum channel. The longitudinal electric field along the accelerator channel axis (as recorded by a witness bunch) set up by this pencil-like bunch is the same as in the conventional structure of the CDWA. Moreover, if we interchange the drive bunch and the witness bunch, the witness bunch will register the same axial wakefield. However, the stability of the annular bunch is far superior to that of the pencil bunch.

WEPPP010 FACET: SLAC’s New User Facility – C.I. Clarke (SLAC), F.-J. Decker, R.A. Erickson, C. Hast, M.J. Hogan, S.Z. Li, Y. Nosochkov, N. Phinney, J.T. Seeman, J. Sheppard, U. Wienands, M.Woodley, G. Yocky (SLAC) A. Seryi (JAI)
FACET (Facility for Advanced Accelerator and Experimental Tests) is a new User Facility at SLAC National Accelerator Laboratory. The first User Run started in spring 2012 with 23 GeV, 3 nC electron beams. The facility is designed to provide short (20 um) bunches and small (10 um wide) spot sizes, producing uniquely high power beams. FACET supports studies from many fields but in particular those of Plasma Wakefield Acceleration and Dielectric Wakefield Acceleration. The creation of drive and witness bunches and shaped bunch profiles is possible with "Notch" Collimation. FACET is also a source of THz radiation for material studies. Positrons will be available at FACET in future user runs. We present the User Facility and the available tools and opportunities for future experiments.

WEPPP012 High-Gradient THz-Scale Two-Channel Coaxial Dielectric-Lined Wakefield Accelerator – S.V. Shchelkunov (Yale University, Beam Physics Laboratory), M.A. LaPointe (Yale University, Beam Physics Laboratory) J.L. Hirshfield, T.C. Marshall (Omega-P, Inc.) G.V. Sotnikov (NSC/ KIPT)
A mm-scale THz CoaxialDielectricWakefield Accelerator structure is currently under study by Yale University Beam Physics Lab and collaborators for its performance with annular drive bunches. With our recent successful experiments with the cm-scale GHz rectangular module at AWA/Argonne (USA) and planned activity there with yet another cm-scale GHz coaxial structure, the program of new research has two objectives. The first is to design a structure to produce acceleration gradients approaching 0.35 GeV/m per each nC of drive charge when excited by an annular like bunch; has an attractive feature that the drive and accelerated bunches both have good focusing and stability properties; and also exhibits a large transformer ratio. The second goal is to build and test the structure at FACET/SLAC (USA). At FACET the structure can be excited only with the available pencil-like drive bunch, but the reciprocity principle allows one to observe some of the properties that would be seen if the excitation were to be by an annular drive bunch. This presentation shows our latest findings, discusses related issues, and discusses our plans for experiments.

WEPPP038 Plasma Wakefield Experiments at FACET – M.J.Hogan (SLAC), E. Adli, R.J. England, J.T. Frederico, S.J. Gessner, S.Z. Li, M.D. Litos, D.R. Walz (SLAC) W. An, C.E. Clayton, C. Joshi, K.A.Marsh,W.B.Mori (UCLA) P.Muggli (MPI)
FACET, the Facility for Advanced Accelerator and Experimental Tests, is a new facility in sector 20 of the SLAC linac to study beam driven plasma wakefield acceleration. The facility was commissioned in the summer of 2011 and the experimental program will begin in April 2012. The nominal FACET beam parameters are 23GeV, 3nC electron or positron bunches compressed to~20 micron long and focused to ~10 micron wide. The intense fields of the FACET bunches will be used to field ionize neutral metal alkali vapors produced in a heat pipe oven. Previous experiments at the SLAC FFTB facility demonstrated 50GeV/m gradients in an 85cmfield ionized lithium plasma where the interaction distance was limited by head erosion. Simulations indicate the lower ionization potential of cesium and rubidium will decrease the rate of head erosion and increase single stage performance. The initial experimental program will compare the performance of lithium and rubidium plasma sources with single and double bunches. Later experiments will investigate improved performance with a pre-ionized plasma. The status of the experiments and expected performance are reviewed.

WEPPP046 Nonlinear Dielectric Wakefield Experiment for FACET – P. Schoessow(Euclid TechLabs, LLC), S.P. Antipov, C.-J. Jing, A. Kanareykin (Euclid TechLabs, LLC) S. Baturin (LETI)
Recent advances in ferroelectric ceramics have resulted in new possibilities for nonlinear devices for particle accelerator and rf applications. The new FACET (Facility for Advanced Accelerator Experimental Tests) at SLAC provides an opportunity to use the GV/m fields from its intense short pulse electron beams to perform experiments using the nonlinear properties of ferroelectrics. Simulations of Cherenkov radiation in the THz planar and cylindrical nonlinear structures to be used in FACET experiments will be presented. Signatures of nonlinearity are clearly present in the simulations: superlinear scaling of field strength with beam intensity, frequency upshift, and development of higher frequency spectral components.

WEPPP052 Self-modulation of Long Particle Bunches in Plasmas at SLAC – P. Muggli (MPI) Y. Fang (USC) R.A. Fonseca (ISCTE - IUL) M.J. Hogan (SLAC) W.B. Mori (UCLA) L.O. Silva, J. Vieira (IPFN)
The transverse self-modulation (SM) of ultra-relativistic, long particle bunches can lead to the generation of large amplitude wakefields. In this work we show that the physics of SM could be investigated with the long electron and positron bunches available at SLAC. The propagation of SLAC electron and positron bunches in 1 meter plasmas was modeled with OSIRIS. 3Dsimulations reveal that hosing may limit SM, but that shaped bunches with a hard-cut front ensure that saturation of SM can be reached. Cylindrically symmetric simulations show that the blowout regime can be achieved using these shaped bunches. Accelerating gradients in excess of 20 GeV/m are generated, and up to 10 GeV energy gain and loss are observed in the simulations at the 1% charge level after one meter of plasma. Because the blowout regime is reached, positron driven wakes lead to accelerating gradients that can be less than half than those of electrons. Simulations results outlining the SM results expected with the SLAC-FACET beam parameters will be presented.

WEPPP055 High Transformer Ratio Drive Beams for Wakefield Accelerator Studies – R.J. England (SLAC), J.T. Frederico, M.J. Hogan, M.D. Litos (SLAC) W. An, C. Joshi, W. Lu, W.B. Mori (UCLA) P.Muggli (MPI)
For wakefield based acceleration schemes, use of an asymmetric (or linearly ramped) drive bunch current profile has been predicted to enhance the transformer ratio and generate large accelerating wakes. We discuss plans and initial results for producing such bunches using the 23 GeV electron beam at the FACET facility at SLAC National Accelerator Laboratory and sending them through plasmas and dielectric tubes to generate transformer ratios greater than 2 (the limit for symmetric bunches). The scheme proposed utilizes the final FACET chicane compressor and transverse collimation to shape the longitudinal phase space of the beam.

WEPPP056 Positron PWFA Simulations for FACET – S.J. Gessner (SLAC), E. Adli, S. Corde, R.J. England, J.T. Frederico,M.J.Hogan, S.Z. Li, M.D. Litos,D.R.Walz, Z.Wu (SLAC)W. An,W.B.Mori (UCLA)
When a positron beam enters a plasma, plasma electrons are drawn in toward the beam axis, creating a region of extremely large charge density with complicated, nonlinear fields. Few analytic solutions exist to describe these fields, and this necessitates the use of simulations to model positron beam and plasma interactions. This presentation should cover recent work on positron PWFA simulations using the QuickPIC particle-in-cell code. I will discuss the computational challenges associated with positron PWFA and specific applications of the simulations for future experimental tests at the FACET user facility at SLAC.

WEPPR040 Intensity Effects of the FACET Beam in the SLAC Linac – F.-J. Decker (SLAC), N. Lipkowitz, J. Sheppard, G.R. White, U.Wienands,M.Woodley, G. Yocky (SLAC)
The beam for FACET (Facility for Advanced aCcelerator Experimental Tests) at SLAC requires an energy-time correlation ("chirp") along the linac, so it can be compressed in two chicanes, one at the midpoint in sector 10 and one W-shaped chicane just before the FACET experimental area. The induced correlation has the opposite sign to the typical used for BNS damping, and therefore any orbit variations away from the center kick the tail of the beam more than the head, causing a shear in the beam and emittance growth. Any dispersion created along the linac has similar effects due to the high (> 1.2% rms) energy spread necessary for compression. The initial huge emittances could be reduced by a factor of 10, but were still bigger than expected by a factor of 2-3. Normalized emittance of 2 um-rad in Sector 2 blew up to 150 um-rad in Sector 11 but could be reduced to about 6-12 um-rad for the vertical plane although the results were not very stable. Investigating possible root causes for this, we found locations where up to 10 mm dispersion was created along the linac, which were finally verified with strong steering and up to 7mmsettling of the linac accelerator at these locations.

THPPR029 A New Control Room for SLAC Accelerators – R.A. Erickson (SLAC),M. Stanek, Z. Van Hoover (SLAC)
We propose to construct a new control room at SLAC to unify and improve the operation of the LCLS, SPEAR3, and FACET accelerator facilities, and to provide the space and flexibility needed to support the LCLS-II and proposed new test beam facilities. The existing control rooms for the linac and SPEAR3 have been upgraded in various ways over the last decade, but their basic features have remained unchanged. We propose to build a larger modern Accelerator Control Room (ACR) in the new Research Support Building (RSB), which is currently under construction at SLAC. Shifting the center of control for the accelerator facilities entails both technical and administrative challenges. In this paper, we describe the motivation and design concept for the ACR and the remaining challenges to completing this project.