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AAC'04 Agenda
AAC'04 Poster
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EM Structure-Based Accelerators Working Group
Vitaly Yakimenko: In Vacuum Laser Acceleration of Electrons at the Brookhaven ATF
  Phase velocity of the electric field for accelerated electrons is larger than the speed of light when the electrons travel over long distance larger than 3 times laser Rayleigh length. This present major difficulty in using laser fields in the vacuum to accelerate the electrons. Expected acceleration is small and one of the challenges of such experiment is to detect acceleration. We use Inverse Cherenkov based Accelerator as a starting point and gradually demonstrate change in the acceleration gradient as a function of gas pressure. This way we can predict much more precisely acceleration at the vacuum level and judge if it is sufficiently high when compared to the energy spectrometer resolution. Recent experimental results will be presented.
Feng Zhou: Proof-of-principle experimental test for the novel vacuum electron-laser acceleration at the BNL-ATF
  A recently developed theoretical model [Y.K.Ho, et al., Phys. Rev. E, 66, 066501 (2002)] demonstrated the injection electrons with low energy (5-20 MeV) and small incident angle (q~0.1) relative to the laser propagation direction are captured and significantly accelerated in a strong laser field (a0³4), and the accelerating gradient can reach GeV/cm with a0>30. To verify this mechanics, we propose to use the BNL-ATF upgrading Tera-Watters CO2-laser and high-brightness electron beam to carry out a proof-of-principle experiment. Complete simulation results with ATF parameters (including both electron and laser beams) are presented. Layout for this experiment including the laser injection optics and electrons extraction system is described. Diagnostics to measure angular distribution and energy spectrum of output electrons are discussed.
  A tall, dielectric-lined rectangular wake field microstructure is analyzed as a possible element of an advanced linear wake field accelerator. This accelerator would be driven by a train of 3fs 500MeV electron microbunches that would be chopped out of a longer bunch using e.g. a powerful CO2 laser, and then formed into a train of rectangular-profile bunches using a quadrupole. The bunches set up a periodic wake field in the microstructure that can be built up to 1100 MV/m, for example, using a train of ten 2pC 3-fs bunches spaced by the period of the wake fields in the structure. Stability is examined for drive and accelerated bunches using computations of test particle orbits in the longitudinal and transverse wake fields excited by the drive bunches. It is found that nearly all test electrons in the drive bunches are confined within the structure for a travel distance of 30 cm or more, while test electrons located in an accelerated bunch can have stable motion over at least 100cm without passing through the structure walls, thereby gaining >1GeV.
A tall, dielectric-lined rectangular wake field microstructure is analyzed as a possible element of an advanced linear wake field accelerator. This accelerator would be driven by a train of 3fs 500MeV electron microbunches that would be chopped out of a longer bunch using e.g. a powerful CO2 laser, and then formed into a train of rectangular-profile bunches using a quadrupole. The bunches set up a periodic wake field in the microstructure that can be built up to 1100 MV/m, for example, using a train of ten 2pC 3-fs bunches spaced by the period of the wake fields in the structure. Stability is examined for drive and accelerated bunches using computations of test particle orbits in the longitudinal and transverse wake fields excited by the drive bunches. It is found that nearly all test electrons in the drive bunches are confined within the structure for a travel distance of 30 cm or more, while test electrons located in an accelerated bunch can have stable motion over at least 100cm without passing through the structure walls, thereby gaining >1GeV
Benjamin Cowan: Photonic Crystal Laser-Driven Accelerator Structures
  We discuss simulated photonic crystal structure designs, including two- and three-dimensional planar structures and fibers. The discussion of 2D structures demonstrates guiding of a speed-of-light accelerating mode by a defect in a photonic crystal lattice and reveals design considerations and trade-offs. With a three-dimensional lattice, we introduce a candidate geometry, discuss beam dynamics in that structure, and present possible manufacturing techniques. In addition we discuss W-band scale tests of a fiber structure. The computational methods are also discussed.
John Power: Observation of Multipactor in an Alumina-based Dielectric-Loaded Accelerating Structure
  We report a new regime of single-surface multipactor that was observed during high-power testing of an 11.424-GHz alumina-based dielectric-loaded accelerating structure. Previous experimental observations of single-surface multipactor on a dielectric occurred in cases for which the rf electric field was tangential and the rf power flow was normal to the dielectric surface (such as on rf windows), and found that the fraction of power absorbed at saturation is ~1%, independent of the incident power. In this new regime, in which strong normal and tangential rf electric fields are present and the power flow is parallel to the surface, the fraction of power absorbed at saturation is an increasing function of the incident power, and more than half of the incident power can be absorbed. A simple model is presented to explain the experimental results.
Wayne Kimura: Detailed Model Comparisons with STELLA Experimental Results
  High-trapping efficiency and narrow energy spread in a staged laser acceleration system was demonstrated during the STELLA experiment [1]. The experiment used inverse free electron lasers (IFELs) driven by the BNL ATF CO2 laser. The 1st IFEL modulated the electron beam energy. A subsequent chicane created a train of ~3 fs-long microbunches separated by 10.6 microns. These microbunches are trapped and accelerated in a 2nd IFEL where up to 80% trapping efficiencies and energy spreads down to 0.36% (1-sigma) were measured. This paper presents the detailed model comparisons with the data including results as a function of the phase delay time between the microbunches entering the 2nd IFEL and the laser field. The model also provides information on the energy-phase space and longitudinal density distribution of a microbunch. The energy-phase plots are particularly useful for understanding the observed energy spectra, and the processes occurring to achieve high-trapping efficiency and narrow energy spread.

[1] W. D. Kimura, et al., Phys. Rev. Lett. 92, 054801 (2004).

  We present a design for a slab-symmetric optical accelerating structure which is to be resonantly excited at a wavelength of 340 micron but can be scaled to other frequency ranges. The device consists of a vacuum gap between dielectric-lined conducting walls and partakes of the well-known advantages of slab symmetry (including suppression of transverse beam wakefields and low power density). Accelerating fields on the order of 100 MeV/m are predicted when the structure is powered by a high-power (~100 MW) FIR radiation source now in development at UCLA. Optimal strategies for power coupling into the structure will be discussed. Scaling questions lead to the investigation of an entirely nonmetallic design, which has significant effects on the accelerating modes.
Stanislav Zhilkov: Driven radiation of ribbon electron beam as a tool for optical modulation
  The amplitude of Cherenkov radiation (CR) or Smith-Purcell radiation (SPR), which produced by electrons that compactly move along surface, can be controlled by the deflector of double-metal-plates type. The height of passing of ribbon electron beam (EB) over surface is regulated depending on voltage that applied to the deflector’s plates. If EB produces suitable radiation at the same time when optical beam (OB) is in the interaction region, then due to superposition the amplitude of OB is changed. So as the deflector’s voltage regulates EB-radiation, the amplitude modulation of OB occurs. Three schemes – CR dielectric grating FEL, SPR metal grating FEL and flat-layered dielectric unidirectional optical amplifier (UOA) – have been analyzed in accordance with their effectiveness to provide optical modulation. Theory predicts that the rate of modulation can be 5% of frequency of OB (or 1000 Gbps for visible light); technical means assume that driven voltage is of order 1V. Feasibility of the concept can be tested for THz range by modified SP FEL in Dartmouth.
Steven Gold: Development of a 20-MeV Dielectric-Loaded Accelerator Test Facility*
  This paper will describe a joint project by the Naval Research Laboratory (NRL) and Argonne National Laboratory (ANL), in collaboration with the Stanford Linear Accelerator Center (SLAC), to develop a dielectric-loaded accelerator (DLA) test facility powered by the high-power 11.424-GHz magnicon that was developed by NRL and Omega-P, Inc. The magnicon can presently produce 25 MW of output power in a 250-ns pulse at 10 Hz, and efforts are in progress to increase this to 50 MW [1]. The facility will include a 5-MeV electron injector being developed by the Accelerator Laboratory of Tsinghua University in Beijing, China. The DLA test structures are being developed by ANL, and some have undergone testing at NRL at gradients up to ~8 MV/m [2]. SLAC is developing a means to combine the two magnicon output arms, and to drive an injector and accelerator with separate control of the power ratio and relative phase. RW Bruce Associates, working with NRL, is developing means to join short ceramic sections into a continuous accelerator tube by ceramic brazing using a millimeter-wave beam. The installation and testing of the first dielectric-loaded test accelerator, including injector, DLA structure, and spectrometer, should take place within the next year. The facility will be used for testing DLA structures using a variety of materials and configurations, and also for testing other X-band accelerator concepts. The initial goal is to produce a compact 20-MeV dielectric-loaded test accelerator.

* Work supported by DoE and ONR. (a) Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375(b) LET Corporation, Washington, DC 20007(c) High Energy Physics Division, Argonne National Laboratory, Argonne, IL 60439(d) Stanford Linear Accelerator Center, Menlo Park, CA 94025(e) Tsinghua University, Beijing 100084, China(f) RW Bruce Associates, Arnold, MD 21012[1] O.A. Nezhevenko et al., Proc. PAC 2003, p. 1128.[2] S.H. Gold et al., AIP Conf. Proc. 691, p.

Levi Schachter: Optical Bragg Acceleration Structure
  We have made extensive calculations recently regarding wakes in dielectric structures. In an attempt to optimize the efficiency a unique structure has been designed. It resembles an optical fiber except that contrary to the latter, the dielectric coefficient is unity (vacuum) in the center due to the need of ensuring electron propagation. The similarity between the two is the fact that for field confinement the effective dielectric coefficient of the surrounding structure is smaller than in the center – in our case it is smaller than unity. This is accomplished by a series of periodic dielectric layers forming a Bragg structure. In spite of the complexity of the structure it was possible to establish some analytical estimates of the interaction impedance, group velocity, maximum electric field and most importantly of the wake. A general approach of evaluating wake fields in dielectric structures has been developed and will be presented.
Evgenya Smirnova: Photonic Band Gap structures for accelerator applications
  A photonic band gap (PBG) structure is a one-, two- or three-dimensionalperiodic metallic and/or dielectric system (for example, of rods), whichacts like a filter, reflecting rf fields in some frequency range andallowing rf fields at other frequencies to transmit through. PBG structureshave many promising applications in active and passive devices at millimeterwave and higher frequencies. Metal PBG structures can be employed at X andKu-band accelerators to suppress wakefields whenever dielectric PBGstructures are attractive at higher frequency for construction of low-losslaser-driven accelerators. For both applications two-dimensional (2D) PBGstructures are of main interest. In this talk I present a review oftheoretical studies and computer modeling of 2D metal and dielectricstructures. Also world-wide experimental efforts on constructing and testingmetal and dielectric PBG accelerators and microwave devices are discussed.
Chunguang Jing: High power rf test for MCT based 11.424GHz dielectric loaded structures
  Recently we carried out two high power RF tests of dielectric-loaded accelerating structures based on a ceramic material made of Magnesium Calcium and Titanium (MCT), which has advantages over alumina-based tubes due to higher dielectric constant (around 20). In both experiment, rf breakdown occurred at small gaps in the joints between ceramic tube sections. The RF power applied to the tube was limited to 1.2 MW due to permanent arcing which occurred when the incident power reached 1 MW. Based on numerical simulation, the peak surface electric field gradient at this power level was equivalent to greater than 65MV/m because of field enhancement at the ceramic gap. This suggests that MCT-based dielectric loaded accelerating structures may handle 60~~80MV/m electric field without breakdown. Also, during the process of raising the RF power to the point of breakdown, the s-parameters behaved similarly to that observed for the alumina-loaded tube test, suggesting that MCT undergoes a similar multipactor process at high levels of RF input power. However, the MCT structure did not show any visible light emission at the point where the transmission coefficient of the tube began to fall, which occurred at an incident power of 90 kW (equivalent to 1.7MV/m E-field). We began to observe light emission at 440 kW incident power. We have fabricated additional MCT taper sections with their small dimension modified for better RF power transmission efficiency, and a corresponding copper module is being machined. We anticipate better performance will be achieved in the near future.
Anatoly Blanovsky: Conceptual Design of Dielectric Accelerating Structures for Intense Neutron and Monochromatic X-ray Sources
  Bright compact photon sources, which utilizing electron beam interaction with periodic structures, may benefit a broad range of applications in the medical, industrial and scientific fields. A class of dielectric-loaded periodic structures for hard and soft X-ray production has been proposed to provide a high accelerating gradient when excited by an external RF and/or primary electron beam. It is demonstrated that target-distributed accelerators (TDA), in which an additional electric field compensates for lost beam energy in internal targets, provide all necessary means to drive a high flux subcritical reactor (HFSR) for nuclear waste transmutation. In addition to the prime concept, the TDA for Boron Neutron Capture Therapy, nuclear isomer-based compact energy sources, monochromatic computer tomography and X-ray lithography are proposed. At present, synchrotron radiation from an electron storage ring is practically the only high-quality source of monochromatic x-rays with intensities that are adequate for these applications. However, their high cost, large size and low x-ray energies are serious limitations. In the proposed quasi-monochromatic X-ray sources, the combination of a low atomic number of internal targets and low prime-accelerator voltage reduces the intensity of continuous spectrum to the point at which characteristic radiation assumes a greater importance.One of the early assumptions of the theory of dielectric wake-field acceleration was that, in electrodynamics, the vector potential was proportional to the scalar potential. The analysis takes into consideration a wide range of TDA design aspects including wave model of observed phenomena, layered compound separated by a Van der Waals gap and a high energy density source powered by fission electric cells (FEC) with a multistage collector. The FEC is essentially a high-voltage power source that directly converts the kinetic energy of the fission fragments into electrical potential of about 2MV.
Alex  Kanareykin: Enhanced Transformer Ratio Experiment at Argonne Wakefield Accelerator
  In this talk we present an experimental program and our group recent results for the design, development and demonstration of an Enhanced Transformer Ratio Dielectric Wakefield Accelerator (ETR-DWA). We present here an experimental design of a 13.625 GHz dielectric loaded accelerating structure, a laser multisplitter producing a ramped bunch train demostration, and simulations of the bunch train parameters required. Test results of the accelerating structure bench testing and ramped pulsed train generation with the laser multisplitter are shown as well. The principal goal of the project is to increase the transformer ratio, the parameter that characterizes the energy transfer efficiency from the accelerating structure to the accelerator electron beam.
Chris Sears: IFEL-Chicane based Microbuncher at 800 nm
  As a first stage to net acceleration in a laser based EM structure RF electron pulses must be microbunched to match the laser wavelength. We report on the design of an undulator and chicane for microbunching at 800nm using an inverse free electron laser (IFEL) interaction. This includes design considerations for the hardware itself, the laser IFEL interaction and bunching performance, and a full 3D particle tracking simulation to study the focusing effects and possible emittance growth due to the fringe fields of the magnets. The talk will close with a discussion of laser-electron beam diagnostics for overlap in the undulator and for diagnosing microbunching performance.
Wei Gai: 1 GeV acceleration using electron beam driven wakefields in structures
  In this talk, we first discuss the transformer ratio problem in collinear wakefield acceleration. Then, we review three wakefield schemes that have potential to accelerate a witness beam to 1 GeV: 1) Collinear wakefield acceleration with non-symmetric drive bunch shape or bunch trains; 2) Collinear wakefield acceleration with multiple drive beam which uses staging techniques; 3) Two beam acceleration scheme where the drive and the witness beam travel in different but RF connected structures. We will discuss the drive beam and structure requirements for each scheme that can support > 100 MV/m gradient and sustained acceleration of > 10 meter length. Particular examples that use dielectrics as the wakefield structure for each scheme will be given. Finally, we will discuss possible future research plans that will achieve the goal of 1 GeV acceleration.
Gerald Dugan: Advanced accelerator requirements for synchrotron radiation facility linacs
  The general electron beam requirements of few GeV electron linacs for the production of synchrotron radiation will be reviewed, with an emphasis on the requirements which could be satisfied using advanced accelerator systems. The talk will cover the beam requirements needed for the generation of the synchrotron radiation in conventional storage rings, wigglers and undulators driven by an energy-recovery linac, and SASE free-electron lasers.
Wayne Kimura: Conceptual Design for a 1-GeV IFEL Accelerator
  A conceptual design for an inverse free electron laser (IFEL) electron accelerator with a net energy gain of 1-GeV will be presented. This design was developed using various models available at STI Optronics and UCLA. These models have been anchored with previous successful IFEL experiments. The electron beam characteristics while being accelerated will be examined including trapping efficiency, phase-space distribution, and preservation of the microbunch length.
Yu Ho: Exploring Acceleration Channel of Vacuum Laser Beams
  A new scheme of the electron laser acceleration in vacuum has been studied with theoretical investigation and 3D simulations. In this scheme, relativistic electrons injected with small incident-angle relative to the laser propagation direction are not expelled by the laser beam as predicted by the ponderomotively scattering model, but are captured and significantly accelerated in the strong laser field region. We call this new scheme CAS (capture and acceleration scenario). It has been found that there exists a lower wave phase velocity region (less than c) for any focused laser beam propagating in vacuum. Thus the overlapping regions of the lower phase velocity region with that of the axial electric field component form ideal acceleration channels. Relativistic electrons injected into this channel can be trapped in the acceleration phase and remain in phase with the laser field for sufficient long times, thereby receiving considerable energy from the field. The crucial conditions for CAS to work are the laser intensity should be strong enough, the electron incident angle is sufficiently small, and the optimum incident electron energy should be in the range of 5-15 MeV. The main features of CAS are the principal acceleration force is the axial electric field component and the maximum energy gain is linearly proportional to the field strength. A typical value of the energy gain is 100 MeV (a = 10), 2 GeV (a = 100). There is an intensity threshold, which is critically dependent on the beam width (thereby the phase velocity in the acceleration channel). The output beam properties of the CAS scheme have been investigated in quantitatively detail, such as the energy spectra, angular distributions, energy-angle correlation, fractions of accelerated electrons in the total incident electron bunch, emittances, etc. In CAS, optics and medium placed near the laser focal region are not necessary, thereby allowing use of high intensity lasers and large energy gain.
Chieh Sung: Study of a THz IFEL prebuncher for laser-plasma accelerators
  For monoenergetic acceleration of electrons, the injected particles need to be bunched with the same periodicity as the accelerating structure. In the Plasma Beat Wave Acceleration experiment in the Neptune laboratory, the accelerating structure has a periodicity of 340 μm. The plasma wave is phase-locked to the CO2 beat-wave used to drive it. We are proposing to use the same beat-wave to generate a high power 340 μm EM radiation via difference frequency mixing in GaAs. This radiation would have the same phase relationship as the plasma wave and therefore can be used to prebunch an existing nominally 10MeV electron beam based on an IFEL concept. We will present the design of such a prebuncher which uses a 50 cm long undulator. The injected 4ps long electron beam is expected to bunch in a series of microbunches each only 45 μm long containing over 40% of the injected current after 1.5 m drift space following the undulator.
Chieh Sung: Single stage GeV-class IFEL accelerator
  A high power 0.8 μm laser, in a diffraction-dominated mode is used to self-trap and to accelerate electrons. 1D and 3D simulation codes are used to explore the feasibility of a single-stage, 1GeV IFEL accelerator.
  A new S-band Plane-Wave-Transformer (PWT) photoinjector with capabilities to achieve high vacuum is under development at DULY Research Inc. The PWT is equipped with NEG pumps and a cathode load lock which is designed to handle semiconductor photocathodes such as GaAs. Polarized electrons with high charge, low emittance and high rep rate can be produced from the PWT, suitable for future linear colliders and other high brightness beam applications. Ultra short bunches can also be produced from the PWT for advanced accelerators and light sources.

*Work supported by DOE SBIR grant number DE-FG02-03ER83846