|Broadly Tunable Free-Electron Laser for Four-wave Mixing Experiments with Soft X-ray Pulses
|This paper examines a FEL design for the production of three soft x-ray pulses from a single electron beam suitable for four-wave mixing experiments. Independent control of the wavelength, timing and angle of incidence of the three ultra-short, ultra-intense pulses with exquisite synchronization is critical. A process of selective amplification where a chirped electron beam and a tapered undulator are used to isolate the gain region to only a short fraction of the electron beam is explored in detail. Numerical particle simulations are used to demonstrate the essential features of this scheme in the context of the LCLS-II design study.
|Observation of Smith-Purcell Radiation at 32 GHz from a Multi-channel Grating with Sidewalls
|In a demonstration experiment at 5 GHz, we found copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave, when the grating had sidewalls. Reaching higher frequencies requires a reduction in the size of the grating, which leads to a considerable reduction in power. To partially compensate this, we suggested superposing several copies of the reduced grating in parallel. A test of this concept has been performed with a seven-channel grating, at a frequency near 32 GHz. The SP radiation signals were observed directly with a fast oscilloscope. Power levels were of order 5 kW, in fair agreement with three-dimensional simulations made using the code "MAGIC".
|Flying RF Undulator
|A new concept for the room-temperature rf undulator, designed to produce coherent X-ray radiation by means of a relatively low-energy electron beam and pulsed mm-wavelength radiation, is proposed. The “flying” undulator is a high-power short rf pulse co-propagating together with a relativistic electron bunch in a helically corrugated waveguide. The electrons wiggle in the rf field of the -1st spatial harmonic with the phase velocity directed in the opposite direction in respect to the bunch velocity, so that particles can irradiate high-frequency Compton’s photons. A high group velocity (close to the speed of light) ensures long cooperative motion of the particles and the co-propagating rf pulse. This work is supported by the Russian Foundation for Basic Research (Projects 14-08-00803 and 14-02-00691).
|Poster TUP040 [0.189 MB]
|High Efficiency Lasing with a Strongly Tapered Undulator
Funding: This work was supported by DOE grant DE-FG02-92ER40693, Defense of Threat Reduction Agency award HDTRA1-10-1-0073 and University of California Office of the President award 09-LR-04-117055-MUSP.
Typical electrical to optical energy conversion efficiencies for FELs are limited by the Pierce parameter to 10-3 or smaller. Undulator tapering schemes have enabled extraction of as much as 1 or 2% of the electron energy. Recently, the UCLA BNL helical inverse free electron laser (IFEL) experiment at ATF demonstrated energy doubling and acceleration of 30% of an electron beam from 52 to 93 MeV with a modest 1011 W power CO2 laser pulse. By reversing and retuning the undulator, the electrons may be violently decelerated, thereby transferring energy from the beam to the laser pulse. Simulations show that by sending a 1 kA, 70 MeV electron beam and 100 GW laser into a prebuncher and the reversed undulator, 41% of the electron beam energy should be converted to radiation, allowing the laser pulse power to grow to 127 GW.
|IFEL Driven Micro-Electro-Mechanical System Free Electron Laser
|The Free Electron Laser has provided modern science with a tunable source of high frequency, high power, coherent radiation. To date, short wavelength FEL's have required large amounts of space in order to achieve the necessary beam energy to drive the FEL process and to reach saturation of the output radiation power. By utilizing new methods for beam acceleration as well as new undulator technology, we can decrease the space required to build these machines. In this paper, we investigate a scheme by which a tabletop XUV FEL might be realized. Utilizing the Rubicon Inverse Free Electron Laser (IFEL) at BNL together with micro-electro-mechanical system (MEMS) undulator technology being developed at UCLA, we propose a design for a compact XUV FEL.
|Terahertz FEL based on Photoinjector Beam in RF Undulator
|Photoinjectors, which can produce picosecond electron bunches of MeV-level, are attractive for THz generation. Fortunately, a long distance to reach scattering power saturation in FEL is not necessary, if bunch length is shorter than the produced THz half-wavelength. However, the energy of several MeVs does not allow providing long traveling of the flying bunch without longitudinal divergence. That is why, we suggest using specific RF undulator in a form of the normal wave in the helical waveguide at 3 cm wavelength. The mentioned wave has the -1st space harmonic with transverse fields and negative phase velocity (responsible for particle wiggling). This wave has also the 0th harmonic with longitudinal field and positive phase velocity equal to bunch velocity. Due to the synchronous 0th harmonic one can channel low-energy bunches (due to longitudinal focusing field) as far as several meters distance. One might also inject electron bunches in slightly accelerating field, in this case the output THz pulse obtain nearly linear frequency modulation. Such long THz pulses with the mentioned modulation of the frequency can be effectively compressed by pair of diffraction gratings.
|Poster TUP046 [2.914 MB]
|Chirped Pulse Superradiant Free-electron Laser
Funding: This work is supported by Ministry of Science and Technology under Contract NSC 102-2112-M-007-002-MY3
When a short electron bunch traverses an undulator and radiates a wavelength significantly longer than the bunch length, the electrons quickly loses energy through so-called superradiance and generate a negatively chirped radiation frequency at the output. In this paper, we develop a theory to describe this chirped-pulse radiation and numerically demonstrate pulse compression by using a quadratic phase filter. As a design example at THz, a photoinjector/linac system generates a 15 MeV electron bunch containing 15-pC charge in a 60-fs duration. The electrons radiate a chirped pulse at 2.5 THz from a 1.5 m long undulator with a period of 5.6 cm and undulator parameter of 1.7. By using a grating pair, the output THz field can be compressed from 27 to 3 cycles. As another example at EUV, a future dielectric laser accelerator  is assumed to generate a 100 MeV electron bunch containing 75-fC charge in 1-nm long length. The electrons radiate a chirped EUV pulse at 13.5 nm from a 15.8 cm long dielectric laser undulator  with a period of 1.05 mm and undulator field of 3.3 T. By using a quadratic phase filter as a pulse compressor, the peak power of the EUV radiation is increased from 0.7 to 10 kW.
*Y.C. Huang and R.L. Byer, Appl. Phys. Lett. 69 (15), (1996) 2185-2177.
**T. Plettner, R. L. Byer., Phys. Rev. ST Accel. Beams 11, (2008) 030704.
|Thoughts on the New Technologies for Compact FEL Devices
|The possibility to merge different solutions exploiting wave undulators, laser plasma acceleration and other schemes, allows the design of compact Free Electron Laser (FEL) sources operating in the extreme UV-X-ray region of the spectrum. Newly developed high peak power lasers have opened indeed effective possibilities of driving coherent light sources operating with laser plasma accelerated beams and wave undulators. We combine different ideas including also the concept of sheared beam configuration to achieve compact and reliable FEL devices. We finally comment on the perspective use of Radio Frequency (RF) undulators, which seems to be fairly good candidates for opening a new technological strategy for the design of FEL oscillators, including bi-harmonic and multi-frequency operation.
|Storage Ring XFEL with Longitudinal Focusing
|In present work we investigate the possibility of running a high gain FEL on a storage ring using a longitudinally focusing insertion to compress bunches passing an undulator. If integrated into a storage ring similar to PETRA III such device could potentially produce continuous ∼1ps pulses of photons in the nm range with peak pulse powers of tens of GW. Even without operating in FEL saturation mode the longitudinal focusing can provide means to increase the brightness and shorten the photon pulse length
|Coherent Radiation Sources Driven by Superconducting Spoke Accelerators
Funding: This work has been supported by the Office of Naval Research and the High Energy Laser Joint Technology Office.
The NPS FEL Group and Niowave are collaborating to design several FEL systems using superconducting spoke RF cavities. Accelerators reaching electron beam energies of 2MeV to 75MeV with milliamps of average current are considered to drive FELs using a short ten period undulator, approximately 30cm long. The coherent radiation ranges from THz to infrared depending on the electron beam energy. In the 2MeV accelerator, the radiation process can be super-radiant due to the long millimeter wavelength, while the 40MeV accelerator generates infrared wavelengths in a conventional FEL oscillator configuration. An intermediate 8MeV accelerator powers an FEL oscillator with short picosecond pulses providing a kilowatt of THz power in a compact source. Compton backscattering is described to generator incoherent, but monochromatic x-rays for medical applications.
|Short-period Undulators based on Laser-driven Wakefields in Plasma Channels
Funding: Work supported by the U.S. DOE under Contract No.DE-AC02-05CH11231.
We present a novel type of short-period undulator based on controlling the focusing forces on a beam in a plasma wave excited by an intense laser pulse undergoing centroid oscillations in a plasma channel. The period of such a plasma undulator is proportional to the Rayleigh length of the laser pulse and can be sub-mm with an effective undulator strength parameter of order unity. The undulator period can further be controlled and reduced by beating laser modes, or using multiple colors, in the plasma channel. We present analytic expressions for the electron trajectories in the plasma undulator. Analytic work is compared to numerical modeling. Examples are presented of short-period laser-driven plasma undulators based on available laser and plasma channel parameters.
|Status of Electron Beam Slicing Project at NSLS-II, BNL
|The Electron Beam Slicing (e-beam slicing) at NSLS-II, Brookhaven National Laboratory, supported by the Laboratory Directed Research and Development (LDRD) Program, is focused on the development of the new method to generate ultra-short x-ray pulses using focused short low energy (∼20 MeV) electron bunches to create short slices of electrons from the circulating electron bunches in a synchrotron radiation storage ring. The e-beam slicing activities are staged in 3 main phases. In Phases 0, the theory of e-beam slicing is developed, the low energy linac compressor is simulation designed, the radiation separation between the satellite and core is analyzed by simulation and the properties of the e-beam slicing system are discussed and compared with other ultra-short x- ray sources. Phase 0 has completed successfully, Phase 1 is under way. This paper presents an update on the status of Phase 0.
|Electron Beam De-chirping in a Plasma
|Beam-driven plasma wakefield acceleration (PWFA) is a promising and evolving technique for next-generation, compact light sources. By propagating a dense drive-beam through a plasma, expelled plasma electrons can excite wakefields supporting electric fields in excess of 10 GV/m. Like in a conventional radio-frequency cavity, the acceleration strongly depends on the injection phase with respect to the wakefield. Different injection schemes have been proposed recently, all having in common a significant time-energy correlation, the so-called energy-chirp. These energy-chirps typically lead to energy bandwidths of up to 10% in PWFA, hampering efficient beam transport and eventually any kind of free-electron laser (FEL) application. Analogous to short-range wakefields generated by electron beams in a resistive beam pipe, the wakefields generated by the electron beam in a plasma can be used to compensate the initial energy-chirp. Here we present the idea of electron beam de-chirping in a plasma and show the conditions for optimal de-chirping with simultaneous beam quality preservation. This is the basis for efficient beam transport and thus paves the way for FELs utilizing PWFA.
|Regenerative Laser Undulator X-ray Free-Electron Laser (ReLUX-FEL)
We present a new high-gain x-ray FEL concept based on a nearly co-propagating sheared laser undulator  and regenerative amplification [2,3] of a single x-ray pulse via successive interactions between a single electron bunch and the same laser undulator pulse. This new x-ray FEL configuration is called the Regenerative Laser Undulator X-ray FEL which uses a relativistic electron beam crossing a sheared TW laser beam at small angles. The generated x-ray pulse follows a zigzag path between Bragg reflectors and repetitively interacts with the laser and electron pulses. Between interactions, the electron bunch is advanced longitudinally to maintain its overlap with the x-ray pulse. The laser pulse follows a different path that includes an optical delay to synchronize it with the electron and x-ray pulses. The laser wave-front tilt and width are designed to increase the interaction time to provide many oscillation cycles. We present a ReLUX-FEL that can produce coherent 12-keV photons with 2 GeV electron beams. Time-independent Genesis simulations show that with 3 kA peak current and an FEL rho of 0.0007, the ReLUX-FEL saturates after 12 interactions.
 J.E. Lawler et al., J. Phys. D: Appl. Phys. 46 (2013) 325501.
 D.C. Nguyen et al., NIMA 429 (1999) 125.
 Z. Huang et al., PRL 96 (2006) 144801
|Poster TUP071 [0.618 MB]
|Experiment Preparation Towards a Demonstration of Laser Plasma Based Free Electron Laser Amplification
|One direction towards compact Free Electron Laser is to replace the conventional linac by a laser plasma driven beam, provided proper electron beam manipulation to handle the value of the energy spread and of the divergence. Applying seeding techniques also enables to reduce the required undulator length. Rapidly developing Laser Wakefield Accelerators (LWFA) are already able to generate synchrotron radiation. With the presently achieved electron divergence and energy spread an adequate beam manipulation through the transport to the undulator is needed for FEL amplification. A test experiment for the demonstration of FEL amplification with a LWFA is under preparation in the frame of the COXINEL ERC contract in the more general context of LUNEX5. Electron beam transport follows different steps with strong focusing thanks to variable strength permanent magnet quadrupoles, demixing chicane with conventional dipoles, and a second set of quadrupoles for further focusing in the undulator. Progress on the equipment preparation and expected performance are described.
|Phase-merging Enhanced Harmonic Generation for Seeded Free-electron Lasers
Funding: This work was partially supported by the Major State Basic Research Development Program of China (2011CB808300) and the National Natural Science Foundation of China (11175240, 11205234, and 11322550)
The recently proposed phase-merging enhanced harmonic generation (PEHG) mechanism [*, **] holds promising prospect for seeded free-electron lasers (FEL) in short-wavelength. A remarkable bunching efficiency at high harmonics can be accomplished by using PEHG mechanism. In this paper, the theoretical descriptions and numerical simulations of PEHG will be presented, including the more recent theoretical progresses and a potential proof-of-principle experiment at Shanghai deep ultraviolet FEL test facility.
* Haixiao Deng, Chao Feng, Phys. Rev. Lett. 111 (2013) 084801.
** Chao Feng, Haixiao Deng, Dong Wang, Zhentang Zhao, New Journal of Physics,16 (2014) 043021.
|Slides FRB01 [29.808 MB]
|A Collinear Wakefield Accelerator for a High Repetition Rate Multi-beamline Soft X-ray FEL Facility
Funding: Supported by U.S. Department of Energy under Contract No. DE-AC02-06CH11357 and by the U.S. Department of Energy Laboratory LDRD program at Los Alamos National Laboratory.
A concept is presented for a multi beamline soft x-ray free-electron laser (FEL) facility where several FEL undulator lines are driven by an equal number of high repetition rate single-stage collinear wakefield accelerators (CWA). A practical design of the CWA, extending over 30 meters and embedded into a quadrupole wiggler, is considered. The wiggler’s structure of alternating focusing and defocusing quadrupoles is used to control single-bunch breakup instability. It is shown that practical restrictions on the maximum attainable quadrupole field limit the maximum attainable charge in the drive bunch whose sole purpose is to produce a high accelerating field in the CWA for the following main bunch. It is also pointed out that the distance between drive and main bunches varies along the accelerator, causing a measurable impact on the energy gain by the main bunch and on the energy spread of electrons in it. Means to mitigate these effects are proposed and results are presented for numerical simulations demonstrating the main bunch with plausible parameters for FEL application including a relatively small energy spread. Finally, results are presented for the expected FEL performance using an appropriately chosen undulator.
|Slides FRB02 [6.512 MB]
|Tapering Enhanced Stimulated Superradiant Amplification
|The electrical to optical conversion efficiency of FELs is typically limited to less than 1 percent. Efforts to improve conversion efficiency have generally involved undulator tapering to drive the interaction beyond saturation in combination with focusing the electron beam to compensate gain guiding losses within undulator. Here we propose a scheme whereby a coherent radiation seed is focused into a strongly tapered undulator to violently decelerate electrons, thereby converting as much as 70 percent of e-beam energy to coherent radiation. By tapering the undulator to accommodate the radiation growth, a modest input seed may be used to drive the FEL interaction far beyond saturation in order to achieve high electrical to optical conversion efficiency. The scheme relies on a prebunched beam and a seed laser focused into a strongly tapered undulator and is therefore called tapering enhanced stimulated superradiant amplification (TESSA).
|Slides FRB03 [1.100 MB]
|Divergence Reduction and Emittance Conservation in a Laser Plasma Acceleration Stage
Plasma accelerators promise a compact source of highly relativistic electron beams. Driven by high-intensity lasers or high-energetic electron beams, the longitudinal and transverse electric fields inside the plasma cavitiy support the generation of GeV electron beams over m-scale distances, while measured emittances on the order of 0.1 mm.mrad have been reported from plasma-driven accelerators. However, it remains challenging to conserve this excellent emittance when coupling from the plasma into vacuum and a subsequent beam optics, especially when considering the large energy spread, typically accumulated during the off-crest acceleration inside the plasma. Recently, we presented an analytical solution  to describe an adiabatic matching from the plasma into vacuum. Further elaborating this concept , we will discuss the generation of low-divergence electron beams from a tailored plasma target in order to preserve the emittance generated within the plasma. We will apply our concept to an externally injected electron bunch, that is matched in and out of a tailored plasma target, generating a GeV-level electron beam with low divergence and good emittance.
* K. Floettmann, Phys. Rev. ST - Accel. Beams 17, 054402 (2014)
** I. Dornmair, K. Floettmann, and A. R. Maier, submitted (2014)