Alan M. M. Todd

The new IR and THz FEL facility at the Fritz Haber Institute in Berlin

A mid-infrared oscillator FEL has been commissioned at the Fritz Haber Institute. The accelerator consists of a thermionic gridded gun, a subharmonic buncher, and two S-band standing-wave copper structures. It provides a final electron energy adjustable from 15 to 50 MeV, low longitudinal (< 50 keV ps) and transverse emittance (< 20 πmm mrad), at more than 200 pC bunch charge with a micro-pulse repetition rate of 1 GHz and a macro-pulse length of up to 15 µs. Pulsed radiation with up to 100 mJ macro-pulse energy at about 0.5% FWHM bandwidth is routinely produced in the wavelength range from 4 to 48 µm. A characterization of the FEL performance in terms of pulse energy, bandwidth, and micro-pulse shape of the IR radiation is given. In addition, selected user results are presented. These include, for instance, spectroscopy of bio-molecules (peptides and small proteins) either conformer selected by ion mobility spectrometry or embedded in superfluid helium nano-droplets at 0.4 K, as well as vibrational spectroscopy of mass-selected metal-oxide clusters and protonated water clusters in the gas phase.

Megawatt-class free-electron laser concept for shipboard self-defense

An efficient MW-class free electron laser (FEL) directed energy weapon (DEW) system holds promise for satisfying shipboard self-defense (SSD) requirements on future generations of Navy vessels because of the potential for high- power operation and the accessibility to all IR wavelengths. In order to meet shipboard packaging and prime power constraints, the power efficiency and high real-estate gradient achievable in a FEL driven by a superconducting rf accelerator is attractive. Configuration options and the key development issues for such a system are described.

A Compact THz Source: 100/200 GHz Operation of a Cylindrical Smith–Purcell Free-Electron Laser

We report first operation in the terahertz regime of a cylindrical grating Smith-Purcell free-electron laser. Propagation of an annular electron beam in proximity to a cylindrical grating causes strong electron bunching due to a beam-surface wave interaction. Electromagnetic radiation results from the bunching (fundamental) and, at bunch harmonics, the Smith-Purcell effect. In the experiment, over 2.5 kW was generated at 100 GHz (fundamental) and over 100 W at 200 GHz (Smith-Purcell). The results illustrate the potential of this configuration for generation of high-power terahertz radiation.

High-power electron beam injectors for 100 kW free-electron lasers

A key technology issue on the path to high-power FEL operation is the demonstration of reliable, high-brightness, high-power injector operation. We describe two ongoing programs to produce 100 mA injectors as drivers for 100 kW free-electron lasers. In one approach, in collaboration with the Thomas Jefferson National Accelerator Facility, we are fabricating a 750 MHz superconducting RF cryomodule that will be integrated with a room-temperature DC photocathode gun and tested at the Laboratory. In the other approach, in collaboration with Los Alamos National Laboratory, a high-current 700 MHz, normal-conducting, RF photo-injector is being designed and will undergo thermal management testing at the Laboratory. We describe the design, the projected performance and the status of both injectors.

A compact, high-power THz source

Summary form only given. This paper presents a source concept capable of generating high power in the terahertz (THz) range. The source utilizes a Smith-Purcell-type interaction between an annular electron beam and a cylindrical grating. The Smith-Purcell interaction has long been explored for generation of high frequency rf. Two problem areas have been discrepancies in expected and observed spectra and low output power. Recent research [1] has shown the grating dispersion lies below the Smith-Purcell range and, hence, cannot directly radiate at Smith-Purcell frequencies. The observed grating radiation is thought to be caused by end-effects and harmonics resulting from beam-rf nonlinearities. Recent simulations and experiments [2,3] support this interpretation. The low output power is a result of the exponential decay of the electric field away from the grating surface. This surface-mode characteristic requires electron beams to be thin and close to the grating to interact efficiently. Hence, standard pencil beams are ill suited for power generation while sheet beams present generation and focusing problems. The power limitation can be overcome with an annular electron beam propagating near a cylindrical grating. Annular beams are compatible with standard electron gun design and magnetic focusing techniques.

High-power THz source development

We describe a joint program between Advanced Energy Systems (AES) and Jefferson Laboratory (JLab), first to design, construct and commission a high-power, broadband, THz laboratory at the JLab free electron laser (FEL) facility, and secondly to develop a more compact, transportable, high-power THz source. The former facility can today deliver over 100W of broadband THz radiation up to several THz to user laboratories. The latter device, which has about a 50 GHz bandwidth and is tunable, is targeted to deliver on the order of 50 watts average power with a MW of peak power. It is planned for delivery to the JLab facility in 2006.

Beam diagnostics and modeling in CIRFEL

Abstract In this paper results of phase stability measurements of the photocathode drive laser at different points in the drive laser system, as well as the electron current micro-pulse phase stability measurements results are reported. Summary results of CIRFEL bend modeling and emittance measurements are also described. Result of a 5th order perturbation model for wiggler focusing is also reported.

Free Electron Lasers in 2017

Forty-one years after the first operation of the free electron laser (FEL) at Stanford University, there continue to be many important experiments, proposed experiments, and user facilities around the world. Properties of operating and proposed FELs in the terahertz (THz), infrared (IR), visible, ultraviolet (UV), and X-ray regimes are tabulated and discussed. LIST OF FELS IN 2017 The following tables list existing (Tables 1 and 2) and proposed (Tables 3 and 4) relativistic free electron lasers (FELs) in 2017. Some FELs in Tables 1 and 2 may not be currently operating, but are still included until we have been notified they are decommissioned. Tables 2 and 4, denoted as “Short Wavelength”, contain FELs that are designed to operate in the UV and X-ray regimes (400-nm or shorter wavelength), while Tables 1 and 3, denoted as “Long Wavelength”, contain all other FELs. The first column lists a location or institution, and the FEL’s name in parentheses. References are listed in Tables 5 and 6; another useful reference is the following website: http://sbfel3.ucsb.edu/www/vl_fel.html. The second column of each table lists the operating wavelength , or wavelength range. The longer wavelength FELs are listed at the top and the shorter wavelength FELs at the bottom of each table. The seven orders of magnitude of operating wavelengths indicate the flexible design characteristics of the FEL mechanism. In the third column, tb is the electron bunch duration (FWHM) at the beginning of the undulator, and ranges from almost continuous-wave to short sub-picosecond time scales. The expected optical pulse length in an FEL oscillator can be several times shorter or longer than the electron bunch depending on the optical cavity Q, the FEL desynchronism and gain. The optical pulse can be many times shorter in a high-gain FEL amplifier, or one based on self-amplified spontaneous emission (SASE). Also, if the FEL is in an electron storage ring, the optical pulse is typically much shorter than the electron bunch. Most FEL oscillators produce an optical spectrum that is Fourier-transform limited by the optical pulse length. The electron beam kinetic energy E and peak current I are listed in the fourth and fifth columns, respectively. The next three columns list the number of undulator periods N, the undulator wavelength 0, and the rms undulator parameter /2 (cgs units), where e is the electron charge magnitude, B is the rms undulator field strength, m is the electron mass, and c is the speed of light. For an FEL klystron undulator, there are multiple undulator sections as listed in the N-column; for example, 2x7. Some undulators used for harmonic generation have multiple sections with varying N, 0, and K values as shown. Some FELs operate at a range of wavelengths by varying the undulator gap as indicated in the table by a range of values for K. The FEL resonance condition,  = 0(1+K)/2, relates the fundamental wavelength  to K, 0, and the electron beam energy E = ( – 1)mc2, where  is the relativistic Lorentz factor. Some FELs achieve shorter wavelengths by using coherent harmonic generation (CHG), high-gain harmonic generation (HGHG), or echo-enabled harmonic generation (EEHG). The last column lists the accelerator types and FEL types, using the abbreviations listed after Table 4. The FEL optical power is determined by the fraction of the electron beam energy extracted and the pulse repetition frequency. For a conventional FEL oscillator in steady state, the extraction can be estimated as 1/(2N); for a high-gain FEL amplifier, the extraction at saturation can be substantially greater. In a storage-ring FEL, the extraction at saturation is substantially less than this estimate and depends on ring properties. In an FEL oscillator, the optical mode that best couples to the electron beam in an undulator of length L = N0 has a Rayleigh length z0  L/121/2 and has a fundamental mode waist radius w0  (z0/). An FEL typically has more than 90% of its power in the fundamental mode. At the 2017 FEL Conference, new lasings were reported at DESY, PSI, SACLA, Pohang, and SINAP. These are all large X-ray FEL facilities, showing there is significant worldwide interest in short wavelength FEL applications. Various other facilities reported updated parameters for existing FELs, and there are several newly proposed short-wavelength FELs around the world. ACKNOWLEDGMENT The authors are grateful for the support of Compass Scientific Engineering. ______________________________________________________________________________________________________________________________________________________________________ * pneyman@ccicms.com 38th International Free Electron Laser Conference FEL2017, Santa Fe, NM, USA JACoW Publishing ISBN: 978-3-95450-179-3 doi:10.18429/JACoW-FEL2017-MOP066

Experimental demonstration of a high power smith-purcell source using a cylindrical grating

Summary form only given. Many applications of THz radiation remain impractical or impossible due to an absence of compact sources with sufficient power. A source in which the interaction occurs between an annular electron beam and a cylindrical grating has been shown in simulations to be capable of generating very high THz power in a very compact package. The grating surface-wave produces strong beam bunching and generates significant power at the fundamental frequency and harmonics. A collaboration between Advanced Energy Systems and CEA/CESTA has been ongoing in performing proof-of-principle tests on cylindrical grating configurations producing millimeter wave radiation. Testing has been performed with a 6 mm period grating, producing power at the fundamental frequency of 15 GHz, second harmonic power at 30 GHz and although not measured, simulations show meaningful third harmonic power at 45 GHz. Comparison between simulations and the experimental results will be presented. Future plans will increase the frequency of operation to 100 GHz.

Compact, high-power Terahertz source using cylindrical gratings

Summary form only given. A promising compact source of high power THz radiation which utilizes an annular electron beam and a cylindrical grating is being studied. The cylindrical grating configuration exhibits substantially enhanced performance in comparison with the classical 2-D planar gratings. In the studied source, the grating surface-wave produces strong beam bunching and generates significant power at the fundamental surface-wave frequency and at harmonics as Smith-Purcell radiation. A collaboration between Advanced Energy Systems and CEA/CESTA has been ongoing in performing proof-of-principle tests on cylindrical grating configurations producing millimeter wave radiation. Testing has been performed with a 6 mm period grating, producing power at the fundamental frequency of 15 GHz, second harmonic power at 30 GHz and although not measured, simulations show meaningful third harmonic power at 45 GHz. To date simulations have been performed at frequencies up to the THz frequency range. Simulations are ongoing using both MAGIC and VORPAL. Comparisons between these two codes will be presented and compared to the experimental results. Additionally, a three dimensional dispersion relation has been developed. A comparison between the dispersion relation and three dimensional simulations shows the potential higher order modes.