Hans P. Bluem

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.

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.

The polarized SRF gun experiment

RF electron guns are capable of producing electron bunches with high brightness, which outperform DC electron guns and may even be able to provide electron beams for the ILC without the need for a damping ring. However, all successful existing guns for polarized electrons are DC guns because the environment inside an RF gun is hostile to the GaAs cathode material necessary for polarization. While the typical vacuum pressure in a DC gun is better than 10{sup -11} torr the vacuum in an RF gun is in the order of 10{sup -9} torr. Experiments at BINP Novosibirsk show that this leads to strong ion back-bombardment and generation of dark currents, which destroy the GaAs cathode in a short time. The situation might be much more favorable in a (super-conducting) SRF gun. The cryogenic pumping of the gun cavity walls may make it possible to maintain a vacuum close to 10{sup -12} torr, solving the problem of ion bombardment and dark currents. Of concern would be contamination of the gun cavity by evaporating cathode material. This report describes an experiment that Brookhaven National Laboratory (BNL) in collaboration with Advanced Energy Systems (AES) is conducting to answer these questions.

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.

Superconducting RF injector for high-power free-electron lasers (FEL)

A key technology issue on the path to high-power FEL operation is the demonstration of reliable, high-brightness, photo-cathode injector operation. The physics and engineering conceptual design of a high-current superconducting RF injector has been completed and will be presented. The system, which is an outgrowth of the existing injector on the Jefferson Lab IR FEL, consists of an integrated room temperature DC photocathode gun and a 500 MHz superconducting RF accelerator. The device is compact and produces high-brightness beams. After DC acceleration in the gun, emittance compensation techniques are utilized to reduce the RMS normalized emittance by over a factor of two to /spl sim/2 /spl pi/ mm-mrad at the output of the RF accelerator. The design is based upon the existing geometry of the Jefferson Lab DC gun and will be capable of operation at 100 mA average beam current.

Measurements on the frequency effects and coupling of slots and irises for the APT CCDTL

In preparation for the fabrication of the coupled-cavity drift tube linac portion of the Low-Energy Demonstration Accelerator for the Accelerator Production of Tritium project, the effects of coupling slots/irises have been measured and compared to theoretical calculations. A number of multi-cavity cold models have been constructed corresponding to various sections of the coupled-cavity drift tube linac. These models have been used to measure the average cavity to cavity coupling, the frequency effects of the cavity to cavity coupling slots, the coupling into the cavity structures from a waveguide through an iris, and the effect of the iris on the frequency of the cavity being driven. The results of these measurements have been compared to the theoretical values. The effect of a shorting plate placed in one of the feed waveguides was also investigated. This was used to approximate the position of a closed vacuum valve which would be used in the event of a vacuum window failure.

A compact, high-power THz source: Concept & simulation

Many applications of THz radiation remain impractical or impossible due to an absence of compact sources with sufficient power. This paper presents a source concept capable of high power in the THz range. The interaction occurs between an annular electron beam and a cylindrical grating. The grating surface-wave produces strong beam bunching and generates significant high-frequency rf. Parametric studies using the MAGIC simulation code explored grating geometry, beam current, voltage, magnetic field, tuning bandwidth, harmonic generation and controlled feedback. Several multi-grating configurations were also simulated. In contrast to planar grating, simulations showed rf growth at current densities as low as 50 A/cm2. Results indicate a compact device could generate multiple tens of watts output above 1THz.

Compact, scalable THz source

A vacuum electronic device is under investigation as a compact, high-power terahertz source. Particle-in-cell simulations have been performed to predict general RF performance for a range of device parameters. Results to date indicate the potential for operation beyond 1 THz at power levels in excess of 50 W in a highly compact configuration.

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.