In the FEL facility solid state physics, superconductivity and spectroscopic experiments have been performed in the FIR and mm-wave region.
Free Electron Lasers are very flexible sources of coherent radiation, due to their wide range tunability and high brightness. The physical mechanism that produces coherent emission in a Free Electron Laser is the interaction between a relativistic electron beam and a magnetostatic field with a particular spatial configuration, while in a conventional laser the stimulated emission from an atomic or a molecular system is exploited. FELs have been built all around the world and, up to now, they cover the electromagnetic spectrum from the millimeter waves up to the vacuum ultraviolet. FELs can provide high peak power and short pulses or narrow linewidth in long pulse operation, depending on the characteristics of the apparatus. Electronic efficiency as high as 30% has been demonstrated in systems employing tapered undulators, and wide range tunability has been achieved in many FELs making them appealing for a variety of spectroscopic applications. Many applications have been made possible by the peculiar features of FELs in the fields of spectroscopy, solid state physics, biology and medicine.
A Free Electron Laser is essentially composed of three parts: an electron accelerator, a magnetic undulator and an optical resonator (fig. 1). The electrons are forced by the magnetic field on an oscillating trajectory, thus emitting synchrotron radiation. In the electron frame reference the process can also be seen as a scattering between the electron beam and the virtual photons of the undulator. If an external field is present, the radiation is emitted in phase with this external field. The interaction between the laser field, the static magnetic field of the undulator and the electron beam has as a final effect the spatial bunching of the electrons on the scale of the radiation wavelength, and the transfer of energy from the electron beam to the laser field. The undulator can be considered as the equivalent of the "active medium" of a conventional laser system, while the electron beam is the equivalent of the "pumping system"
Due to their peculiar characteristics Free Electron Lasers can, in principle, cover most of the electromagnetic spectrum from the microwave region to the vacuum ultraviolet: the wavelength of the emitted radiation depends on the electron energy and on the magnitude and periodicity of the undulator magnet field according to the following relation:
where lu is the undulator period, g is the relativistic factor of the electrons g= ( 1-b2)-1/2 and K is the so-called undulator parameter, proportional to the magnetic field inside the undulator.
It is important to notice that lu and K have some limitations in their practical ranges; they are used to tune the FEL around a central frequency, mainly determined by the energy parameter g of the electrons, that depends on the choice of the electron accelerator. It is thus clear that, in order to achieve emission at short wavelength it is necessary to use high energy electron accelerators, that imply high costs and large size of the facility.
The appealing features of FELs (e.g. tunability, high output power) are usually counterbalanced by some drawbacks, such us large size, high cost and system complexity. Nevertheless, there is a wavelength range, not covered by conventional laser sources, where the FEL can meet the requirement of compactness: the far-infrared (FIR) and submillimeter (sub-mm) region. Many applications have been proposed in this spectral region, ranging from solid state physics to biophysics and nuclear physics.
The development of FELs in this spectral region benefits of the following advantages:
The main advantage of operating in a waveguide is the possibility of having an optical mode of small and constant cross section, which can be propagated with low attenuation along the waveguide. This results in a better filling factor with respect to the electron beam. It also allows the design of an undulator with a small gap between the magnet poles, so that high values of the magnetic field can be obtained even with short undulator periods. Both effect increase the FEL gain. For electron energies in the range between 2 and 5 MeV significant gain is obtained also for a small number of periods, thus making possible the FEL operation with a short undulator and a short resonator.
An important requirement for a radio frequency (RF) driven FEL is the matching between the round trip time of the optical pulses in the cavity and the electron bunch spacing. Exploiting the dispersion properties of the waveguide it is possible to slow down the wave velocity allowing the superposition of the electron bunch and the light pulse all over the interaction region (zero slippage operation). In this situation the gain curve has a broad bandwidth (Dl/l ~ 1/N1/2), the FEL efficiency is doubled, and the gain is less sensitive to the e-beam quality, compared to the free-space case.
On the basis of the above considerations, a mm-wave FEL-facility was built at Frascati in the years 1987-91, utilizing a RF driven 5 MeV microtron as electron source. A compact Cherenkov FEL has been operating during the years 1989-90. An undulator FEL was then built in 1991.
The experimental layout is shown in fig. 1 and the FEL operating parameters are reported in table 1.
COMPACT MM-WAVE FEL DESIGN PARAMETERS
|Electron Energy (MeV)||2.3||Energy spread||1%|
|I-peak (A)||4||Und. period (cm)||2.5|
|I-av (A)||0.2||N (num. of periods)||8|
|Micropulse duration (ps)||15||Undulator par. K||1|
|Macropulse duration (ms)||4||Waveguide hor. gap (mm)||10.67|
|Norm. emittance (cm rad)||0.02||Waveguide vert. gap (mm)||4.32|
After achieving the first oscillation at l=2.43 mm (~ 120 GHz) with a fixed resonator length configuration in December 1991, the emission performance of this compact waveguide FEL has been optimized in term of output power and tunability. The input mirror can now be moved, allowing a cavity length detuning up to 15 mm, with an accuracy of 5 mm. The relative spectral bandwidth can vary from less then 0.1 % up to 8 % in the wavelength range from 2 to 3.5 mm and an output power up to 1.5 kW in 4 ms macro-pulses is now available. The parameters of the emitted radiation are reported in table 2 and the tuning capability of the system is shown in fig. 3.
EMITTED RADIATION CHARACTERISTICS
|2 - 3.5 mm|
|> 50 ps||Relative
|0.6 - 8 %|
|up to 1.5 kW||Micropulse
|up to 10 kW|
Fig. 4a: FEL output power vs resonator detuning
|In fig. 4 the FEL output power is reported as a function of the cavity detuning, together
with the Fabry Perot interferogram of the radiation emitted at two different cavity lengths.
The Fabry Perot interferogram has been used to analyze the spectral contents of the emitted radiation taken at two different cavity length, indicated by the arrows. Spectra are composed by discrete frequencies at harmonic of the driving RF (fig. 4b). The two spectra are dramatically different, both in the peak frequency and in the bandwidth. This is due to the non linear dispersion relation in the waveguide: for the e.m. modes to be amplified in the FEL they must be simultaneously an harmonic of the driving RF (3 GHz) and they must be longitudinal modes of the resonator. Being the dispersion relation non linear, changing the cavity length and taking into account the finesse of the resonator, a different number of discrete frequencies can be "sustained" by the FEL resonator, as shown on the graph.
Other experiments have been performed with the same apparatus: evidence has been shown of the so called coherent spontaneous emission, i.e. the radiation emission from short electron bunches, when passing through a magnetic undulator, without the presence of a resonator. The intensity of this emission is much higher than the conventional spontaneous emission, and this allows better performance when using it as a seed for FEL operation, as evidenced studying the growth of the FEL output power during the macropulse.
Fig. 4b: FP interferograms of the FEL Radiation taken at the resonator length indicated by the arrows
The FEL radiation has been used for different applications in solid state physics (fig. 5): measurements of the long wavelength behavior of different solid state detectors, including photon drag detectors and electronic bolometers, have been performed in collaboration with the University of Essex (UK). Of particular interest are the studies on the high temperature behavior of an InSb detector, used as electron bolometer: a new detection process has been observed at 77 K, with an effect opposite in sign and in competition with the usual bolometric response.
Another collaboration is in progress with the University of Rome "La Sapienza" and with the Energy Department of ENEA to study the spectral behavior of High-Tc superconducting films at low frequencies. These measurements will allow a better understanding of the physical processes involved in superconductivity at high temperature.
order to satisfy the increasing interest of the scientific community on FELs as coherent radiation user sources,
in 1994 ENEA has built a new experimental area devoted to external user applications. At present the facility provides
500 h/year of beam time to users. With the operation of this facility, the European laboratories will be able to
provide wavelengths in a wide spectral range (2 mm - 3 mm) (fig.
During the years 1993/1995 a considerable effort has been devoted to the design of a new Far Infrared FEL, with extended tunability down to 200 mm and increased output power up to 30 kW. This project was aimed at the realisation of a coherent source to be utilized, in collaboration with INFN - Trieste and Paul Scherrer Institute - Villigen (CH), in a fundamental physics experiment, to measure the QED corrections to the energy levels of a muonic hydrogen atom. The requirements on the FIR radiation source needed for the spectroscopic measurement, namely compactness, high power, long temporal light pulse, tunability between 200 and 500 mm and capability to provide full power within 1.5 ms from a trigger, rule out the use of conventional laser sources for this experiment, leaving the FEL as the only possible choice. In order to perform preliminary low power tests, a new "prototype" is presently in construction at ENEA Frascati. This FEL makes use of a new 16 periods undulator (fig. 7) and utilizes the present electron source with a new 15 MW klystron as RF source. In the last months of 1995 this RF system has been tested and now it is possible to switch between the magnetron and the klystron with a one day off-time.
Tests have been performed on the new undulator and on the vacuum chamber, in order to verify the vacuum performance of the system. A minor modification has been made on the microtron in order to extract the electrons at the energy of 5.5 MeV. With this configuration emission and lasing will be possible in the range between 400 and 850 mm, allowing preliminary experimental studies on the ETM efficiency and optical pulse stretching at shorter wavelengths. Measurements have been performed on the coherent spontaneous emission from this system.
In the final configuration a RF linac should be used as electron source, in order to achieve higher currents and higher output power. The design parameters of this new FEL are reported in table 3.
FIR FREE ELECTRON LASER DESIGN PARAMETERS
|Electron Energy (MeV)||5.5||5.5|
|Peak Electron Current I (A)||40||40|
|Average Current on Macro-Bunch (A)||0.45 A||0.45 A|
|Micro-Bunch Duration (FWHM)||3.7 ps||3.7 ps|
|Macro-Bunch Duration||2-7 ms||2-7 ms|
|Electron Normalized Emittance||16 p mm mrad||16 p mm mrad|
|Undulator Period lu (cm)||2.5||2.5|
|Number of Periods N||16||16|
|Undulator Parameter K||1.412-1.425||1.180-1.232|
|Waveguide horizontal gap a (cm)||3||3|
|Waveguide vertical gap b (cm)||0.188||0.4|
|Waveguide length L (cm)||50||50|
|Power macropulse (kW)||35||35|
Table 3 : DESIGN PARAMETERS OF THE NEW FIR FEL
New Experiments: The Phase Matching System
The FEL Staff
EC Project MAIFEL
EC Proposal THz