In this Letter we report the first experimental observation of the double emittance minimum effect in the beam dynamics of high-brightness electron beam generation by photoinjectors; this effect, as ...predicted by the theory, is crucial in achieving minimum emittance in photoinjectors aiming at producing electron beams for short wavelength single-pass free electron lasers. The experiment described in this Letter was performed at the SPARC photoinjector site, during the first stage of commissioning of the SPARC project. The experiment was made possible by a newly conceived device, called an emittance meter, which allows a detailed and unprecedented study of the emittance compensation process as the beam propagates along the beam pipe.
Pulsed laser deposition (PLD) has been proposed several years ago as a suitable technique to deposit a pure Mg film over a radio frequency (rf) gun Cu backflange in order to obtain a high efficiency ...photocathode surface for the generation of high brightness electron beams. In this paper we report preliminary experimental results on the emission properties of a PLD grown Mg film within the high electric field gradients of a rf gun showing the effects of the rf conditioning process on the cathode surface. Even though a laser cleaning process should be performed on the sample surface in order to remove contaminated layers, the results presented here are very promising for the realization of a final Mg-based photocathode.
The new generation of linac injectors driving free electron lasers in the self-amplified stimulated emission (SASE-FEL) regime requires high brightness electron beams to generate radiation in the ...wavelength range from UV to x rays. The choice of the injector working point and its matching to the linac structure are the key factors to meet this requirement. An emittance compensation scheme presently applied in several photoinjectors worldwide is known as the “Ferrario” working point. In spite of its great importance there was, so far, no direct measurement of the beam parameters, such as emittance, transverse envelope, and energy spread, in the region downstream the rf gun and the solenoid of a photoinjector to validate the effectiveness of this approach. In order to fully characterize the beam dynamics with this scheme, an innovative beam diagnostic device, the emittance meter, consisting of a movable emittance measurement system, has been designed and built. With the emittance meter, measurements of the main beam parameters in both transverse phase spaces can be performed in a wide range of positions downstream the photoinjector. These measurements help in tuning the injector to optimize the working point and provide an important benchmark for the validation of simulation codes. We report the results of these measurements in the SPARC photoinjector and, in particular, the first experimental evidence of the double minimum in the emittance oscillation, which provides the optimized matching to the SPARC linac.
Advanced high brightness radio frequency (RF) gun injectors require photocathodes with a fast response, high quantum efficiency (QE) and good surface uniformity. Metal films deposited by various ...techniques on the gun back wall could satisfy these requirements. A new deposition technique has been recently proposed, i.e. pulsed laser ablation. Several Mg samples have been deposited by this technique: the emission performance and morphological changes induced on the cathode surface during laser activation are compared and discussed.
In this study, pulsed laser ablation of pure Mg targets (99.99%) using an XeCl excimer laser (
λ
=
308
nm,
τ
=
30
ns) was used to grow Mg coatings on Cu substrates. A thin layer of Mg (100–200
nm) ...was deposited in UHV of 5
×
10
−6
Pa to ensure high purity. A second ultra-thin layer of MgO of 10–20
nm was deposited in oxygen at 10
−2
Pa pressure to protect the previous layer from atmospheric contamination.
Measurements for determining the electron yield of the deposited samples under UV irradiation were performed on a diode structure in another UHV apparatus evacuated to 10
−6
Pa.
Ablated Mg films with a graphite cover as photocathodes Perrone, A.; Cultrera, L.; Pereira, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2005, Letnik:
554, Številka:
1
Journal Article
Recenzirano
Mg films deposited by pulsed laser ablation on a Cu substrate were tested as photocathodes in a DC diode. They showed features of robustness and a quantum efficiency of
3
×
10
-
4
at low accelerating ...field when illuminated by 266
nm light at normal incidence. A graphite protective layer of about 20
nm thickness deposited by laser ablation just after the Mg deposition made the cathode stable and of easy handling. The cathodes could stay for several weeks in air before being inserted in the test chamber. The preparation procedure and the tests are presented.
The influence of the adsorbed gases on the photoelectron performance of Mg thin film-based photocathodes was investigated. Mg thin films were deposited on Silicon and Copper substrates by pulsed ...laser ablation technique. The samples deposited on Silicon substrate were used to deduce the morphology, the structure and the thickness by using different diagnostic techniques. On the contrary, the samples deposited on Copper substrate were tested as photocathodes in a DC photodiode cell. The quantum efficiency of the Mg photocathodes was systematically improved by laser cleaning treatments, which appears to be necessary before any use of a metallic photocathode. The role of the hydrogen and oxygen absorption process on the emission properties is presented and discussed.
Photoemission from polycrystalline diamond films irradiated with the harmonics of a Nd:YAG laser indicates that the amorphous or quasi-amorphous zones (patches) of sp-C on the external surface of the ...diamond grains make a dominant contribution to the emission yield. This is deduced from: (a) the strong reduction of the yield as a consequence of the partial removal of amorphous carbon (a-C) patches by etching with acid and (b) the correlation between the yield and the a-C content determined by Raman spectroscopy. The junction between the a-C patches and the diamond determines a configuration of the energy bands, which allows one-photon emission with an energy of 4.7 eV (4th harmonic). We have measured samples with variable a-C content and found an efficient emission, with values of quantum efficiency on the order of 10. A phenomenological picture of the electron emission process leads us to locate it at the a-C--diamond--vacuum triple border. This indicates a high local efficiency and suggests that a diamond film with an appropriate distribution of a-C patches on the surface could be a very efficient material. Moreover it could work in moderate vacuum and have a high laser-radiation damage-threshold. Such a material would therefore be interesting as a robust photocathode .
The photoemission behaviour of a series of diamond-based polycrystalline films irradiated by the second (2.3 eV), third (3.5 eV) and fourth (4.7 eV) harmonics produced by a Q-switched-mode-locked Nd: ...Yag laser has been investigated and related to the structural and compositional characteristics of the layers. The materials were polycrystalline undoped diamond films as well as Nd- and N-containing diamond films grown by CVD techniques, diamond-like and amorphous carbon layers. The morphological and structural characteristics of the films were investigated by electron microscopy, Raman spectroscopy and electron diffraction. The analysis of the photoemission curves does not evidence any improvement of the emission efficiency in the case of Nd-containing films nor for the diamond films grown in the presence of N
2. The results evidence conversely a strong correlation between the characteristics of the photoemission process at sub-band gap energies and the presence of amorphous sp
2-C patches located at the diamond film surfaces. The photoemitting properties of our samples are discussed and rationalized by considering charge emission occurring at the sp
2-diamond-vacuum border and the emission process governed by the ratio of amorphous sp
2-C to crystalline sp
3-C. The rather high values of quantum efficiency measured in the course of the present research at 3.5 and 4.7 eV suggest that a proper distribution of amorphous carbon onto a good quality diamond surface is the key factor for the preparation of efficient and stable photocathode materials.
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