Conference Paper

Status of Nb-Pb superconducting RF-GUN cavities

TJNAF, Newport News
DOI: 10.1109/PAC.2007.4440949 Conference: Particle Accelerator Conference, 2007. PAC. IEEE
Source: IEEE Xplore


We report on the progress and status of an electron RF* gun made of two superconductors: niobium and lead [1]. The presented design combines the advantages of the RF performance of bulk niobium superconducting cavities and the reasonably high quantum efficiency of lead. The design of RF-gun and performance of 3 test cavities without and with the emitting lead spot are reported in this contribution. Measured quantum efficiency for lead at 2 K is presented briefly. More details are reported in [9].

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    • "The study is of great interest for the R&D of photocathodes and in particular for Nb superconducting radio-frequency guns (SRF), which combine the advantages of photoassisted production of high brightness and short electron pulses with reduced electrical losses and continuous wave operation [8] [9]. SRF cavities present a main drawback in the low quantum efficiency (QE) of the material used for their fabrication (QE Nb ~2×10 -5 @ 250 nm) with respect to other metallic photocathodes, reducing the possibility of obtaining electron beams of high current [10]. "
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    ABSTRACT: Pb thin films were prepared by the ns pulsed laser deposition technique on Si (100) and polycrystalline Nb substrates for photocathode application. As the photoemission performances of a cathode are strongly affected by its surface characteristics, the Pb films were grown at different substrate temperatures with the aim of modifying the morphology and structure of thin films. An evident morphological modification in the deposited films with the formation of spherical grains at higher temperatures has been observed. X-ray diffraction measurements showed that a preferred orientation of Pb (111) normal to the substrate was achieved at 30 °C while the Pb (200) plane became strongly pronounced with the increase in the substrate temperature. Finally, a Pb thin film deposited on Nb substrate at 30 °C and tested as the photocathode showed interesting results for the application of such a device in superconducting radio-frequency guns.
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    • "The operation of thin lead film superconducting photocathodes has been accepted as a way to construct superconducting RF electron injectors for linear superconducting accelerators able to deliver up to 1 mA average current with 1nC bunches to free electron laser undulators [1] [2] [3] [4] [5]. A technological development in lead film coating was started by a successful proof-of-principle experiment showing the quantum efficiency of 5.4·10 -3 for a lead photocathode excited with a laser ultraviolet beam of 195 nm wavelength [2]. This photocathode had been deposited using ultra high vacuum (UHV) cathodic arc onto the back wall of a TESLA-type electron gun [6]. "
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    ABSTRACT: We report the efforts undertaken at NCBJ and some of its collaborating laboratories dedicated to prepare pure and flat lead films coated onto niobium to operate as superconducting photocathodes. Three approaches to lead cathodic arc deposition have been implemented and tested: active plasma flux filtering, passive filtering and unfiltered flux. None of them allowed us to find a proper balance between thickness and surface roughness of a cathode. At that point efforts were taken to establish post-deposition heat treatment of lead films.
    Full-text · Article · Nov 2013 · Proc SPIE
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    • "The conclusion from many QE tests, both at 300 K and 4.2 K, was that such a SRF injector has the potential to deliver ~1 mA average current and ~1 nC charge/bunch. Figure 1 shows layout of the 1.3 GHz SRF injector as proposed in [2]. Figure 1: 1.6-cell SRF-injector cavity with 2 HOM couplers and an input coupler. "
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    ABSTRACT: In this contribution, we report Quantum Efficiency (QE) test results with a hybrid lead/niobium superconducting RF (SRF) photoinjector at 2K and new Pb arc deposited cathodes at 300K. The ultimate goal of our effort is to build a Nb injector with the superconducting cathode made of lead, which, as reported in the past, demonstrated superior QE compared to other metallic superconducting elements. At first, we present the test results obtained with a 1.6-cell high purity Nb cavity with the emitting lead spot in the center of the back plate. The QE test results at room temperature and the SEM surface analysis of eight Pb cathodes, deposited recently under various conditions, are discussed in the second part of this contribution.
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