We investigated a diode-pumped multipass disk Yb:KGW amplifier intended for amplifying a train of 3D ellipsoidal pulses of a laser driver for a photocathode of a linear electron accelerator. The multipass amplification geometry permitted increasing the energy of broadband (about 10 nm) pulses with a repetition rate of 1 MHz from 0.12 µJ to 39 µJ, despite large losses (two orders of magnitude) introduced by a beam shaper of 3D ellipsoidal beam. The distortions of the pulse train envelope were minimal due to optimal delay between the moment of pump switching on and arrival of the first pulse of the train.

https://pitz.desy.de/sites2009/site_pitz/content/e145700/e183655/e183656/infoboxContent210183/Gacheva_Paper_OpticsExpress_oe-23-8-9627.pdf

Disk Yb:KGW amplifier of profiled pulses of laser driver for electron photoinjector

We demonstrate a simple single grating beam modulation technique, which enables the use of a highly intense neutron beam for differential phase and dark-field contrast imaging and thus spatially resolved structural correlation measurements in full analogy to interferometric methods. In contrast to these interferometric approaches our method is intrinsically achromatic and provides unprecedented flexibility in the choice of experimental parameters. In particular the method enables straight forward application of quantitative dark-field contrast imaging in time-of-flight mode at pulsed neutron sources. Utilizing merely a macroscopic absorption mask unparalleled length scales become accessible. We present results of quantitative dark-field contrast imaging combining microstructural small angle scattering analyses with real space imaging for a variety of materials.

/pdf/achromatic-non-interferometric-single-grating-neutron-dark-1798sgjkkn.pdf

Achromatic Non-Interferometric Single Grating Neutron Dark-Field Imaging.

Acceleratori di particelle di nuova generazione, come CLIC (Compact LInear
Collider), necessitano di fasci di elettroni ad elevata brillanza che potrebbero essere generati attraverso l'uso di un fotoiniettore (sorgente di fotoelettroni attivata da un fascio laser). Fotocatodi di Cs2Te illuminati con fasci laser nell'ultravioletto (UV) sono attualmente utilizzati in diversi fotoiniettori, ma le specifiche tecniche delle sorgenti di elettroni per acceleratori innovativi come CLIC sono significativamente piu' difficili da realizzare. In particolare, il requisito piu' stringente per il drive beam di CLIC e' quello di riuscire ad ottenere allo stesso tempo impulsi di elettroni ad elevata carica (8.4 nC) ed alta frequenza di ripetizione (500 MHz), treni di impulsi di lunga durata (140 us) e fotocatodi con una vita utile ragionevolmente estesa. In particolare l'energia del treno di impulsi laser nel UV, per una tale estesione temporale, e' al momento limitata a causa del deterioramento del fascio laser durante il processo di generazione di quarta armonica. L'utilizzo della seconda armonica (fascio laser nel verde), a condizione che questa sia accoppiata ad un fotocatodo con una soglia di fotoemissione bassa, rappresenterebbe una soluzione a questa limitazione. L'antimoniuro di Cesio (Cs3Sb), essendo
un materiale con buone proprieta' fotoemissive nelle lunghezze d'onda del
visibile, risulta essere un materiale di interesse in questo ambito.
L'obiettivo principale di questa tesi di dottorato e' quello di fornire uno studio sperimentale approfondito dell'uso di tale materiale per applicazioni in fotoiniettori. Le attivita' sperimentali volte a studiare la possibilita' di produrre catodi Cs3Sb di elevata qualita', comprendono la produzione, attraverso tecniche di deposizione di film sottili, e la caratterizzazione delle proprieta' fotoemissive in una linea di accelerazione DC. Le prestazioni dei fotocatodi di Cs-Sb sono state studiate all'interno di un fotoiniettore a radiofrequenza, permettendo cosi' di caratterizzarne le funzionalita' piu' rilevanti per queste applicazioni. Analisi di spettroscopia di fotoemissione a raggi x (XPS) sono state svolte per correlare le proprieta' fotoemissive osservate alla composizione chimica dei fotocatodi e per studiare i processi di deterioramento del catodo, a cui esso e' soggetto durante il funzionamento del fotoiniettore. Inoltre, e' stato condotto un confronto con le prestazioni raggiunte da catodi di Cs2Te.
Questo lavoro di tesi fornisce una dettagliata caratterizzazione delle prestazioni di fotocatodi in Cs-Sb e rappresenta percio' un importante contributo allo sviluppo di fotocatodi per applicazioni in fotoiniettori a radiofrequenza ed elevata carica.

Characterization of Cs-Sb cathodes for high charge RF photoinjectors

The Compact Linear Collider (CLIC) study proposes a multi-TeV, high luminosity, electron-positron linear collider in order to fulfill the current need for a lepton collider. The study has been started in the late 80s at CERN and currently is a joint effort with a collaboration of 40 institutes. An innovative scheme of high peak RF power production for the high accelerating gradient has been proposed for CLIC. The so called "two-beam scheme" consists of two beams that are running parallel to each other. One of the beams is to be accelerated for the collision experiments and called "the main beam". The second beam of the CLIC scheme is "the drive beam" and will be employed for the power production. The quality of the main beam acceleration depends on the stability of the power that is generated by the drive beam. Therefore, the optimization of the drive beam production with the proper time structure and within the required beam dynamics tolerances is one of the most important accelerator physics aspects of the project. Currently in the conceptual level, the baseline design of the drive beam injector consists of a thermionic gun. This electron source has to be combined with a sub-harmonic bunching system in order to provide the required time structure of the drive beam. However, a big disadvantage of this scheme is the parasitic satellite bunches that are produced due to the sub-harmonic bunching system. PHIN photoinjector has been raised as another option in order to replace the existing thermionic gun of CLIC test facility (CTF3) and to form the bases of a source for the CLIC drive beam. The PHIN project is in the framework of the European CARE (Coordinated Accelerator Research in Europe) program.

http://iopscience.iop.org/article/10.1088/1742-6596/347/1/012036/pdf

PHIN photo-injector as the CLIC drive beam source

The high-average current electron gun low energy electrons from a thermionic cathode at high intensity is the prototype of the drive beam source for the Compact Linear Collider project. The electromagnetic design of the gun was done originally with 2D simulations to minimize the beam emittance for high-current operation (of the order of 8 A) at 140 keV. This paper reports on the first experimental measurements of beam extraction and transport. For the first time, 3D tracking simulations, which take into account precisely the cathode–grid geometry, have been compared with experimental optical transition radiation images of the beam, and we obtained impressive agreement. In addition, we have confirmed that the details of the cathode–grid assembly can impact significantly the beam emittance.

Operation of a high-current drive beam electron gun prototype for the Compact Linear Collider.

The future Compact Linear Collider (CLIC) e − /e + collider is based on the two-beam acceleration concept, whereby interleaving electron bunches of the drive beam through a delay loop and combiner rings as well as high peak RF power at 12 GHz are created locally to accelerate a second beam, the main beam. One of the main objectives of the currently operational CLIC Test Facility (CTF3) is to demonstrate beam combination from 1.5 GHz to 12 GHz, which requires satellite-free fast phase-switching of the drive beam with sub-ns speed. The PHIN photo-injector, with the photo-injector laser, provides flexibility in the time structure of the electron bunches produced, by direct manipulation of the laser pulses. A novel fiber modulator-based phase-switching technique allows clean and fast phase-switch at 1.5 GHz. This paper describes the switching system based on fiber-optic modulators, and the measurements carried out on both the laser and the electron beam to verify the scheme.

Fast phase switching within the bunch train of the PHIN photo-injector at CERN using fiber-optic modulators on the drive laser

The high charge PHIN photo-injector was developed within the framework of the European CARE program to provide an alternative to the drive beam thermionic gun in the CTF3 (CLIC Test Facility) at CERN. In PHIN 1908 electron bunches are delivered with bunch spacing of 1.5 GHz and 2.33 nC charge per bunch. Furthermore the drive beam generated by CTF3 requires several fast 180 deg phase-shifts with respect to the 1.5 GHz bunch repetition frequency in order to allow the beam combination scheme developed at CTF3. A total of 8 subtrains, each 140 ns long and shifted in phase with respect to each other, have to be produced with very high phase and amplitude stability. A novel fiber modulator based phase-switching technique developed on the laser system provides this phase-shift between two consecutive pulses much faster and cleaner than the base line scheme, where a thermionic electron gun and sub-harmonic bunching are used. The paper describes the fiber-based switching system and the measurements verifying the scheme. The paper also discusses the latest 8nC charge production and cathode life-time studies on Cs2Te.

/pdf/high-charge-phin-photo-injector-at-cern-with-fast-phase-22sw19241n.pdf

High charge phin photo injector at cern with fast phase switching within the bunch train for beam combination

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Fast phase switching within the bunch train of the PHIN photo-injector at CERN using fiber-optic modulators on the drive laser

High charge phin photo injector at cern with fast phase switching within the bunch train for beam combination