Slice emittance measurements at the SLAC gun test facility

Dowell, D.H.; Bolton, Paul R.; Clendenin, J. E.; Emma, P.; Gierman, S.M.; Graves, W.; Limborg, C.; Murphy, Brendan; Schmerge, John

doi:10.1016/s0168-9002(03)00939-2

Cited by 22 publications

(10 citation statements)

References 6 publications

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“…Such a beam can, in principle, be realized in an "idealized" RF photoinjector with a perfectly working emittance compensation technique [18] that allows one to align slices in transverse phase space. For real beam, the variation in the space charge forces can be significant and cannot be properly compensated with solenoidal emittance compensation that was observed in different measurements [19][20][21]. In addition, CSR-related effects in bunch compressors can lead to further deviations from the simple model.…”

Section: Discussionmentioning

confidence: 99%

A simple method for the determination of the structure of ultrashort relativistic electron bunches

Saldin

Schneidmiller

Yurkov

2005

Nuclear Instruments and Methods in Physics Research Section A:

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In this paper we propose a new method for measurements of the longitudinal profile of 100 femtosecond electron bunches for X-ray Free Electron Lasers (XFELs). The method is simply the combination of two well-known techniques, which where not previously combined to our knowledge. We use seed 10-ps 1047 nm quantum laser to produce exact optical replica of ultrafast electron bunches. The replica is generated in apparatus which consists of an input undulator (energy modulator), and the short output undulator (radiator) separated by a dispersion section. The radiation in the output undulator is excited by the electron bunch modulated at the optical wavelength and rapidly reaches 100 MW-level peak power. We then use the now-standard method of ultrashort laser pulse-shape measurement, a tandem combination of autocorrelator and spectrum (FROG -frequency resolved optical gating). The FROG trace of the optical replica of electron bunch gives accurate and rapid electron bunch shape measurements in a way similar to a femtosecond oscilloscope. Real-time single-shot measurements of the electron bunch structure could provide significant information about physical mechanisms responsible for generation ultrashort electron bunches in bunch compressors. The big advantage of proposed technique is that it can be used to determine the slice energy spread and emittance in multishot measurements. It is possible to measure bunch structure completely, that is to measure peak current, energy spread and transverse emittance as a function of time. We illustrate with numerical examples the potential of the proposed method for electron beam diagnostics at the European X-ray FEL.

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Section: Discussionmentioning

confidence: 99%

A simple method for the determination of the structure of ultrashort relativistic electron bunches

Saldin

Schneidmiller

Yurkov

2005

Nuclear Instruments and Methods in Physics Research Section A:

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“…This value is on a par with state-of-the-art externally illuminated cathodes 22 and very good when compared to earlier results [23][24][25] or to the theoretical prediction made by the three-step model for photoemission. 26 The transverse coherence length of the electron bunch at the slit position can be estimated from the measured emittance and beam size using the relation 22,27 Using this equation, we obtain L x ¼ 4:1 nm. For the 100 lm core diameter fiber, a similar measurement was carried out, at a higher acceleration voltage of V acc ¼ À1500 V and a slightly changed geometry (see the supplementary material for details).…”

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confidence: 99%

Optical fiber-based photocathode

Casandruc

Bücker

Kassier

et al. 2016

Applied Physics Letters

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“…Consequently, it becomes important to extract electrons from a photoinjector with the lowest possible emittance. Extensive worldwide photoinjectorrelated R&D aimed at emittance reduction has been performed for more than two decades [4][5][6][7][8][9][10][11]. Many studies suggest that the drive laser pulse must be uniform in time and space at the photocathode to produce the best emittance beam [12,13], and this was the approach first pursued at the LCLS.…”

mentioning

confidence: 99%

Impact of the spatial laser distribution on photocathode gun operation

Zhou

Brachmann

Emma

et al. 2012

Phys. Rev. ST Accel. Beams

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It is widely believed that a drive laser with uniform temporal and spatial laser profiles is required to generate the lowest emittance beam at the photoinjector. However, for a given 3 ps smooth-Gaussian laser temporal profile, our recent simulations indicate that a truncated-Gaussian laser spatial profile produces an electron beam with smaller emittance. The simulation results are qualitatively confirmed by later analytical calculation, and also confirmed by measurements: emittance reduction of $25% was observed at the linac coherent light source (LCLS) injector with a truncated-Gaussian laser spatial profile at the nominal operating bunch charge of 150 pC. There was a significant secondary benefit-laser transmission through the iris for the truncated-Gaussian profile was about twice that compared to the nearly uniform distribution, which significantly loosens the laser power and quantum efficiency requirements for drive laser system and photocathode. Since February 9, 2012, the drive laser with the truncated-Gaussian spatial distribution has been used for LCLS routine user operations and the corresponding free electron laser power is at least the same as the one when using the nearly uniform spatial profile. I. OVERVIEWIt is well known that the cost and performance of the x-ray free-electron-laser (FEL) [1][2][3] depend critically on the emittance of the electron beam. Modern linear accelerators, such as the Linac Coherent Light Source (LCLS) [1], efficiently preserve the electron beam emittance through acceleration. Consequently, it becomes important to extract electrons from a photoinjector with the lowest possible emittance. Extensive worldwide photoinjectorrelated R&D aimed at emittance reduction has been performed for more than two decades [4][5][6][7][8][9][10][11]. Many studies suggest that the drive laser pulse must be uniform in time and space at the photocathode to produce the best emittance beam [12,13], and this was the approach first pursued at the LCLS. The LCLS commissioning and early operations started with 3 ps pulses stacked to form 6.5 ps nearly flattop pulses [14] using a split and delay arrangement, which adds complexity to the laser system. A nearly uniform laser spatial profile was obtained by overfilling an iris located far upstream of the cathode. Most of the laser beam was lost on the iris. As such, a very high laser power from the laser amplifier was required to keep sufficient laser energy at the cathode. The laser beam size on the iris can be adjusted by tuning the telescope upstream of the iris.The LCLS team is constantly making efforts to improve the injector emittance as well as to simplify the drive laser system in order to increase system availability. In the spring of 2010, one of the stacked lasers pulses was removed, resulting in no obvious emittance change [15]. Since then, 3 ps single polarization with Gaussian temporal profile is used for the user operations. In late 2010, simulations [16] indicated that the emittance could be improved using a truncated-Gaussian laser s...

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Slice emittance measurements at the SLAC gun test facility

Cited by 22 publications

References 6 publications

A simple method for the determination of the structure of ultrashort relativistic electron bunches

A simple method for the determination of the structure of ultrashort relativistic electron bunches

Optical fiber-based photocathode

Impact of the spatial laser distribution on photocathode gun operation

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