To generate the electrons to be accelerated, a photoinjection laser is used at the linac-based test facility FLUTE (Ferninfrarot Linac- Und Test Experiment) at the Karlsruhe Institute of Technology (KIT). The properties of the laser pulse, such as intensity, laser spot size or temporal profile, are the first parameters to influence the characteristics of the electron bunches. In order to control the initial parameters of the electrons in the most flexible way possible, the laser optics at FLUTE are therefore supplemented by additional setups that allow transverse and longitudinal laser pulse shaping by using so-called Spatial Light Modulators (SLMs). In the future, the control of the SLMs will be integrated into a Machine Learning (ML) supported feedback system for the optimization of the electron bunch properties. In this contribution the first test experiments and results on laser pulse shaping at FLUTE on the way to this project are presented

Müller, A-S.

Nabinger, M.

Nasse, M. J.

Sax, C.

Schäfer, J.

Widmann, C.

Xu, C.

English

KITopen

TRANSVERSE AND LONGITUDINAL MODULATION OFPHOTOINJECTION PULSES AT FLUTEM. Nabinger , C. Sax , M. J. Nasse, J. Schäfer, C. Widmann, C. Xu, A.-S. Müller∗† ∗Karlsruhe Institute of Technology, Karlsruhe, GermanyAbstractTo generate the electrons to be accelerated, a photoin-jection laser is used at the linac-based test facility FLUTE(Ferninfrarot Linac- Und Test Experiment) at the KarlsruheInstitute of Technology (KIT). The properties of the laserpulse, such as intensity, laser spot size or temporal profile,are the first parameters to influence the characteristics ofthe electron bunches. In order to control the initial parame-ters of the electrons in the most flexible way possible, thelaser optics at FLUTE are therefore supplemented by ad-ditional setups that allow transverse and longitudinal laserpulse shaping by using spatial light modulators (SLMs). Inthe future, the control of the SLMs will be integrated into amachine learning (ML) supported feedback system for theoptimization of the electron bunch properties. In this contri-bution, the first test experiments and results on laser pulseshaping at FLUTE on the way to this project are presented.INTRODUCTIONModern and future accelerator projects require electronbeams with an exceptional combination of high brightnessand low emittance. As a consequence, in recent years therehas been a strong interest in the improvement of RF pho-toinjectors [1–3]. Of particular interest is the tailoring ofthe photoinjection pulse properties [4]. Arbitrary shapingof the drive laser pulse in both transverse and longitudinaldirections provides a powerful tool to significantly influenceand control the electron bunch characteristics.Among the most promising devices for realizing indi-vidual pulse shaping are liquid crystal SLMs, pixel-basedelectro-optical components that allow a modulation in phaseand/or intensity of the incident light. The working principleof the phase-only modulating SLMs, which are used in thiswork, is shown in Fig. 1. The birefringent liquid crystalmolecules (LCM) are embedded in several cells/pixels be-tween pairs of electrodes on a silicon substrate, protected bya glass layer on top. In such a setup - also known as liquidcrystals on silicon (LCoS) - the orientation of the LCM canbe changed by applying a voltage to the cell, which leads to achange of the effective refractive index seen by the incidentlight wave. As a result, the phase of the reflected wave canbe modulated for every pixel independently.In order to create arbitrary images, the voltages corre-sponding to the phase shift for each single pixel in the arraymust be controlled entirely. This is achievable by computingand applying a 2D phase distribution or phase mask, alsoknown as a computer generated hologram (CGH). For the∗ Both authors have contributed equally.† matthias.nabinger@kit.eduFigure 1: Working principle of an SLM with embeddedLCM. By applying a voltage, the orientation of the LCMcan be changed and thus the phase of the incident light canbe modulated.calculation of a CGH, several algorithms can be used, forinstance the Gerchberg-Saxton (GS) algorithm, which isbased on inverse Fourier transforms.At the linac-based test facility FLUTE at KIT, a 800 nmTi:Sa laser with 35 fs bandwith-limited pulse length is usedfor photoinjection. The copper photocathode requires a con-version of the wavelength to 266 nm by third harmonic gen-eration. Such nonlinear processes and also the subsequentrelatively complex beam path with many optical elementsare difficult to take into account in the conventional calcula-tion of CGHs. This leads to distortions of the final imageon the photocathode. Hence, a simple application of the GSalgorithm is insufficient. Therefore, first approaches to ML-assisted control of pulse modulation by SLMs have beenintroduced [5, 6]. Recently, promising attempts have beeninvestigated and further developed at FLUTE [7].In this paper, we present two test setups and the achievedresults for transverse and longitudinal pulse shaping using aLCoS-SLM at FLUTE at KIT. Here, a commercially avail-able Hamamatsu LCoS-SLM X13138-02 optimized for awavelength range of 800 nm ± 50 nm with a resolution of1024×1272 pixels was used. In addition, a first integration ofan SLM into the photoinjection path of FLUTE was realized.We present the first transversely modulated photoinjectionpulses at FLUTE and the resulting double electron beam.PULSE SHAPING TEST SETUPSTransverse ModulationAs schematically shown in Fig. 2a, the transversal testsetup was realized by using a widened linearly polarizedred alignment laser of 638 nm wavelength to allow simpleinitial optimization of the CGH calculations. Despite thewavelength mismatch in terms of optimal use, the SLM13th Int. Particle Acc. Conf. IPAC2022, Bangkok, Thailand JACoW PublishingISBN: 978-3-95450-227-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2022-TUPOPT068TUPOPT068ContentfromthisworkmaybeusedunderthetermsoftheCCBY4.0licence(©2022).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI1174MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT25: LasersFigure 2: (a) Schematic drawing of the transverse pulse modulation test setup at FLUTE. The laser light (here 638 nm)is modulated by the SLM using a calculated hologram. Afterwards it is displayed as an amplitude image via a Fouriertransform in the focal plane of a converging lens. (b) Amplitude image of the ’KIT’ logo with clearly visible zero-orderbeam (unmodulated light). (c) Blazed grating phase mask and (d) resulting splitting into the different diffraction orders. (e)Amplitude image of the ’KIT’ logo with suppressed zero-order beam by superpositioning the phase mask from (c) and onlyusing the first diffraction order of (d).Figure 3: (a) Schematic drawing of the longitudinal pulse modulation test setup at FLUTE. The incident laser pulse is firstsplit into its spectral components via a transmission grating and then imaged by a cylindrical lens onto the SLM. Aftermodulating the imaged line spectrum, the pulse is spectrally reassembled and measured. (b) Measurement result (blue withuncertainty band) of generated spectral top-hat pulse shape. The unmodulated pulse is shown in green, the difference in red.still functions acceptably in the visible red spectrum. Aftercoupling the laser light onto the SLM surface, a CGH wascalculated with an in-house developed Python program usingthe GS algorithm and applied to the device. Subsequently,the reflected beam was sent through a lens, here acting asa Fourier lens, resulting in an amplitude image in the focalplane. By this, arbitrary manipulations in the image planecan be achieved, for example a picture of the ’KIT’-logo(Fig. 2b).In order to eliminate the strong unmodulated light com-ponent (also called zero-order beam) visible in the mid-dle of the amplitude image generated in this way, differ-ent approaches [8] were tested. One possibility is to use ablazed-grating phase mask (Fig. 2c) and consequently splitthe light into different diffraction orders (Fig. 2d). Applyingthe superposition-principle, i.e. adding up the CGH fromFig. 2a and the grating from Fig. 2c would result in multiplecopies of the image, one for each diffraction order. Sinceonly the zeroth diffraction order contains unmodulated com-ponents, these can be successfully suppressed by filtering ahigher order (Fig. 2e).Longitudinal ModulationA longitudinal modulation of the laser pulse shape witha phase-only SLM can be achieved via a modulation of thedifferent spectral components of the laser pulse. To do this,the spectral components are imaged on the pixels of theSLM. For the longitudinal tests at FLUTE we used the setupillustrated in Fig. 3a. The ultra-short laser pulse is spatiallysplit in its spectral components with a transmission grating.The spectrum is imaged via a cylindrical lens onto the SLMsurface and can then be modulated individually.The subsequent reassembling of the pulse is realized bya mirrored imaging system, so that all optical componentsform a well known zero-dispersion-4  -system [9]. After-wards, diagnostics like an autocorrelator can be used tomeasure and characterize the temporal properties of themodulated pulse or a spectrometer to analyze the spectralshape. For the ideal arrangement of all components shownin Fig. 3a simulations were performed in advance to achievethe best possible resolution for the available SLM size (notshown here, see [10]).We used this setup to generate a spectral top-hat pulse,as shown in Fig. 3b. The measured unmodulated referencelaser pulse from the FLUTE photoinjection Ti:Sa laser sys-tem (green curve) with a spectral range from 760 nm to13th Int. Particle Acc. Conf. IPAC2022, Bangkok, Thailand JACoW PublishingISBN: 978-3-95450-227-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2022-TUPOPT068MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT25: LasersTUPOPT0681175ContentfromthisworkmaybeusedunderthetermsoftheCCBY4.0licence(©2022).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI830 nm was modulated using a blazed grating phase maskwith horizontal blaze lines, leading to a diffraction in verticaldirection. Vertical stripes with the horizontal grating lineson the SLM allow the control of each wavelength individ-ually. By looking at a side order, one can not only avoidinterfering unmodulated (zero-order beam) parts, but alsoregulate the intensity of the spectral components by choos-ing the appropriate blaze angle. Thus, the frequency rangeof interest can be attenuated and for example fringe regionsof the spectrum can be eliminated. The next step planned isto move from a spectral top-hat pulse shape to a temporalone. To do this, the spectrum must be modulated with a sincfunction, which corresponds to the Fourier transform of thetop-hat function from time to the frequency domain.PHOTOINJECTION PULSE MODULATIONRecently, initial experiments on modulation of photoinjec-tion pulses have been performed at FLUTE. For this purpose,an LCoS-SLM was integrated into the beam path using apair of flip mirrors to allow easy switching between beamoperation with and without SLM. In this setup, the SLM wasplaced in the beam path to the electron gun approximately40 m before the photocathode surface.For the first photoinjection pulse modulation experiments,a blazed grating phase mask was used again. Here the pro-grammed grating properties, like grating constant and blazeangle, were chosen in such a way that the zeroth and firstdiffraction order could both be observed on the cathodesurface with a similar intensity and a separation in the sub-millimeter range. After modulation, the effects on the result-ing electron beam were observed.The results of the experiment are shown in Fig. 4 as false-color images of the laser intensity on the cathode surface(left column) and the electron density on a YAG screen (rightcolumn), respectively. The screen for electron detection isplaced at the end of the beamline at about 3 m after the cath-ode. As expected, a division of the laser spot into 2 focalpoints could be observed on the cathode. As a guide to theeye, a dashed white line was drawn through the referencespot to show the left-shifted modulated first diffraction or-der. Since the overall intensity was not changed, the weakerpulses can be explained by the intensity splitting as well asmodulation losses.As a direct consequence of the two laser pulses on the cath-ode, the generation of two co-propagating electron beamswas observed (Fig. 4 bottom right). Similar to the laser in-tensity, less charge per electron bunch could be detected andthe first order beam was a bit weaker than the zeroth orderbeam. Also noticeable is the vertical offset of the secondbunch, which is due to the off-axis propagation relative tothe solenoid’s magnetic axis. This effect is to be expected ifthe beam axis does not coincide with the magnetic axis [11],and could be used to perform specific and efficient studies onthe alignment of a solenoid magnet in comparable machines.Figure 4: False color images of the photoinjection UV laserpulse on the cathode seen by a camera (left column) andthe resulting electron beam visualized on a YAG screen(right column). In the top row, the unmodulated laser andelectron spot is shown. In the bottom row, the laser pulse wastransversely modulated via a blazed-grating phase mask onthe SLM causing the emergence of a second laser spot (firstdiffraction order), which in the end is resulting in a doubleco-propagating electron beam. Corresponding referencelaser pulses or electron bunches (zeroth diffraction order)were marked with a white dashed line as a guide to the eye.SUMMARY AND OUTLOOKAt the linac-based test facility FLUTE at KIT, two lasershaping setups were realized for transverse and longitudinalpulse modulation. Here, a calculation of CGHs for the gen-eration of arbitrary amplitude images was implemented. Inaddition, measures for the reduction of unmodulated lightcomponents were tested and a spectral top-hat pulse shapecould be successfully generated. Furthermore, a first imple-mentation of an SLM in the photoinjection path at FLUTEwas realized, resulting in the creation of spatially displacedco-propagating electron bunches. This represents a crucialstep for future advanced beam modulation projects to studybunch and machine characteristics, and also paves the way to-wards an ML-supported feedback system including injectionlaser and machine parameters.ACKNOWLEDGEMENTSWe thank E. Bründermann for valuable ideas and com-ments. M. N., J. S. and C. X. acknowledge support by theDoctoral School "Karlsruhe School of Elementary and As-troparticle Physics: Science and Technology" (KSETA).REFERENCES[1] R. Akre et al., "Commissioning the Linac Coherent LightSource injector", Phys. Rev. ST Accel. Beams, vol. 11,13th Int. Particle Acc. Conf. 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IPAC2022, Bangkok, Thailand JACoW PublishingISBN: 978-3-95450-227-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2022-TUPOPT068MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT25: LasersTUPOPT0681177ContentfromthisworkmaybeusedunderthetermsoftheCCBY4.0licence(©2022).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI

Transverse and Longitudinal Modulation of Photoinjection Pulses at FLUTE

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Transverse and Longitudinal Modulation of Photoinjection Pulses at FLUTE

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