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  • 1. Björklund Svensson, Jonas
    et al.
    Guénot, Diego
    Ferri, Julien
    Ekerfelt, Henrik
    Gallardo González, Isabel
    Persson, Anders
    Svendsen, Kristoffer
    Veisz, Laszlo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundh, Olle
    Low-divergence femtosecond X-ray pulses from a passive plasma lens2021In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 17, no 5, p. 639-645Article in journal (Refereed)
    Abstract [en]

    Electron and X-ray beams originating from compact laser-wakefield accelerators have very small source sizes that are typically on the micrometre scale. Therefore, the beam divergences are relatively high, which makes it difficult to preserve their high quality during transport to applications. To improve on this, tremendous efforts have been invested in controlling the divergence of the electron beams, but no mechanism for generating collimated X-ray beams has yet been demonstrated experimentally. Here we propose and realize a scheme where electron bunches undergoing focusing in a dense, passive plasma lens can emit X-ray pulses with divergences approaching the incoherent limit. Compared with conventional betatron emission, the divergence of this so-called plasma lens radiation is reduced by more than an order of magnitude in solid angle, while maintaining a similar number of emitted photons per electron. This X-ray source offers the possibility of producing brilliant and collimated few-femtosecond X-ray pulses for ultra-fast science, in particular for studies based on X-ray diffraction and absorption spectroscopy.Main

  • 2. Buck, A.
    et al.
    Wenz, J.
    Xu, Jiancai
    Khrennikov, K.
    Schmid, K.
    Heigoldt, M.
    Mikhailova, J. M.
    Geissler, M.
    Shen, B.
    Krausz, F.
    Karsch, S.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Shock-Front Injector for High-Quality Laser-Plasma Acceleration2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 18, article id 185006Article in journal (Refereed)
    Abstract [en]

    We report the generation of stable and tunable electron bunches with very low absolute energy spread (ΔE≈5  MeV) accelerated in laser wakefields via injection and trapping at a sharp downward density jump produced by a shock front in a supersonic gas flow. The peak of the highly stable and reproducible electron energy spectrum was tuned over more than 1 order of magnitude, containing a charge of 1–100 pC and a charge per energy interval of more than 10  pC/MeV. Laser-plasma electron acceleration with Ti:sapphire lasers using this novel injection mechanism provides high-quality electron bunches tailored for applications.

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  • 3. Cardenas, D. E.
    et al.
    Ostermayr, T. M.
    Di Lucchio, L.
    Hofmann, L.
    Kling, M. F.
    Gibbon, P.
    Schreiber, J.
    Veisz, Laszlo
    Umeå University, Faculty of Science and Technology, Department of Physics. Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Sub-cycle dynamics in relativistic nanoplasma acceleration2019In: Scientific Reports, E-ISSN 2045-2322, Vol. 9, article id 7321Article in journal (Refereed)
    Abstract [en]

    The interaction of light with nanometer-sized solids provides the means of focusing optical radiation to sub-wavelength spatial scales with associated electric field enhancements offering new opportunities for multifaceted applications. We utilize collective effects in nanoplasmas with sub-two-cycle light pulses of extreme intensity to extend the waveform-dependent electron acceleration regime into the relativistic realm, by using 106 times higher intensity than previous works to date. Through irradiation of nanometric tungsten needles, we obtain multi-MeV energy electron bunches, whose energy and direction can be steered by the combined effect of the induced near-field and the laser field. We identified a two-step mechanism for the electron acceleration: (i) ejection within a sub-half-optical-cycle into the near-field from the target at >TVm−1 acceleration fields, and (ii) subsequent acceleration in vacuum by the intense laser field. Our observations raise the prospect of isolating and controlling relativistic attosecond electron bunches, and pave the way for next generation electron and photon sources.

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  • 4.
    Cardenas, Daniel
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Max-Planck-Institut für Quantenoptik, Garching, Germany; Ludwig-Maximilian-Universität München, Am Coulombwall 1, 85748, Garching, Germany.
    Chou, Shao-Wei
    Umeå University, Faculty of Science and Technology, Department of Physics. Max-Planck-Institut für Quantenoptik, Garching, Germany; Ludwig-Maximilian-Universität München, Am Coulombwall 1, 85748, Garching, Germany; Center for High Energy and High Field Physics, National Central University, Chungli 32001, Taiwan .
    Wallin, Erik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Xu, J.
    Buck, A.
    Schmid, K.
    Rivas, D.E.
    Shen, B.
    Gonoskov, A.
    Marklund, M.
    Veisz, Laszlo
    Umeå University, Faculty of Science and Technology, Department of Physics. Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Electron bunch evolution in laser-wakefield acceleration2020In: Physical Review Accelerators and Beams, E-ISSN 2469-9888, Vol. 23, article id 112803Article in journal (Refereed)
    Abstract [en]

    We report on systematic and high-precision measurements of the evolution of electron beams in a laser-wakefield accelerator (LWFA). Utilizing shock-front injection, a technique providing stable, tunable and high-quality electron bunches, acceleration and deceleration of few-MeV quasimonoenergetic beams were measured with cutting-edge technology sub-5-fs and 8-fs laser pulses. We explain the observations with dephasing, an effect that fundamentally limits the performance of LWFAs. Typical density dependent electron energy evolution with 57–300  μm dephasing length and 6–20 MeV peak energy was observed and is well described by a parabolic fit. This is a promising electron source for time-resolved few-fs electron diffraction.

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  • 5.
    Chadi, Abd Alrahman
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mejean, G.
    Grilli, R.
    Romanini, D.
    Note: Simple and compact piezoelectric mirror actuator with 100 kHz bandwidth, using standard components2013In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 84, no 5, p. 056112-Article in journal (Refereed)
    Abstract [en]

    We propose a mounting scheme to control the displacement of a mirror (or other small object) by a cylindrical piezoelectric actuator, giving uniform response and no phase lag up to high frequencies. This requires a simple ring holder, and unmodified off-the-shelf components. In our implementation, the piezo-mirror assembly has its first mechanical resonance around 120 kHz, close to the resonance for the bare piezo. The idea is to decouple the fundamental elongation mode of the piezo-mirror assembly from the holder by side-clamping the assembly at its zero-displacement plane for this mode. The main drawback is a reduced mirror displacement, by a factor 2 in our case (mirror displacement is similar to 2.5 mu m). Also, the mirror needs to be light with respect to the piezo: still, we use a standard half-inch mirror. The resulting system is very compact as it fits inside a 1-in. commercial steering mirror post.

  • 6.
    Forssén, Clayton
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Characterization of Multi Plate Field Mill for Lunar Deployment2018Independent thesis Advanced level (degree of Master (Two Years)), 300 HE creditsStudent thesis
    Abstract [en]

    During the Apollo 10 and 17 missions NASA astronauts reported that they saw streamers emanating from the surface of the moon. They concluded that the streamers were produced by light scattering from dust particles. The particles are believed to be transported by an ambient electric field. This theorized electric field has never been measured directly, although the electric potential on the surface and above it has. The exact behavior and origin of the electric field is unknown, but has been approximated to be between 1 and 12 V/m. To measure this electrical field a new type of instrument, called Multi Plate Field Mills (MPFM) has been developed. This type of instrument is capable of measuring both the amplitude and directionality of the electrical field. Three of these instruments will be mounted on a 1U CubeSat to be lunched with the PTS mission to the moon scheduled to Q4 2019. In this work the MPFM were characterized. The precision of the instrument for electrical fields applied along the z, y and x axis was found to be 0.6, 1.3, 1.4 (V/m)/(Hz)^(1/2) respectively for measurements in air and 0.14, 0.6, 0.6 (V/m)/(Hz)^(1/2) for measurements in vacuum. This sensitivity outperforms the current state of the art Field Mills and, in addition to that, it provides an assessment of the directionality of the electrical field.

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    Characterization of Multi Plate Field Mill for Lunar Deployment
  • 7. Gonoskov, A.
    et al.
    Wallin, Erik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Polovinkin, A.
    Meyerov, I.
    Employing machine learning for theory validation and identification of experimental conditions in laserplasma physics2019In: Scientific Reports, E-ISSN 2045-2322, Vol. 9, article id 7043Article in journal (Refereed)
    Abstract [en]

    The validation of a theory is commonly based on appealing to clearly distinguishable and describable features in properly reduced experimental data, while the use of ab-initio simulation for interpreting experimental data typically requires complete knowledge about initial conditions and parameters. We here apply the methodology of using machine learning for overcoming these natural limitations. We outline some basic universal ideas and show how we can use them to resolve long-standing theoretical and experimental difficulties in the problem of high-intensity laser-plasma interactions. In particular we show how an artificial neural network can "read" features imprinted in laser-plasma harmonic spectra that are currently analysed with spectral interferometry.

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  • 8. Guillaume, E.
    et al.
    Döpp, A.
    Thaury, C.
    Ta Phuoc, K.
    Lifschitz, A.
    Grittani, G.
    Goddet, J.-P.
    Tafzi, A.
    Chou, S.-W.
    Veisz, László
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Malka, V.
    Electron Rephasing in a Laser-Wakefield Accelerator2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 15, article id 155002Article in journal (Refereed)
    Abstract [en]

    An important limit for energy gain in laser-plasma wakefield accelerators is the dephasing length, after which the electron beam reaches the decelerating region of the wakefield and starts to decelerate. Here, we propose to manipulate the phase of the electron beam in the wakefield, in order to bring the beam back into the accelerating region, hence increasing the final beam energy. This rephasing is operated by placing an upward density step in the beam path. In a first experiment, we demonstrate the principle of this technique using a large energy spread electron beam. Then, we show that it can be used to increase the energy of monoenergetic electron beams by more than 50%.

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  • 9. Götzfried, J.
    et al.
    Döpp, A.
    Gilljohann, M. F.
    Foerster, F. M.
    Ding, H.
    Schindler, S.
    Schilling, G.
    Buck, A.
    Veisz, Laszlo
    Umeå University, Faculty of Science and Technology, Department of Physics. Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Karsch, S.
    Physics of High-Charge Electron Beams in Laser-Plasma Wakefields2020In: Physical Review X, E-ISSN 2160-3308, Vol. 10, no 4, article id 041015Article in journal (Refereed)
    Abstract [en]

    Laser wakefield acceleration (LWFA) and its particle-driven counterpart, particle or plasma wakefield acceleration (PWFA), are commonly treated as separate, though related, branches of high-gradient plasma-based acceleration. However, novel proposed schemes are increasingly residing at the interface of both concepts where the understanding of their interplay becomes crucial. Here, we present a comprehensive study of this regime, which we may term laser-plasma wakefields. Using datasets of hundreds of shots, we demonstrate the influence of beam loading on the spectral shape of electron bunches. Similar results are obtained using both 100-TW-class and few-cycle lasers, highlighting the scale invariance of the involved physical processes. Furthermore, we probe the interplay of dual electron bunches in the same or in two subsequent plasma periods under the influence of beam loading. We show that, with decreasing laser intensity, beam loading transitions to a beam-dominated regime, where the first bunch acts as the main driver of the wakefield. This transition is evidenced experimentally by a varying acceleration of a low-energy witness beam with respect to the charge of a high-energy drive beam in a spatially separate gas target. Our results also present an important step in the development of LWFA using controlled injection in a shock front. The electron beams in this study reach record performance in terms of laser-to-beam energy transfer efficiency (up to 10%), spectral charge density (regularly exceeding 10  pC MeV−1), and angular charge density (beyond 300  pC μsr−1 at 220 MeV). We provide an experimental scaling for the accelerated charge per terawatt (TW) of laser power, which approaches 2 nC at 300 TW. With the expanding availability of petawatt-class (PW) lasers, these beam parameters will become widely accessible. Thus, the physics of laser-plasma wakefields is expected to become increasingly relevant, as it provides new paths toward low-emittance beam generation for future plasma-based colliders or light sources.

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  • 10. Khrennikov, K.
    et al.
    Wenz, J.
    Buck, A.
    Xu, Jiancai
    Heigoldt, M.
    Veisz, Laszlo
    MPI für Quantenoptik, Garching, Germany.
    Karsch, S.
    Tunable All-Optical Quasimonochromatic Thomson X-Ray Sourcein the Nonlinear Regime2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 114, no 19, article id 195003Article in journal (Refereed)
    Abstract [en]

    We present an all-laser-driven, energy-tunable, and quasimonochromatic x-ray source based on Thomson scattering from laser-wakefield-accelerated electrons. One part of the laser beam was used to drive a few-fs bunch of quasimonoenergetic electrons, while the remainder was backscattered off the bunch at weakly relativistic intensity. When the electron energy was tuned from 17–50 MeV, narrow x-ray spectra peaking at 5–42 keV were recorded with high resolution, revealing nonlinear features. We present a large set of measurements showing the stability and practicality of our source.

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  • 11.
    Lindahl, Jonatan
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Designing radiation protection for a linear accelerator: using Monte carlo-simulations2019Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The department of Radiation Sciences at Umeå University has obtained an old linear accelerator, intended for educational purposes. The goal of this thesis was to find proper reinforced radiation protection in an intended bunker (a room with thick concrete walls), to ensure that the radiation outside the bunker falls within acceptable levels. The main method was with the use of Monte Carlo-simulations.

    To properly simulate the accelerator, knowledge of the energy distribution of emitted radiation was needed. For this, a novel method for spectra determination, using several depth dose measurements including off-axis, was developed. A method that shows promising results in finding the spectra when measurements outside the primary beam are included. The found energy spectrum was then used to simulate the accelerator in the intended bunker. The resulting dose distribution was visualized together with 3D CAD-images of the bunker, to easily see in which locations outside the bunker where the dose was high.

    An important finding was that some changes are required to ensure that the public does not receive too high doses of radiation on a public outdoor-area that is located above the bunker. Otherwise, the accelerator is only allowed to be run 1.8 hours per year. A workaround to this problem could be to just plant a thorn bush, covering the dangerous area of radius 3m. After such a measure has been taken, which is assumed in the following results, the focus moves to the radiation that leaks into the accelerator’s intended control room, which is located right outside the bunker’s entrance door.

    The results show that the accelerator is only allowed to be run for a maximum of 6.1 or 3.3 hours per year (depending on the placement of the accelerator in the room), without a specific extra reinforced radiation protection consisting mainly of lead bricks. With the specific extra protection added, the accelerator is allowed to be run 44 or 54 hours per year instead, showing a distinct improvement. However, the dose rate to the control room was still quite high, 13.7 μGy/h or 11.2 μGy/h, compared to the average dose received by someone living in Sweden, which is 0.27 μGy/h. Therefore, further measures are recommended. This is however a worst case scenario, since the leakage spectrum from the accelerator itself was simulated as having the same energy spectrum as the primarybeam at 0.1 % of the intensity, which is the maximum leakage dose according to the specifications for the accelerator. This is probably an overestimation of the intensity. Also, the energy spectra of the leakage is probably of lower energy than the primary beam in at least some directions. Implementing more knowledge of the leak spectra in future work, should therefore result in more allowed run hours for the accelerator.

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    Exjobb_JonatanLindahl
  • 12.
    Marklund, Mattias
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ilderton, Anton
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundin, Joakim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Probing new physics using high-intensity laser systems2011In: DIODE-PUMPED HIGH ENERGY AND HIGH POWER LASERS ELI: ULTRARELATIVISTIC LASER-MATTER INTERACTIONS AND PETAWATT PHOTONICS AND HIPER: THE EUROPEAN PATHWAY TO LASER ENERGY / [ed] Hein, J; Silva, LO; Korn, G; Gizzi, LA; Edwards, C, SPIE - International Society for Optical Engineering, 2011, article id 80801HConference paper (Refereed)
    Abstract [en]

    Current high-intensity laser sources offer a multitude of research, experiment and application possibilities, ranging from e. g. ionisation studies of atomic and molecular systems to particle acceleration for medical purposes. Planned upgrades of existing laser sources will further increase the deliverable intensities and make certain low-intensity (as compared to the Schwinger field) tests of quantum electrodynamics viable. Moreover, secondary sources of radiation, and planned future facilities, offer several-orders-of-magnitude increases in intensities. Thus, it is highly relevant to ask what kind of physics that may be probed using future light sources.

  • 13.
    Nilsson, Adam
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Measuring Athletic Performance UsingAccelerometers2023Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    With help from a 3D-camera Photon Sports has developed reliable and easy-to-use technology tomeasure and give feedback to athletes during performance tests. This technology makes it possibleto directly extract a number of parameters Key Performance Indicators (KPIs).In most field sports, multidirectional acceleration and deceleration are key areas to improveto become quicker as an athlete. However, it has become clear that only using the 3D-camera hassome limitations. This project aims to investigate if accelerometers can provide us with this extrainformation and thereby complement measurement technology to the measurements PhotonSports are doing today.We have done a three-part project where we first focused on time synchronization betweenclocks from the 3D-camera and the accelerometers. In the second part, we investigated how wellwe can extract the ground contact time (gct) during sprint using accelerometers attached to thefeet of the athlete. In the third and last part, we studied how we can combine both the 3D-cameraand the accelerometers to extract new KPIs that we were unable to extract before.From our developed time-synchronization algorithm, we found that the result is promisingif one looks at how the measured acceleration data from both devices are matching each othervisualy, however, it does not agree with the reference data we have extracted using a Software-Development-Kit (SDK) and Photon Sports already developed application. The gct was evaluatedto values in most cases either ranging between 100 ms and 200 ms or 300 ms and 400 ms. Whencomparing to earlier studies it seems like gct in the 100−200 ms interval are closer to the correctvalue whilst there has been some error in the calculations in the later interval. By combiningthe camera and the accelerometers we were able to extract leg stiffness and reactivity index as newKPIs. The KPIswere evaluated to a reasonable value, however, it became apparent both fromcalculationsof the newKPIs and the gct that the lowsampling rate of the camera and the accelerometersare an obstacle to reliably compute these parameters. We could therefor conclude that if PhotonSports want to introduce new tests where accelerometers are used they should be aware of the limitationsthat comes with i low sampling rate and be sure they try to compute KPIs that are possibleto evaluate with the sampling rate they have.

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  • 14.
    Persson, Leif
    et al.
    Umeå University, Faculty of Science and Technology, Department of Mathematics and Mathematical Statistics. Swedish Defence Research Agency, FOI CBRN Defence and Security, SE-901 82 Umeå, Sweden.
    Boso, Jonas
    Nylén, Torbjörn
    Ramebäck, Henrik
    Application of a Monte Carlo method to the uncertainty assessment in in situ gamma-ray spectrometry2018In: Journal of Environmental Radioactivity, ISSN 0265-931X, E-ISSN 1879-1700, Vol. 187, p. 1-7Article in journal (Refereed)
    Abstract [en]

    In situ gamma-ray spectrometry has since the introduction of portable germanium detectors been a widely used method for the assessment of radionuclide ground deposition activity levels. It is, however, a method that is most often associated with fairly large and, more important, poorly known combined measurement uncertainties. In this work an uncertainty analysis of in situ gamma ray spectrometry in accordance with the Guide to the Expression of Uncertainty in Measurements is presented. The uncertainty analysis takes into account uncertainty contributions from the calibration of the detector system, the assumed activity distribution in soil, soil density, detector height and air density. As a result, measurement results from in situ gamma spectrometry will serve as a better basis for decision-making in e.g. radiological emergencies.

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