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The 329859 NOVEXSOURCE project aimed at developing a novel, table-top, ultra-fast hard X-ray coherent light source, by utilising a compact, high power laser source. The main concept behind the project was to use the same laser system, so that any electronic jitter issue would be excluded and reduced to fsec optical jitter level, to firstly generate relativistic electrons via the Laser Wakefield Acceleration mechanism (LWFA), subsequently generate High Harmonic radiation (HHG) and eventually scatter the HHG radiation of the relativistic electron beam, where via Thomson upshifting hard X-ray photons would be created. LWFA involves the use of ultra-intense and powerful lasers so that highly non-linear electron density wakes are created from the interaction of the laser pulse with sufficiently underdense gas targets. Due to this non-linearity, background electrons can be trapped in the accelerating structure of the wakes and gain relativistic energies with the current record reaching 4.2 GeV with <10% energy spread. On the other hand, HHG requires much lower laser intensities to the point the ionisation of the medium is under certain conditions not favourable and the use of different geometry but still underdense gas targets. By scattering the HHG photon beam from the relativistic electron beams, Thomson upshifting of the photon energy occurs, albeit at small scattered photon numbers, reaching the multi keV energy regime.
The NOVELXSOURCE project was driven by the intense scientific and industrial interest shown in the past decade in compactisation of novel particle accelerators and tunable light sources at the higher end of the energy spectrum. This quest for table-top particle and light sources has greatly benefited from parallel technological breakthroughs in laser development manifested in the form of commercialisation of multi 10s TW laser systems, albeit at <100 Hz repetition rates. The novelty of this project, which in parallel was one of the greatest challenges faced, was the demonstration of relativistic electron generation with a <TW, ultrafast laser system operating at the 1 kHz repetition rate level and the simultaneous use of the same system to create the photon sources. The main objectives of the project were:

I) controlled generation of an ionisation induced relativistic LWFA electron beam
ii) controlled generation of a HHG beam of optimum quality using the same laser system
iii) Thomson upshifting to higher photon energies of the HHG beam off the LFWA beam, which will be a first demonstration of the potential offered for an ultra-compact, ultra-fast novel hard X-ray sources.

Given the great complexity in the experimental setup, the Thomson upshifting part of the project has not yet been completed and the demonstration of such a photon source remains top-priority.

A major part of the project was devoted in developing the primary photon source, i.e. the laser system to be used for LWFA electron acceleration, HHG photon generation and eventually Thomson upshifting. In collaboration with Dr. Mecseki (a post-doctoral associate in the Plasma Physics Group at Imperial College) partial redesign and rebuild was performed and overall improvement of the final energy output, pulse duration and stability of the system was achieved. A major issue was the diode pumped high repetition rate amplifier, which due to the kHz pumping of the medium did not allow enough energy to be extracted and pump the Optical Parametric Amplifer (OPA) stages. Once this issue was realised, the laser system had been redesigned to accommodate 3 flash-lamp pumped YLF rod amplifiers, albeit at reduced operational repetition rate (10 Hz). A major breakthrough was achieved where by the use of different doubling crystals in the final OPA stage in a multipass geometry, laser pulses of 3 mJ, <9 fs were delivered at the output of the system (publication in preparation). Should the diode pump amplifier section be replaced by state-of-the-art high repetition rate modules, a 1 kHz OPCPA, few mJ, <10 fs laser system can be realised.

For the proposed experimental objectives, the following actions were performed. A new ultra compact 0.53 m3), stainless steel, vacuum target chamber, with all the necessary feedthroughs (gas lines and computer control lines) to accommodate both LWFA and HHG experiments was designed and constructed. For the same purpose, innovative gas jet and gas cell targets designs were implemented in collaboration with the JAI group at Imperial College, while for harmonic photon detection a new flat-field grating spectrometer was assembled.
The gas cell targets’ and electron diagnostics’ performance was tested in experiments performed on the Astra Facility at Central Laser facility, at the Rutherford Appleton Laboratory. Refined versions of both the cell and diagnostics were fielded on the 300 TW Astra Gemini laser, which allowed consistent 2 GeV electron beams to be produced, achieving a European record for LWFA produced electrons, with parallel unprecedented imaging results of large biological samples (publication in preparation).
The experiments performed at Imperial College involving the OPCPA <TW laser system produced inconclusive results with a small signal-to-noise ratio as the electron charge of the beams is expected to be very low (~fC). However, extensive 2D and 3D modelling of the LWFA process, using the Particle-In-Cell code EPOCH at Imperial College’s HPC facilities, coupled with home built MATLAB visualisation routines, suggest that with the experimental conditions the electrons should gain energies up to 20 MeV albeit at the expected ~fC charge level (publication in preparation). The experiments and simulations are performed based on the ionisation induced injection method, which utilises high Z gas as the target, whereby the lower ionisation states provide the background plasma electrons and the higher ionisation states provide the accelerated electrons (hence the low accelerated charge). This method greatly reduces the laser intensity threshold for the generation of relativistic electron beams allowing table-top electron accelerators with sub-TW laser systems. The experimental results are subject to further analysis involving advanced algorithm development for weak signal extraction from low signal-to-noise ratio datasets.

By the completion of the project three main achievements can be identified. One is the realisation of a ~kHz, ~mJ, <10 fsec, OPCPA compact systems, second is the demonstration in simulation that such a <0.5 TW laser systems can be used as a compact accelerator, and the third was contribution to development of the targetry that enabled achieving a stable >2 GeV electron, which is the current European record for LWFA produced electron beams. Further to this laser work, and through participation in International Conferences (CLEO 2015), new contacts with European laser developers (VENTEON, CLASS 5 PHOTONICS) have been established for future R&D projects such as furthering the energy output of these systems based on new amplifier technologies (thin disk amplifiers, fibre amplifiers etc). Moreover, through a number of invited seminars (ROleMAK), new communication lines have been opened with the future users of these novel particle and light sources, such researchers in Material Sciences for future material probing at the hard X-ray, fsec regime.

The area of photonics with its socio-economic impact and cross-industrial innovation potential has been recognised by the EU by being included in Europe's Key Enabling Technologies (KETs) of the 21st Century. Numerous EU funded networks have been formed during the past years promoting the dissemination of lasers and next generation light and particle sources in industry and fundamental research (Laserlab-Europe, EUCALL, EuCARD2, EuroNNAC2, EuPRAXIA). Furthermore, the importance placed on laser based particle and photon sources is highlighted by the decision of the EU to incorporate in the 2006 ESFRI roadmap, and proceed to their full implementation, two large scale projects aiming at creating the next generation of these sources, the European XFEL and Extreme Light Infrastructure (ELI).

As such, the NOVELXSOURCE project managed to contribute to advancements in the field of laser photonics as well as in the field of next generation particle and photon sources. The closer cooperation between industry and academia will lead to a timely realisation of such sources, which in turn will lead to new cost-effective and efficient photonics and photonics based technologies, being one of the promising routes which the EU can benefit from by creating new markets and hence new job opportunities in the research and industrial sector.

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