On the implementation of the project PN-II-ID-PCE-2011-3-0958 entitled “ Electron acceleration and polaritonic transport in laser-plasma interaction in new capillary geometries” during October 2011 - September 2016

On the implementation of the project PN-II-ID-PCE-2011-3-0958 entitled “ Electron acceleration and polaritonic transport in laser-plasma interaction in new capillary geometries” during January 2015 - December 2015

On the implementation of the project PN-II-ID-PCE-2011-3-0958 entitled “ Electron acceleration and polaritonic transport in laser-plasma interaction in new capillary geometries” during October 2011- December 2014

Introduction

The main interest of the project lies in developing and achieving new capillary optics geometries suited to emphasize new mechanisms that are responsible for the acceleration and propagation of laser accelerated electron beams, with an aim to validate novel, very recent theories [M. Apostol and M. Ganciu, “ Polaritonic pulse and coherent X and gamma rays from Compton (Thomson) backscattering”, selected as Research Highlight by the Journal of Applied Physics in 2011]. The theory is based on the assumption of a polaritonic propagation regime for a bunch of relativistic electrons, spatially and temporally synchronized by means of a very high power laser pulse matched to the plasma parameters.

The basic aspects approached throughout this period have spanned both theoretical and experimental aspects, in an attempt to identify the most relevant experimental issues that guarantee successful achievement of the project objectives. In particular, a special attention was paid to the identification of novel capillary discharge geometries, with an aim to enhance the reproducibility, the energy and the quality of the laser accelerated electron beams.

Theoretical aspects

Starting from a phenomenological analysis pertaining to the interaction between high power laser beams and plasmas whose frequencies are the same as the laser frequency, we might consider that in case of this resonant coupling the group speed is negligible (close to zero) and the electromagnetic energy is rediscovered in a phase plasma oscillation (the phase speed tends to be infinite). Under such conditions the plasma impedance can be considered as infinite, the electromagnetic field is negligible and the energy density associated to the electric field is periodically exchanged into kinetic energy density associated to electrons. Under conditions of ultra-intense electric field, relativistic effects can no longer be discarded, which induces a nonlinear behaviour of such coupling.

We also try to extend such theory for the case of resonant coupling between the laser beam and a bunch of electrons that travel with the same speed as the group speed of the ultra-intense laser beam. Certainly, if we consider that the laser frequency is decreased in the reference system related to the electrons (propagation in the same direction) with 1⁄2γ (relativistic Doppler effect), then the resonance condition is similar to the polaritonic transport condition. Such remark represents the foundation of an investigation performed in 2013, entitled “Coupling of ultra–relativistic atomic nuclei with photons”, that extends our hypotheses to ultra-relativistic ion bunches that interact with laser pulses (with petawatt – PW powers) which propagate from the opposite direction, thus demonstrating that photonuclear reactions can be induced in accelerated nuclei (resulting out of completely ionized atoms) at energies over 1 TeV / nucleon. Thus a symbiosis can be achieved between LHC and ELI-NP, that enables verifying some physical phenomena characteristic to ultrahigh fields in the reference system of particles accelerated at ultra-relativistic energies, corresponding to laser pulse intensities multiplied by ratios larger than (2γ)². A simple numerical estimation for energies around 2 TeV / nucleon, corresponding to a value γ ˜ 2000, for laser intensities around 10²² W/cm² (achievable using PW power lasers), leads to intensity values higher that 1.6 x 10²⁹ W/cm² in the reference system of the nucleus. The same reasoning stands for the case of the polariton, for electron energies around 1 GeV. Our article based on such suggestion has been published [M.Apostol and M. Ganciu, Coupling of (ultra–) relativistic atomic nuclei with photons, AIP Advances 3, 112133 (2013); The perspectives it opens are described in the paper “ELI–NP et le LHC: une combinaison détonante”.

Experimental and technological aspects

Polaritonic transport is associated to a relativistic filamentation process that strongly depends on the initial conditions. Such propagation conditions can be mastered for relatively long distances in the centimeter range (on short and medium term for tenths of cm range) within capillary structures, by inducing a suited plasma profile along the laser pulse propagation duration [S. Abuazoum, S. M. Wiggins, R. C. Issac, G. H. Welsh, G. Vieux, M. Ganciu, D. A. Jaroszynski, Rev Scient. Instrum. 82 (2011) 063505]. Several major issues have been identified in establishing such initial propagation conditions, which we have approached with an aim to overcome them. One of them is related to the non-uniformities that result owing to laser processing of semi-cylindrical channels within sapphire boards that are joined together, with an aim to obtain a capillary type which is used in a large number of laboratories. One of the methods we used to reduce (diminish) such non-uniformities consisted in the deposition of ultra-adherent dielectric thin layers. Preliminary tests were performed using oxidized aluminum targets which simulate the interface with the sapphire (Al₂O₃), by using an original method we have developed in our laboratory, based on Corona discharges and liquid organic precursors of polydimethylsiloxane type. These results have already been published by our group in 2012 [A. Groza, A. Surmeian, C. Diplaşu, C. Luculescu, P. Chapon, A. Tempez, M. Ganciu, Surface and Coatings Technology 212, 145-151 (2012)]. We have also investigated capillary discharges in cylindrical glass structures. A conditioning of the inner surface of the tube in an Ar discharge with a pressure range of tens of Torr was necessary. A discharge configuration was achieved, characterized by a superposition of multiple pulses which enables obtaining reproducible triggering with a temporal precision around 1 ns.

Within the project we have assumed that a strong longitudinal density gradient is required in order to facilitate superposition between the laser pulse and the electron bunch. Such an effect can be obtained through interaction with nanometric structures arranged along the capillary centre. Starting from the expertise of our group in trapping micro and nanoparticles confined in electrodynamic traps, we have discovered a method and a setup that enables trapping of such particles in capillaries, using a suitable adapted configuration. Both the method and setup are described in a patent application submitted to OSIM, number A/00646 from 07. 09. 2012, authors O. Stoican, A. Groza and M. Ganciu. The technical problem solved by this approach consists in confining a solid particle or an ensemble of such particles with micrometer dimensions, in a well-defined and fixed position from space, within an area delimited by a completely closed or partially opened surface. The particle or particles are levitated in a region of space for some period of time, without the need of a sustaining solid component. The system we realized to solve this problem can be used as a component for instruments, setups and equipment intended for investigating, manipulating, measuring or determining the characteristics and physico-chemical properties of micro and nanoparticles, as well as studying the effect of fields of force or external radiation sources upon these. We estimate that the trapping phenomenon enhances the probability of interaction with nanometric particles, of interest not only for the study of establishing the polaritonic transport regime, but also as a new method to generate ion beams with important advantages regarding the process of positioning and changing of targets.

The plasma configuration at the output of the capillary exhibits geometries and densities similar to those of plasmas used for GD-OES. As a matter of fact, the aggressive conditions for such plasmas can be tested by means of GD plasma interaction with different dielectric or metallic targets that simulate both the capillary input conditions and the metallic part of conical electrodes.

An important result of our progress lies in the study and control of filamentary discharges as a specific medium for laser acceleration of electrons, as the European Space Agency (ESA) was interested in our suggestion to test space components using laser accelerated particles. Owing to the exponential energy distribution which is easy to obtain when charged particles are accelerated by means of high power lasers, the aggressive environment characteristic to the (van Allen) radiation belts can be simulated.

A niche of interest was protected through a OSIM patent application [Mihai Ganciu–Petcu, Marius Ioan Piso, Ovidiu Stoican, Octav Marghitu, Răzvan Dabu, Agavni Surmeian, Andreea Julea, Andreea Groza, Constantin Diplaşu, Ion Morjan, Procedeu de testare pentru componente şi sisteme complexe în fluxuri pulsate şi sincronizate de particule accelerate laser, Romanian Patent Application, A/00643, 28. 08. 2013]. As a result of the interest expressed by ESA and NASA, our group has submitted a project for a Competence Centre within our institute, focused on establishing competences and developing novel applications for laser accelerated electron beams in order to perform investigations related to radiation hardness testing of critical space components and systems developed for space missions. The project entitled “Laser–Plasma Acceleration of Particles for Radiation Hardness Testing — LEOPARD”, was financed by the Romanian Space Agency — ROSA.

It has also resulted in several visits of ESA experts to INFLPR, as both parties are interested to extend this new method for the “Radiation Hardness Assurance” program, an important step for the Jupiter Icy Moon Explorer (JUICE) space mission. Significant progress was made on the issue of discharge synchronization in a capillary structure with pre-ionization, which is achieved by means of a generator similar to the one we use to dissociate molecules using high voltage, ultrafast impulses. Such a device was previously realized following a cooperation with French partners and it has resulted in a precise application. The device was adapted for use in synchronized pre-ionization of capillary discharges, of interest for laser acceleration of charged particles. The experimental setup is ready for mounting in the reaction chamber from the CETAL bunker.

Recently, we have also focused on a subject of large impact for our research, with an aim to achieve capillaries with controlled profile and with an inner rugosity value as low as possible. An experiment to achieve such capillaries using the 1 kW CO2 laser from CETAL by means of an auto collimation process within a special optical material was performed.

Another progress we have achieved consists in the submission of an international patent application PCT/RO2104/000022 (System and method for testing components, circuits and complex systems using synchronized and pulsed fluxes consisting of laser accelerated particles, M. Ganciu, M. I. Piso, O.Stoican, B. Mihalcea, C. Diplaşu, O. Marghitu, A. Groza, A. Surmeian, A. Julea, R. Dabu, I. Morjan). As an outcome of it, we have been able to submit a proposal to the European Space Agency (ESA) for a feasibility study on using the CETAL laser in order to generate electrons at hundreds of MeV energies, under an exponential energy distribution. The interest lies in: (1) reproducing the aggressive radiation environment characteristic to the Jovian system and, (2) developing a test and calibration facility in Romania for space components and systems intended to be used for space missions, such as Jupiter Icy Moon Explorer (JUICE).

Project implementation phase at CETAL

The 20 TW laser TEWALAS, located in NILPRP, cannot be used yet to validate our hypotheses due to lack of radiation (radiological) protection for power densities exceeding 10¹⁷ W/cm². As a consequence, we have focused on implementing our experiments using the 1 PW laser from CETAL, able to deliver power values of 45 TW at 10 Hz and 1 PW at 0.1 Hz. The configuration of the experimental zone guarantees an adequate radiation protection, and we consider it to be the best with respect to other facilities that use very high power lasers. Therefore we consider performing complex experiments such as generation and detection of muons and pions. Such a special configuration to satisfy the radiation protection requirements has imposed a complex beam line transport solution.

The location of the compressor in the vicinity of the laser imposes the existence of high vacuum (10^-6 Torr) along the transport area in order to avoid power losses and laser pulse distortions. Setting up the whole experimental setup implies step by step tests and careful monitoring of the possible sources of impurity which might affect the optical quality of the surfaces of the mirrors. Currently we are in an advanced position towards solving such problems. We estimate the first experiments of laser acceleration of electrons at CETAL will be performed in the first semester of the year 2015, when undergoing tests regarding setting up the experimental setup under complex specific conditions characteristic to very high power laser facilities will be finalized. These extremely critical issues have motivated us in pursuing our research objectives to deposit barrier layers and adherence layers on surfaces of interest for the optics of very high power lasers [C. Diplaşu, Romanian Reports in Physics 66, 737 (2014)] and those of atmospheric pressure technologies, respectively, [A. Groza, A. Surmeian, C. Diplaşu, C. Luculescu, C. Negrilă, and M. Ganciu, Journal of Nanomaterials, Article ID 578720 (2014)].

Conclusions