On the design of experiments for the study of extreme field limits in the interaction of laser with ultrarelativistic electron beam

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Abstract

We propose the experiments on the collision of laser light and high intensity electromagnetic pulses generated by relativistic flying mirrors, with electron bunches produced by a conventional accelerator and with laser wake field accelerated electrons for studying extreme field limits in the nonlinear interaction of electromagnetic waves. The regimes of dominant radiation reaction, which completely changes the electromagnetic wave–matter interaction, will be revealed in the laser plasma experiments. This will result in a new powerful source of ultra short high brightness gamma-ray pulses. A possibility of the demonstration of the electron–positron pair creation in vacuum in a multi-photon processes can be realized. This will allow modeling under terrestrial laboratory conditions neutron star magnetospheres, cosmological gamma ray bursts and the Leptonic Era of the Universe.

Introduction

According to a widely accepted point of view, classical and quantum electrodynamics represent well understood and fully complete areas of science whereas one of the forefronts of fundamental physics is in string theory, the predictions of which will hopefully bring experimental physics to novel, higher than ever, levels. In the second half of the 20th century and in the beginning of the 21st century humankind witnessed enormous progress in elementary particle physics with the formulation and experimental proof of the Standard Model and in cosmology with the observational evidence of dark matter and dark energy in the Universe. At the same time classical physics continued its vigorous development, which resulted in the understanding of the nature of chaos in simple mechanical systems with its relation to the turbulence problem and in the achievements in nonlinear wave theory, which led to the formation of a novel area in mathematical physics, thus demonstrating that there cannot be a fully complete area of science, as known by prominent physists [1]. In the beginning of the 21st century there appeared a demand to understand the cooperative behavior of relativistic quantum systems and to probe the quantum vacuum. It was realized that the vacuum probing becomes possible by using high power lasers [2]. With increase in the laser intensity, we shall encounter novel physical processes such as the radiation reaction dominated regimes and then the regime of the quantum electrodynamics (QED) processes. Near the intensity, corresponding to the QED critical electric field, light can generate electron–positron pairs from vacuum and the vacuum begins to act nonlinearly [3].

There are several ways to achieve the higher intensity required for revealing these processes.

One of the methods is based on the simultaneous laser frequency upshifting and pulse compression. These two phenomena were demonstrated within the Relativistic Flying Mirror (RFM) concept, which uses the laser pulse compression, frequency up-shift, and focusing by counter-propagating breaking plasma waves—relativistic parabolic mirrors [4]. In the proof of the principle experiments for this concept a narrow band XUV generation was demonstrated [5] with the high photon number [6].

Another way was demonstrated in large scale experiments [7], where a 50 GeV bunch of electrons from SLAC interacted with a counter-propagating laser pulse of the intensity of approximately 5×1017 W/cm2. Gamma-rays with the energy of 30 GeV produced in the multi-photon Compton scattering then subsequently interacted with the laser light creating the electron–positron pairs. The conclusion on appearance of the gamma-rays with the energy of 30 GeV has been obtained by analysing the spectra of scattered electrons and positrons. The direct evidence of the gamma-ray emission through the spectral measurements, so far, is of particular importance. The direct measurement similar to Ref. [8] would allow the selection of the non-linear processes from the consequent linear photon scattering and, therefore, the quantitative verification of theoretical approaches.

In the present paper we propose table-top experiments on the collision of electromagnetic waves with electron bunch in three configurations, Fig. 1. In the first configuration, Fig. 1a, fast electrons are produced by a compact microtron affording hundred MeV bunches with the duration of picosecond. In the second configuration, Fig. 1b, electron bunches are generated in plasma, where a high-intensity laser pulse excites wake waves accelerating electrons [9]. This laser wake field accelerator (LWFA) uses the so called self-injected electrons, which enter into the accelerating phase of the wake wave due to wave-breaking. LWFA can generate GeV electron bunches with a duration of ten femtoseconds. In both configurations the electron bunch collides with a petawatt power laser pulse. In particular, ideally the spot size of electron beam should be matched to the focused laser spot, the length matched to the Rayleigh length, and relative timing jitter minimized. Electron bunches generated by the LWFA can be optimally synchronized with the counter-propagating petawatt laser pulse. In the third configuration, Fig. 1c, LFWA-generated bunches collide with an extremely intense electromagnetic pulse produced with the Relativistic Flying Mirror (RFM) technique. The RFM reflecting counter-propagating laser pulse upshifts its frequency and shortens its duration due to the double Doppler effect. It can focus the reflected radiation to the spot much smaller than the laser wavelength due to higher reflected radiation frequency [4]. As we discuss later, in these three configurations electron bunches colliding with counter-propagating laser pulses produce gamma-rays via nonlinear Thomson or inverse multiphoton Compton scattering. The third configuration is designed for the most intense interaction, i.e., for paving a way towards extreme field limits in the nonlinear interaction of electromagnetic waves.

The paper is organized as follows. In the second section we present the key dimensionless parameters characterizing the extreme field limits. Then the electromagnetic field parameters required for probing the nonlinear vacuum are briefly discussed. The fourth section contains a consideration of the electron–positron gamma-ray plasma generation via the multi-photon Breit–Wheeler process. Section 5 is devoted to the formulation of possible approaches towards nonlinear vacuum probing and multi-photon electron–positron pair generation with present-day lasers and charged particle accelerator systems. The final section is devoted to conclusions and discussions of the results obtained.

Section snippets

Extreme field limits

Physical systems obey scaling laws, which can also be presented as similarity rules. In the theory of similarity and modeling the key role is played by dimensionless parameters, which characterize the phenomena under consideration [10]. The dimensionless parameters that characterize the high intensity Electromagnetic (EM) wave interaction with matter can also be found in Ref. [11]. The key in the extreme field limit parameters are as follows:

  • 1.

    Normalized dimensionless EM wave amplitudea=eEƛmec2

Electron–positron pair creation from vacuum

Understanding the mechanisms of vacuum breakdown and polarization is challenging for nonlinear quantum field theories and for astrophysics [28]. Reaching the Schwinger field limit under Earth-like conditions has been attracting a great attention for a number of years. Demonstration of the processes associated with the effects of nonlinear QED will be one of the main challenges for extreme high power laser physics [2].

Vacuum nonlinearity is characterized by the normalized Poincare invariantsf=FES

Approaching the Schwinger field limit

The concept of the flying mirror has been formulated in Ref. [4] as a way for approaching the critical QED electric field, the Schwinger field limit. This concept is based on the fact that an EM wave reflected off a moving mirror due to the double Doppler effect undergoes frequency multiplication with the multiplication factor (1+βM)/(1−βM) in the limit βM→1 proportional to the square of the Lorentz factor of the mirror, γM=(1−βM2)−1/2. This makes this effect an attractive basis for a source of

Conclusion

With the concept of the Relativistic Flying Mirror, relatively compact and tunable extremely bright high power sources of ultrashort pulses of x- and gamma-rays become realizable, which will allow for exploring novel physics, for studying the processes of high importance for accelerator physics [50], and for laboratory modeling of processes of key importance for relativistic astrophysics [11], [28]. The experiments in this field will allow modeling under the conditions of a terrestrial

Acknowledgment

We thank G. Dunne, N. Elkina, E. Esarey, D. Habs, T. Heinzl, S. Iso, G. Mourou, M. Murakami, H. Nishimura, H. Ruhl, C. B. Schroeder, T. Tajima, and T. Tauchi for discussions, and appreciate the support from the MEXT of Japan, Grant-in-Aid for Scientific Research (A), 29244065, 2008, and from NSF of USA under Grant no. PHY-0935197.

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