100 GeV Ascent
Kazuhisa Nakajima received Ph. D in 1981 from the University of Tokyo. In 1981 he was involved in superconducting RF accelerator R&D at Cornell University, USA. Since 1983 he devoted himself to TRISTAN electron-positron collider project on accelerator physics at High Energy Accelerator Research Organization (KEK). In 1996 he initiated research on laser acceleration as Group Leader at Advanced Photon Research Center, Japan Atomic Energy Agency. Thereafter his research has been devoted over 20 years to laser plasma acceleration experiments at high power laser facilities in China, Korea and India as well as in Japan. Since 2013 he is involved in the 100 GeV Ascent laser plasma accelerator project as the project manager at IZEST, Ecole polytechnique. Since 2014 he leads the low-density laser plasma group for research on laser plasma electron acceleration at Center for Relativistic Laser Science, Institute for Basic Science in Korea.
IZEST 100 GeV Ascent project is aimed at implementing acceleration of 100 GeV electron beams by means of a laser plasma accelerator (LPA) driven with a multi-PW laser. In 1979, Tajima and Dawson proposed harnessing electric fields of high amplitude plasma density waves driven by intense laser pulses . They showed that for nearly 100% density modulation, acceleration gradients of electric fields due to the charge separation can exceed 100 GV/m for plasma densities around 1018 cm-3. This reduces the accelerator length by more than three orders of magnitude compared with conventional devices.
Over the last decade, rapid progress has been achieved in realizing useful LPAs with energies up to 4.2 GeV by using 1-10s J lasers with 10s fs pulse duration in the plasma density range of 1017-1019 cm-3, demonstrating quasi-monoenergetic electron beams with charge of 1-100s pC and qualities, e.g., emittances of around 1 mm mrad, energy spreads of 0.3% for 100-200 MeV beams, bunch durations of the order of 1 fs. Recently there is a growing interest in laser plasma accelerators driven by PW-class lasers, whereby high-quality electron beams can be accelerated to multi-GeV energies in a centimeter-scale plasma thanks to laser wakefield acceleration (LWFA) mechanism, as reported so far, e.g. 1.8 GeV driven by 130 TW at SIOM, 2 GeV driven by 620 TW at TEXAS, 3 GeV driven by 210 TW at GIST, and 4.2 GeV driven by 380 TW at LBNL-BELLA.
In the community relevant to LPAs, other different approaches to extend the particle energies are currently being pursued: The BELLA project at LBNL is developing 10 GeV stages based on a 30 – 40 J laser with a 1017 cm-3 LWFA to explore staging as a way to reach much higher energies. This has challenges of matching/aligning many stages to reach high energies. The other project, FACET at SLAC, is developing electron beam-driven plasma wakefield accelerators to produce high-quality electron and positron beams with 10s GeV energies in a meter-scale plasma at 1016 – 1017 cm-3 density driven by 20 GeV high-current e-/e+ beams with 10s fs duration from the 2-km linac. The WAKE program at CERN is planned for producing ~ GeV-level energy gain of externally injected electron beams by means of plasma wakefield generated by 450 GeV self-modulated proton bunches from CERN-SPS proton synchrotron. The availability of kJ-level short pulses is significantly important for pushing forward practical applications of laser plasma accelerators to nuclear and high energy physics in the 10s GeV–1TeV regime. In stark contrast to 10s J lasers with 10s fs pulse duration that are adequate for plasma densities around 1017 - 1018 cm-3 and a single stage energy gain of 1-10 GeV, kJ laser pulses can drive plasma wakes at a density of the order of 1016 cm-3 and provide a single-stage energy gain of up to 100 GeV .
The 100 GeV Ascent project is aimed at extending such an energy trend in the capability of laser plasma accelerators toward the uncharted energy region and mastering beam acceleration technologies by means of multi-PW lasers. To our knowledge on laser-plasma based acceleration capable of producing multi-GeV electron beams with qualified properties, we envisage that the ascent project is getting started on the first milestone of 10 GeV acceleration by exploiting a PW-class laser of 800 nm wavelength at plasma density of the order of 1017cm-3, where the focused laser pulse is self-guided in a gas cell. The second milestone of 40 GeV acceleration may be carried out by means of a 800 nm PW-class laser delivering a 200 J, 240 fs pulse to a 2.3 m long plasma with density of the order of 51016cm-3. A target experiment for reaching 100 GeV will be implemented by employing a large-scale laser system named PETAL at CEA Bordeaux, capable of delivering a 3.5 kJ, 500 fs pulse at wavelength λ=1053 nm. According to the energy gain scaling of self-guided LWFAs in the bubble regime, parameters for the electron acceleration experiment toward 100 GeV can be designed for PETAL as normalized field a0 = 3, operating plasma density 1.21016cm-3, accelerator length 12 m, focused spot radius 110 μm, peak power 2.1 PW and pulse energy 1 kJ . A two-staged approach comprising the injector and accelerator stages will be adopted for separately controlling the production and acceleration of high-quality electron beams to the 100 GeV energy level. In the injector stage, electron beams are injected, relying on the self-injection or the ionization-induced injection mechanism with a short mixed gas cell. The accelerator stage comprises a hollow dielectric capillary tube filled with neutral gas for guiding intense short laser pulses over several meters. Since laser is guided by Fresnel reflection at the inner capillary wall, this method relies on neither laser power nor plasma density. Adjusting the capillary tube radius with respect to the laser spot radius can propagate intense laser pulses with peak intensity of the order of 1020 W/cm2 over a 100-m scale .
To date the most popular PW-class lasers at near-infrared wavelengths for the laser-matter interaction research underpin accelerating multi-GeV high-quality electron beams in a centimeter-scale plasma. The energy scaling is capable of predicting results of GeV-class LPA experiments as shown in Fig. 1. It is obvious that the higher normalized field, and the shorter wavelength λL produce the higher energy gain, which scales as a0λL-2. For a given a0, this is attributed to the longer self-guided length. As a result, the operating plasma density at 351 nm can be increased up to 1.71017cm-3 for a0=4 and 2.41017cm-3 for a0=8, respectively, compared to 1.41016cm-3 for a0=4 at 1053 nm. This increase indicates decreasing the threshold of electron self-injection into laser wakefields as well as a decrease of the critical power for self-guiding. This is a motivation to exploit a UV laser pulse for driving wakefields. Assuming a0=4 (8) and normalized spot radius kprL=2 and the minimum pulse duration 136 fs for 100 GeV acceleration, the required peak power and pulse energy of 351 nm laser is estimated by PL = 2.6 (5.1) PW and UL=350 (700) J, respectively. The accelerator length for reaching 100 GeV is estimated as Lacc=3 (1.5) m.
Accordingly, a UV LWFA concept may make the accelerator length 10 times shorter than laser wakefield accelerators driven by a 1053 nm laser pulse. Further embodiment for designing the experiment at PW laser facilities will be in progress.
Figure. 1 Beam energy scaling for self-guided LPAs and the electron energy plots measured by GeV-class LPA experiments.
Laser driven plasma accelerators have evolved to producing ultra-short pulses of multi-GeV electrons and ≈0.1GeV protons. By virtue of the >100GV/m electric fields attainable in plasma media, GeV energies are reached after only centimeters of acceleration.
A key long-term objective of the IZEST framework is the pursuit of accelerators capable of TeV energies to enable investigations of fundamental physics in succession of traditional RF accelerator technology. The first includes: phase of this ascent is the goal of 100 GeV for laser plasma acceleration of electrons.
The “100 GeV Ascent” project within the IZEST framework has been in development since 2012 and is led by K. Nakajima. Here, the challenges include scaling the plasma length from 10−2 → 10 m and the density from 1018 → 1016 cm−3 together with the laser energy from 10 → 103J to achieve a similar magnitude increase in electron energy. These challenges are to be met by employing a strategy of scaling the technology and science in increasing steps at existing laboratories within the IZEST global network. The current strategy of this ascent :
Through such a staged process of R&D and a succession of experiments at intermediate scale facilities, the science and technology can be established and tested before full demonstration at the largest existing facilites such as PETAL at the Laser MegaJoule.
 T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979).
 K. Nakajima et al., Chinese Optics Letters, 11 (2013) 013501.