Strong-Field QED Experiments using the BELLA PW Laser Dual Beamlines M. Turner1e1 S. S. Bulanov1e2 C. Benedetti1e3 A. J. Gonsalves1e4 W. P .

2025-05-02 0 0 951.35KB 22 页 10玖币

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Strong-Field QED Experiments using the BELLA PW

Laser Dual Beamlines

M. Turner *1,e1, S. S. Bulanov 1,e2, C. Benedetti 1,e3, A. J. Gonsalves 1,e4, W. P.

Leemans 2,e5, K. Nakamura 1,e6, J. van Tilborg 1,e7, C. B. Schroeder1,e8, C. G. R.

Geddes1,e9, and E. Esarey 1,e10

1Lawrence Berkeley National Laboratory, Berkeley, USA

2Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany

e1marleneturner@lbl.gov

e2sbulanov@lbl.gov

e3cbenedetti@lbl.gov

e4ajgonsalves@lbl.gov

e5wim.leemans@desy.de

e6knakamura@lbl.gov

e7jvantilborg@lbl.gov

e8cbSchroeder@lbl.gov

e9cgrgeddes@lbl.gov

e10ehesarey@lbl.gov

March 2022

Abstract

The Petawatt (PW) laser facility of the Berkeley Lab Laser Accelerator (BELLA) Center has

recently commissioned its second laser pulse transport line. This new beamline can be operated

in parallel with the ﬁrst beamline and enables strong-ﬁeld quantum electrodynamics (SF-QED)

experiments at BELLA. In this paper, we present an overview of the upgraded BELLA PW facility

with a SF-QED experimental layout in which intense laser pulses collide with GeV-class laser-

wakeﬁeld-accelerated electron beams. We present simulation results showing that experiments

will allow the study of laser-particle interactions from the classical to the SF-QED regime with a

nonlinear quantum parameter of up to χ∼2. In addition, we show that experiments will enable

the study and production of GeV-class, mrad-divergence positron beams via the Breit-Wheeler

process.

*corresponding author

arXiv:2210.09214v1 [physics.acc-ph] 17 Oct 2022

1 Introduction

Classical and quantum electrodynamics have been extensively and successfully veriﬁed for almost

all parameter ranges. However, open questions remain for interactions in strong electromagnetic

(EM) ﬁelds [1, 2, 3, 4]. For example, classical electrodynamics overestimates the radiation reaction

(which is what aﬀects the dynamics of a radiating particle ) and allows for the emission of photons

with energy greater than the particle energy, a problem that can be addressed by switching to the

quantum description. Both open questions and potential applications motivate the study of strong

ﬁeld (SF) interactions in experiments to, e.g.:

1. Develop an experimental framework that provides a consistent way to verify theoretical and

simulation predictions from the classical to the quantum electrodynamics (QED) regime, in-

cluding linear and nonlinear eﬀects as well as multi-staged processes typical of SF-QED en-

vironments. Previous experiments either operated in a parameter space where the nonlinear

quantum parameter χwas clearly below 1 [5, 6], or provided a limited set of data [7, 8]. Due

to the increased availability of high power lasers [9], multiple facilities (as detailed later in

this section) are planning experiments to reach χ > 1 by using higher laser intensities, more

energetic particle beams, and higher repetition rate lasers.

2. Evaluate whether strong-ﬁeld interactions may provide competitive γ-ray or positron sources [3].

Strong EM ﬁelds may be used to produce high-ﬂux γ-rays (see, e.g., Refs. [10, 11, 12]) and

low divergence positron sources [13, 14]. Positron sources are possible bottlenecks for future

TeV-class lepton colliders [15]. Understanding whether strong EM ﬁelds and QED eﬀects

can generate sources that compete with those used in conventional accelerators [16] is a high

priority for the high energy physics community.

The basic building blocks of SF-QED are the Compton eﬀect (photon emission by an electron)

and the Breit-Wheeler eﬀect (photon decay into an electron-positron pair) in strong EM ﬁelds [17].

It is most convenient to characterize these interactions in terms of Lorentz invariant parameters:

F= (E2−c2B2)/E2

cr,(1)

G=cB·E/E2

cr,(2)

χe=γq(E+v×B)2−(E·v/c)2/Ecr,(3)

χγ= (~ω/mc2)q(E+(c2k/ω)×B)2−(E·(ck/ω))2/Ecr,(4)

where cis the speed of light, ~is the Planck constant, and mis the electron mass. Here, Eand B

are the electric and magnetic ﬁelds, respectively, and Fand Gare the Poincar´

e invariants of the

EM ﬁeld [18]. The particle momentum is deﬁned as pµ=γm(c, v), where γ= (1 −v2/c2)−1/2and

vis the particle velocity. The photon momentum is deﬁned as ~kµ= (~ω/c)(1,n), where ωis the

photon frequency, nits propagation direction and the photon is on-shell (kµkµ= 0). Whereas F

and Gcharacterize the ﬁelds itself, χeand χγcharacterize the interaction of charged particles (e.g.,

electrons) and photons, respectively, with the strong ﬁelds (we recall that an EM ﬁeld is considered

strong when it is of the order of the QED critical ﬁeld [18, 19, 20], Ecrit = 1.32 ×1018 V/m or

Bcrit = 4.41 ×109T). All above deﬁned Lorentz invariant parameters are normalized to Ecr, which

provides a natural scale for the onset of quantum eﬀects in the electromagnetic interactions (i.e.,

when F,G,χe,χγ∼1).

Strong EM ﬁelds can be found in diﬀerent environments, including in close proximity of com-

pact astrophysical objects (such as magnetars and black holes) [21, 22], high-Z nuclei [23], dense

particle beams (at the interaction point of high energy particle accelerators) [24], aligned crystals

[25], and in the foci of high power lasers [9]. Some of these environments provide ﬁelds of the order

of the critical strength, but are not accessible in any laboratory in the foreseeable future. Others

can reach the critical strength in the reference frame of a suﬃciently high energy particle or in

ﬁxed plasma targets. At the current state-of-the-art, laboratory SF-QED experiments will require

an interaction between energetic particles and EM ﬁelds (χe=γE/Ecr).

Previous experiments reached a maximum nonlinear quantum parameter (in the following de-

noted as χe,max) of χe,max ∼0.3 in the E144 experiment at SLAC [5, 6], and χe,max ∼0.2 in the

GEMINI experiment at CLF [7, 8]. Experiments using aligned crystals were reported recently

[25, 26, 27], but require speciﬁc analysis techniques and positron beams. Experiments using par-

ticle colliders are proposed [24], but are inaccessible due to the lack of accelerators with necessary

parameters. Therefore, interactions of electrons with high intensity laser pulses provide the most

promising immediate path to increase χeor χγabove unity.

On that path, SLAC is planning the E320 experiment, and DESY is planning the LUXE experi-

ment [28] using conventionally accelerated 10 or 17.5 GeV electron beams in collision with tens of

TW laser pulses. The University of Michigan ZEUS facility will use two laser pulses (with 2.5 PW

and 0.5 PW), one to accelerate electrons in a laser wakeﬁeld accelerator (LWFA) (to either &10 GeV,

or several GeV) and one to provide the EM ﬁeld (with intensity 1021 W/cm2, or 1023 W/cm2). Other

laser facilities with active SF-QED study programs include J-Karen in Japan, Apollon in France,

CORELS in Korea, CALA in Germany, ELI NP in Romania with interaction chambers with collid-

ing 10 PW laser pulses [29, 30], and ELI BL in Czech Republic, SEL in China [31] (for an expanded

list see Ref. [3] and [32] for PW laser facilities).

In this paper, we assess the potential for SF-QED experiments at the BELLA Center of the

Lawrence Berkeley National Laboratory. The BELLA Center hosts a 1 Hz, petawatt (PW) laser

facility called the BELLA PW, and has recently commissioned a second high-power laser beamline

(2BL) that enables SF-QED experiments. Simulation studies (see Sec. 5) show that experiments

on BELLA PW will allow to investigate a wide range of χereaching immediately up to 2, and

potentially up to 4 after optimizations, which is very attractive at the unique 1 Hz repetition rate

of the laser. Additionally, the BELLA Center experimental teams have many years of experience on

laser operation and laser-driven plasma wakeﬁeld acceleration of electron beams [33, 34, 35, 36,

37].

This paper is organized as follows. Section 2 provides an overview of and general introduction

to the BELLA PW facility, Sec. 3 discusses the two basic SF-QED laser-particle interaction geome-

tries, Sec. 4 provides an overview of experimentally achievable electron beam parameters using the

BELLA PW laser, Sec. 5 discusses the scientiﬁc reach of SF-QED experiments based on simulation

results, Sec. 6 explores experimental layouts at BELLA PW, and Sec. 7 closes with a summary and

the conclusions.

2 BELLA PW Experimental Facility and Dual Beamlines

This section provides an overview of the BELLA PW facility, experimental parameters, and planned

experiments. The facility comprises a petawatt laser system, three laser pulse transport lines (1BL,

2BL, and iP2 beamline) and two experimental target chambers. While all parts will be mentioned

brieﬂy, the focus will be on the components required for SF-QED experiments: the BELLA PW

laser system, ﬁrst and second beamline as well as their target chamber, which are illustrated in

Fig. 1.

The core of the BELLA PW facility is a petawatt-class laser system, which provides uncom-

pressed pulses with a total energy up to 60 J per pulse at 1 Hz repetition rate. Pulses are trans-

ported from the laser table to the ﬁrst target chamber via two pulse transport lines (see Fig. 1)

named ﬁrst (1BL) and second (2BL) beamline. Due to losses in the compressor and beamlines, a

total of ∼40 J of pulse energy (or 1.2 PW of maximum power) is available for experiments in the

target chamber (see location 4 on Fig. 1).

Figure 1: Schematic layout of the BELLA Petawatt dual beamlines for SF-QED experiments.

To send laser light into both beamlines, pulses are split after ampliﬁcation and before com-

pression by a beamsplitter on the laser table (see location [1] in Fig. 1). Pulse energy reﬂected by

the beamsplitter is transported in the ﬁrst beamline, the remaining energy is transmitted through

the beamsplitter and transported in the second beamline. The choice of beamsplitter reﬂectivity

deﬁnes the energy splitting ratio and the ratio can be adjusted by exchanging the optic.

Both ﬁrst and second beamline use a deformable mirror (see location [2] on Fig. 1) for beam

shaping and beam proﬁle optimization and a chirped-pulse-ampliﬁcation compressor (see loca-

tions [3] on Fig. 1) for compression down to lengths of τ∼30-40 fs. The two beamlines share a

Dazzler for spectral pulse shaping before compression. A delay line on the ﬁrst beamline as well

as a motorized stage inside the second beamline compressor allow for the adjustment of timing

between ﬁrst and second beamline pulses.

First beamline was commissioned in 2012 together with the BELLA PW laser and has been op-

erating successfully since. The pulse propagating in the ﬁrst beamline is focused by a 13.5 m focal

length oﬀ-axis parabolic mirror (OAP) (see location [5] on Fig. 1) to a focal spot size of w0=53 µm

inside the target chamber. Second beamline commissioning was completed in 2022 and allows for

several focusing options, including f=6.5, 10.4, 13.5 and 18.0 m OAPs or a ﬂat mirror to transport

the collimated beam (up to a diameter of ∼15 cm) into the target chamber (see location [4] on

Fig. 1). In Fig. 1, the layout using the ﬂat mirror (suitable for SF-QED experiments) is shown.

Pulses with a diameter of up to 15 cm can enter the current target chamber, limited by the

location of the chamber support. A new target chamber is required to allow use of the full aperture

beam (diameter ∼20 cm) and could also be designed to facilitate the challenging particle detection

and required radiation shielding for SF-QED experiments.

Not shown in Fig. 1 is the BELLA PW high intensity laser beamline (iP2) and the iP2 target

chamber that is located downstream the target chamber in Fig. 1, as an extension to the ﬁrst beam-

line. The iP2 extension uses the ﬁrst beamline laser pulse that propagates through the target cham-

ber shown in Fig. 1 and provides a laser focus with an intensity of >1021 W/cm2, using a short-focal

length OAP (f/2.5) and is used, e.g., for solid target experiments. However, the iP2 target chamber

provides access for only one laser pulse and therefore it would require a signiﬁcant eﬀort to devise

an interaction conﬁguration, which is suitable for SF-QED experiments [38].

2.1 Dual Beamline Experiments Planned at BELLA PW

The new dual pulse capabilities of the upgraded BELLA PW facility enable a variety of unique ex-

periments. Construction of the second beamline was motivated by plasma staging experiments [39]

enabling research towards a high-energy physics particle collider at the energy frontier. The goal of

these experiments is to demonstrate at the GeV-level that an electron beam accelerated in a LWFA

stage can be further accelerated in a subsequent LWFA stage with high charge capture and high

beam quality. For that, the ﬁrst (second) beamline will be used to drive a wakeﬁeld in the ﬁrst

(second) plasma stage.

Additionally, the BELLA PW facility is planning to use the dual pulse capabilities for single

stage development, e.g., to optimize energy gain. One idea is to guide drive pulses in conditioned

hydrodynamic optically ﬁeld-ionized plasma channels [40, 41]. These experiments will use the

2BL pulse to optically ﬁeld ionize gas and to heat the plasma, leading to the formation of a plasma

channel via hydrodynamic expansion. The 1BL pulse will then drive high amplitude wakeﬁelds in

that channel, which can be used to accelerate electrons to the &10 GeV-level.

Also enabled by the new second beamline, and a natural follow up to the two experiments

mentioned above, are SF-QED studies, which are the topic of this article. The physics reach of

potential experiments is discussed in Sec. 5 and an experimental layout is proposed in Sec. 6.

3 SF-QED Particle and Field Interaction Geometries

This section discusses the motion of charged particles (here we consider electrons) in laser ﬁelds

and in the presence of SF-QED eﬀects. There are two typical interaction geometries: 1) relativistic

electrons - laser and 2) laser - laser. Though it is well known that geometry 2) requires much

higher laser intensities to observe radiation dominance and quantum eﬀects than geometry 1), it

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Strong-FieldQEDExperimentsusingtheBELLAPWLaserDualBeamlinesM.Turner*1,e1,S.S.Bulanov1,e2,C.Benedetti1,e3,A.J.Gonsalves1,e4,W.P.Leemans2,e5,K.Nakamura1,e6,J.vanTilborg1,e7,C.B.Schroeder1,e8,C.G.R.Geddes1,e9,andE.Esarey1,e101LawrenceBerkeleyNationalLaboratory,Berkeley,USA2DeutschesElektronen-Synchrotr...

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Strong-Field QED Experiments using the BELLA PW Laser Dual Beamlines M. Turner1e1 S. S. Bulanov1e2 C. Benedetti1e3 A. J. Gonsalves1e4 W. P .

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