Session PO05: Beam-Plasma Wakefield Accelerators

Performance and preliminary results of the AWAKE experiment in the 2021 proton run

In Run 1 (2016-2018), the AWAKE experiment demonstrated that it was possible to accelerate externally-injected electrons to 2 GeV in plasma wakefields driven by a self-modulated proton bunch. In Run 2 (2021-2028+), AWAKE aims to improve even further the accelerating gradient in the plasma, as well as to demonstrate the scalability of the acceleration process and the beam quality and reliability of the accelerated electrons, in order to demonstrate that it is ready for particle physics applications. During the proton run starting in July 2021--Run 2a--we will study several key ingredients of the Run 2 program: (i) the event-to-event phase reproducible self-modulation of the entire proton bunch seeded by a short electron bunch preceding the protons, (ii) the effect of a phase difference between the front and the back of a proton bunch, (iii) the hosing instability at long wavelengths, and (iv) new diagnostics to measure the intensity of plasma wakefields. The performance of the laser, electron and proton beams in the AWAKE experiment, including their diagnostics and data acquisition systems, will be discussed, and preliminary results of the 2021 run will be presented.   

Physics and Experiments for AWAKE Run 2

The AWAKE experiment at CERN is at the beginning of its Run 2. It will focus on seeding of the self-modulation process with a short electron bunch rather than with a relativistic ionization front (Run 1), and on maximizing the amplitude of wakefields along the plasma using a density step [A. Caldwell et al., Phys. Plasmas 18, 103101 (2011)]. For seeding we will use the 20MeV electron bunch of the Run 1 acceleration experiments.  Phase reproducibility measurements will be similar to those of  [F. Batsch et al., Phys. Rev. Lett. 126, 164802 (2021)]. The density step will be imposed through a temperature step along the laser-ionized rubidium vapor source. We will observe the effects of the density step on the beam halo, on the self-modulation, on electron acceleration and on plasma light emission. Acceleration of an externally-injected electron bunch with a plasma for self-modulation, including density step and electron bunch seeding followed by a plasma for acceleration will focus on bunch quality at the multi-GeV energy level. We are also investigate discharge and helicon plasma sources for acceleration over very long distances. We will present the global physics and experimental plan.    

Projected and Slice Emittance Growth in a Plasma-Based Accelerator

Plasma based accelerators are a promising, compact alternative to conventional electron accelerators. These accelerators must preserve the transverse emittance of the beam in order to satisfy the requirements for collider and light source applications. If not carefully designed, a plasma-based accelerator will increase the beam’s emittance through the process of chromatic phase spreading. We present a general approach to analytically describing the transverse evolution of the witness beam within a nonlinear plasma-based accelerator. Our approach includes the effects of density ramps on the plasma source, energy gain, and wake loading. In addition, our method can handle emittance growth from both beam-plasma mismatch and a transverse offset between the witness beam and the wake. Using this approach, we derive a set of saturation lengths for the projected emittance, longitudinal slice emittance, and energy slice emittance. These analytic expressions can help narrow down the design parameters of a plasma-based accelerator before resorting to expensive simulations.   

Emittance preservation in a single PWFA-LC stage using adiabatic plasma density ramp matching sections in the presence of ion motion

Plasma based acceleration (PBA) is being considered as a building block for a future linear collider (LC). In PBA a short pulse laser or particle beam creates a wakefield and a witness particle beam is accelerated in the wakefield. As the witness beam is accelerated its energy spread must be small and its emittance must be preserved. In some designs the witness beam parameters required by a linear collider are expected to trigger background ion motion which can lead to nonlinear focusing forces which vary along the witness beam. This can lead to emittance growth of the witness beam. To mitigate this, we propose to use an adiabatic plasma density ramp as a matching section. We match the beam to the low density plasma entrance, where the beam has a large matched spot size so the ion motion effects are relatively small. As the beam propagates in the plasma upramp (downramp), it is adiabatically focused (defocused) and its distribution evolves slowly towards an equilibrium distribution including the effects of the adiabatically changing ion motion. We present simulation results using the 3D and quasi-3D quasi-static PIC codes QuickPIC and QPAD. In QPAD the fields are expanded into a small number of azimuthal modes.  Preliminary results show that this method can reduce the projected emittance growth of the witness beam to only 10~20% within a single stage which includes adiabatic matching sections at the entrance and exit.   

Early dynamics of the self-modulation instability growth rate

The self-modulation instability (SMI) is instrumental for single-stage plasma wakefield accelerator concepts with long, high-energy drive bunches. It provides a self-consistent mechanism to reach high-amplitude wakefields despite the driver's length, which would otherwise not excite the plasma resonantly. In recent demonstrations of acceleration with a self-modulated proton driver, the use of a linear plasma density gradient has been a key factor in maximising the energy gain [1]. It is known that a density gradient effectively delays or hastens the growth of the SMI, though this effect has been discussed in the context of asymptotic models that assume small gradients [2], or of the saturation phase of the SMI [3].

We present a new framework for understanding the onset of the SMI, and show that its growth rate varies according to the frequency of the seed (a beam radius perturbation), as shown previously for beam hosing in overdense plasma. This may have implications for the control of the SMI's growth and the associated acceleration process.

[1] AWAKE Collaboration, Nature 561, 363-367 (2018)

[2] C. B. Schroeder, et al., Phys. Plasmas 19, 010703 (2012)

[3] F. Braunmüller, et. al. (AWAKE Collaboration), Phys. Rev. Lett. 125, 264801 (2020)   

Seeded self-modulation of an entire long proton bunch in plasma using a short electron bunch

The AWAKE experiment at CERN [1] relies on the self-modulation of a long proton bunch in plasma [2] to effectively drive wakefields and accelerate an externally injected electron bunch to GeV-level energies [3]. The control of the acceleration requires that the self-modulation process is reproducible from event to event: this is achieved by seeding the instability. Seeding using a relativistic ionization front was demonstrated during AWAKE Run 1 [4], but this method leaves the head of the bunch un-modulated. Therefore we are going to use, for the first time, a 18MeV short electron bunch placed ahead of the proton bunch to seed the self-modulation of the entire bunch [5].  We will present preliminary results of the current experimental campaign where we study the phase reproducibility of the proton bunch self-modulation, varying the parameters of the seed electron bunch at the plasma entrance. 

[1] P. Muggli et al. (AWAKE Collaboration), Plasma Phys. and Contr. Fus., 60(1) 014046 (2017).

[2] M. Turner et al. (AWAKE Collaboration), Phys. Rev. Lett. 122, 054801 (2019).

[3] AWAKE Collaboration, Nature 561, 363 (2018),

[4] F. Batsch et al. (AWAKE Collaboration), Phys. Rev. Lett. 126, 164802 (2021).

[5] P. Muggli et al., J. Phys.: Conf. Ser.1596 012066 (2020).   

Elimination of witness beam hosing in the linear collider regime

Beam driven plasma wakefield acceleration (PWFA) has shown great potential to be the basis for future linear colliders. Recent progress has shown PWFA can acheieve high acceleration gradients with  high energy transfer efficiency while maintaining low energy spread. In the regime relevant for linear colliders, the witness beam emittance is $\sim$ 100 nm and the charge is $\sim$ 1 nC. With these parameters, the ion collapse will be drastic and lead to emittance growth, however An et al. (2017) [1] showed this emittance growth is controllable. Mehrling et al. (2018) [2] showed hosing of the witness beam is suppressed in this regime due to the BNS mechanism. Hildebrand et al. (2018) [3] showed that the on-axis ion column formed by a high density drive beam can fully eliminate the witness beam hosing and realign it with the drive beam. This will occur for unmatched drive beams that oscillate to small sizes but can also be controlled by matching the drive beam with a small spot size.  We will explore the emittance growth of the witness beam and decay time of its centroid as a function of the drive beam spot size and subsequent ion peak formed in the witness beam region.   

Self-stabilizing positron acceleration in a plasma column

Stable acceleration of high-quality positron beams in a plasma-based accelerator is a highly-challenging task, but necessary to realize a plasma-based linear collider. Recently, we proposed a plasma-based positron acceleration scheme in which a wake suitable for efficient, high-quality positron acceleration is produced in a plasma column by means of an electron drive beam [1,2]. 

Here we analyze the performance of the scheme from the point of view of stability when offsets and/or tilts of the driver and witness beams with respect to the plasma column symmetry axis are present. By means of particle-in-cell simulations, we show that propagation of both beams is inherently stable. Finally, we provide tolerances that allow for stable positron acceleration while retaining feasible beam quality.

[1] S. Diederichs, et al. "Positron transport and acceleration in beam-driven plasma wakefield accelerators using plasma columns." Physical Review Accelerators and Beams 22.8 (2019): 081301.

[2] S. Diederichs, et al. "High-quality positron acceleration in beam-driven plasma accelerators." Physical Review Accelerators and Beams 23.12 (2020): 121301.   

Ultra-bright electron bunch injection in a plasma wakefield driven by a superluminal flying focus electron beam

We propose a new method for self-injection of high-quality electron bunches in the plasma wakefield structure in the blowout regime utilizing a "flying focus" produced by an electron drive-beam with an energy chirp. In a "flying focus" the speed of the density centroid of the drive bunch can be superluminal or subluminal by utilizing the chromatic dependence of the focusing optics. We first derive the focal velocity and the characteristic length of the focal spot in terms of the focal length and an energy chirp for the particle beam. We then demonstrate using multi-dimensional particle-in-cell simulations that a plasma wake driven by a superluminally propagating flying focus of an electron beam can generate GeV-level electron bunches with ultra-low normalized slice emittance (∼ 30 nm rad), high current (∼17 kA), low slice energy spread (∼0.1%) and therefore high normalized brightness (>10¹⁹ A/rad²/m²) in plasma of density ~10¹⁹ cm^-3. The injection process is highly controllable and tunable by changing the focal velocity and shaping the drive beam current. Near-term experiments using the new FACET II beam could potentially produce beams with brightness exceeding 10²⁰ A/rad²/m².   

Positron Acceleration in the Elongated Bubble Regime

A new concept is proposed for accelerating positrons in a plasma wakefield accelerator. By loading the nonlinear wakefield (back of the bubble) with a short electron bunch, an elongated area of plasma electron accumulation is created after the first bubble, resulting in a favorable positron accelerating region with simultaneous focusing and accelerating fields. The structure of the focusing field is ideal for emittance preservation: linear in the radial direction and uniform in the longitudinal direction. Scaling laws for the optimal loading parameters are obtained through extensive parameter scans. Owing to the good quality of the focusing field, positron acceleration with emittance preservation can be achieved in this new regime and has been demonstrated in the three-dimensional particle-in-cell simulations.      

Optimization of transformer ratio and beam loading in plasma wakefield accelerator(PWFA) with structure-based algorithm

The PWFA has emerged as a promising candidate for the accelerator technology for a future linear collider and/or light source. For the linear collider application, it is essential that the energy transfer from the drive beam to the wake and from the wake to the trailing beam be efficient, and the energy spread of trailing bunch should be kept low. One way to achieve this is to use longitudinally-shaped bunches. In the linear regime there is an analytical formalism to determine the optimal shapes. However, in the nonlinear blowout regime the theoretical framework is not as well defined. We thus utilize an optimization tool developed at ANL, to efficiently find optimized drive beam and witness beam profiles for PWFA. We parametrize the beam currents as a piecewise-linear longitudinal profile and define optimization objectives for the energy spread. The algorithm converges quickly, and it finds witness beam shapes similar to those calculated by a recent multi-sheath model for nonlinear wakefields. We also obtain optimized drive beam profiles that give high transformer ratios with constraint of fixing the total charge that qualitatively agree with predictions from W. Lu et al.   

Transverse Density Gradients in Blowout Regime Plasma Wakefields

Laser ionized plasma sources are useful for realizing plasma-based acceleration and focusing of relativistic electron beams.  These plasma sources often use a pulsed gas jet that fires into a vacuum environment, after which a region of the gas plume is fully ionized by the passage of a high intensity laser pulse to form the plasma source.  For most applications, a uniform density is desired within the bulk of the plasma source, however gas jet density profiles are not typically uniform when fired into vacuum.  Here we study the contribution to the wakefields of an approximately linear transverse background plasma density gradient and discuss the implications for beam dynamics in plasma lenses and plasma accelerators.  Early experimental results of a laser-ionized plasma lens in a pulsed gas jet at FACET-II are discussed.   

High-degree spin-polarized sub-femtosecond electrons generated in situ in a beam-driven plasma wakefield accelerator

The generation and acceleration of an electron beam with a high degree of spin polarization desired for high-energy collider application is still an outstanding scientific problem for plasma-based accelerators. Based on and further improving our previous work [1], we propose that both the plasma wake and a highly polarized electron beam suitable for injection into such a wake can be produced by using an ytterbium (Yb) plasma. While the nonlinear wake is produced by ionizing both 6s electrons using a drive electron beam, the injection is achieved by ionizing 4f14 electrons of the resultant Yb III ions in-situ using a circularly polarized ultrashort laser pulse. By using the Time-dependent Schrödinger equation (TDSE) and the 3D Particle-in-cell (PIC) code, we show the generation of a sub-femtosecond, high-current (4 kA), low-normalized-emittance (180 nm), and high-energy (15 GeV) electron beam within ~41 cm distance, with up to 56% net spin polarization.

References:

[1] Z. Nie, et. al., Phys. Rev. Lett. 126, 054801 (2021).   

Plasma waves driven by non-neutral fireball beams with applications to positron acceleration

Plasma-based positron acceleration is a long-standing challenge to the advanced accelerators community in the pathway to a compact electron-positron collider. Some of the most prominent solutions rely on hollow laser beams [1] or electrons beams [2] as drivers of the wakefield structure.

One concept that has recently drawn attention in the context of plasma micro-instabilities is the fireball beam [3], which consists of spatial overlap between an electron and a positron beam. This work shows that standard Gaussian, non-neutral fireball beams in certain conditions evolve self-consistently to a hollow shape electron beam with the positron beam focused on-axis, thus enabling conditions for stable positron acceleration studied in [1,2].

We use theory and particle-in-cell simulations to demonstrate stable positron acceleration with the scheme, perform a systematic tolerances study, and study the analogous system with a laser replacing the electron beam.

[1] J.Vieira et al., Physical Review Letters 112, 215001 (2014)

[2] N.Jain et al., Physical Review Letters 115, 215001 (2015)

[3] N Shukla et al., New Journal of Physics 22, 013030 (2020)   

RESISTIVITY AND HEAT CONDUCTION MODELING IN CAPILLARY DISCHARGES

We studied the impact of resistive and thermal transport on capillary waveguide [1] performance as part of an effort to understand and quantify uncertainties in modeling and designing next-generation plasma accelerators. First, the Ji-Held [2] electron transport model was added to the adaptive mesh refinement magnetohydrodynamics code [3], FLASH, and tested against Braginskii [4] and Epperlein-Haines [5] models. The simulations enable us to investigate the performance of argon and hydrogen-filled capillaries discharge. Typically computed diagnostics include azimuthal magnetic field, average ionization state, and temperature fields. We present 2D cylindrical geometry simulations and compare them against theoretical and experimental predictions. Finally, we will discuss how changes to the laser heater affect plasma transport.

1] W. P. Leemans et al., Phys. Rev. Lett.113, 245002.

[2] J.-Y. Ji and E. D. Held, Physics of Plasmas20, 042114 (2013).

[3] B. Fryxell, K. Olson, P. Ricker, F. X. Timmes, M. Zingale, D. Q. Lamb, P. MacNeice, R. Rosner, J. W. Truran, and H. Tufo, ApJS 131, 273 (2000).

[4] S. I. Braginskii, Soviet Journal of Experimental and Theoretical Physics6, 358 (1958).

[5] E. M. Epperlein and M. G. Haines, Physics of Fluids 29, 1029 (1986).