Generating high quality ultrarelativistic electron beams using an evolving electron beam driver

Abstract

A new method of controllable injection to generate high quality electron bunches in the nonlinear blowout regime driven by electron beams is proposed and demonstrated using particle-in-cell simulations. Injection is facilitated by decreasing the wake phase velocity through varying the spot size of the drive beam and can be tuned through the Courant-Snyder (CS) parameters. Two regimes are examined. In the first, the spot size is focused according to the vacuum CS beta function, while in the second, it is focused by the plasma ion column. The effects of the driver intensity and vacuum CS parameters on the wake velocity and injected beam parameters are examined via theory and simulations. For plasma densities of ∼ 10 19 cm −3 , particle-in-cell (PIC) simulations demonstrate that peak normalized brightnesses 10 20 A/m 2 /rad 2 can be obtained with projected energy spreads of 1% within the middle section of the injected beam, and with normalized slice emittances as low as ∼ 10 nm.Over the past few decades, plasma-based acceleration (PBA), driven by either a laser pulse (LWFA) [1] or particle beam (PWFA) [2], has attracted significant interest in compact particle accelerator and x-ray free-electron-laser (XFEL) applications due to the high accelerating fields ∼ 10 − 100 GV/m they generate [3][4][5][6][7][8][9][10][11][12]. While the generation of ultra-relativistic electron beams through selfinjection in an evolving plasma wake has been observed in LWFA experiments [5][6][7][8] and demonstrated in simulations [13][14][15][16], the beams produced to date do not exhibit the sufficiently low energy spreads σ γ and high normalized brightnesses B n = 2I/ǫ 2 n required to drive XFEL devices [17] where I and ǫ n represent the current and normalized emittance, respectively. In recent years, electron injection schemes involving field ionization [18][19][20][21][22][23] or the use of a plasma density down ramp (DDR) [24][25][26][27][28] have shown tremendous potential for high quality beam generation for XFEL applications.In this Letter, we propose and demonstrate a new method of controllable injection using an electron beam driver whose spot size is decreasing in the nonlinear blowout regime to control the wake phase velocity and hence induce electron trapping. As when using a DDR, this proposed method relies on gradually elongating the ion column or cavity length as the drive beam propagates. In this scheme, injection can be achieved by focusing the electron beam driver over spot sizes, σ r , ranging from the scale length of the blowout radius ∼ r b to spot sizes much less than r b . A schematic of this process is shown in in Fig. 1(a). For spot sizes in this range, we will show that the ion column length and blowout radius slightly increase as σ r decreases and become insensitive to variations in σ r when σ r ≪ r b . Therefore, self-injection can be induced by controlling the focusing of the drive beam. This new approach is also a physically simpler alternative to injection methods such as DDR and Ionization, which require sh...

A multi-sheath model for highly nonlinear plasma wakefields

“…While PIC simulations can be used to determine the exact value of ψ min in the nonlinear blowout regime, ψ min can be well approximated by −1 when the maximum bubble radius is sufficiently large, i.e., r m 3. Under these conditions, sheath electrons that trace the bubble r b (ξ) travel near the speed of light, v z ∼ 1, with finite transverse momentum P ⊥ at the rear of the wake 16,20 . From the constant of motion for a plasma particle γ − P z = 1 + ψ 29 , it can be shown that 1…”

“…Recently, there have been many selfinjection schemes proposed to generate high quality electron beams with low energy spread σ γ and normalized emittance ǫ n . The most promising ideas typically involve decreasing the phase velocity γ φ of the plasma wake using either a plasma density down ramp [13][14][15][16][17] or an evolving driver [18][19][20] . In each of these instances, plasma electrons are injected at the very rear of the first bucket of the wake where they can then be accelerated over long periods of time.…”

“…Near the axis, the wake potential ψ must approach −1 if v z approaches c as can be seen from the constant of motion equation γ − P z /mc = 1 + ψ 29 . In fact, many self-injection schemes [13][14][15][16][17][18][19][20] rely on ψ approaching −1 at the rear of the wake in order to satisfy the electron trapping condition γ z > γ φ ≫ 1. Therefore, while the model used by Lu et al 23,24 can predict the bubble trajectory r b (ξ) and longitudinal electric field E z (ξ) in regions where the wake potential ψ(ξ) is sufficiently positive, it will not be accurate near the rear of the bubble where the wake potential becomes negative.…”

Generation of topologically complex three-dimensional electron beams in a plasma photocathode

“…The production of high quality electron beams [13][14][15][16][17] has been instrumental for these advances. Various techniques were developed to controllably inject electrons into a relativistic plasma wake, such as downramp trapping [18][19][20][21][22][23] and ionization injection [23][24][25][26][27].…”

“…Various techniques were developed to controllably inject electrons into a relativistic plasma wake, such as downramp trapping [18][19][20][21][22][23] and ionization injection [23][24][25][26][27]. Recent work on plasma cathodes has opened the possibility of generating femtosecond duration electron beams with MeV energy spreads, peak currents as high as hundreds of kA [28,29] and normalized emittance n as low as 10's of nm [13,14,16,17,[30][31][32]. Besides having the potential to achieve unprecedented beam brightness, plasma cathodes can imprint multi-dimensional spatial structures onto the accelerated beams.…”