Ultralow emittance electron beams from a laser-wakefield accelerator

Abstract

Using quadrupole scan measurements we show laser-wakefield accelerated electrons to have a normalized transverse emittance of 0:21 þ0:01 À0:02 mm mrad at 245 MeV. We demonstrate a multishot and a single-shot method, the mean emittance values for both methods agree well. A simple model of the beam dynamics in the plasma density downramp at the accelerator exit matches the source size and divergence values inferred from the measurement. In the energy range of 245 to 300 MeV the normalized emittance remains constant.Laser-wakefield acceleration (LWFA) [1,2] can deliver ultrarelativistic electron beams in a compact setup with unique features [3][4][5][6]. It is receiving particular attention as a source or driver for ultrashort x-ray beams [7,8] and for its potential for realizing a tabletop free-electron laser (FEL) [9]. The electron bunch duration has recently been measured to be only a few femtoseconds long [10,11] which results in peak beam currents on the order of kiloamperes. An essential parameter for the performance of x-ray sources, FELs, or linear colliders is the transverse electron beam emittance. Previous emittance measurements of LWFA electron beams have used the pepperpot method [12][13][14] giving normalized emittances of $2:2 mm mrad with single shots down to the resolution limit of 1:1 mm mrad. As these measurements are not spectrally resolved, they rely on a low energy spread to give a meaningful normalized emittance. For LWFA beams which fluctuate in energy and energy spread, a simultaneous measurement of the spectrum is required. This technique is also limited to electron energies that can be sufficiently scattered by the pepper-pot mask; to date, measurements of a 508 MeV beam have been carried out [15]. Experiments characterizing the betatron radiation emitted by the electron beam while it is in the plasma suggest the beam size there to be & 1 m [16,17], which in combination with a divergence measurement give an estimated emittance of <0:5 mm mrad [18]. However, inferring the emittance from the electron beam size in the plasma and its downstream divergence in the vacuum can be unreliable as this neglects the plasma-vacuum density transition at the accelerator exit; here the decreasing strength of the plasma focusing forces result in an increase in beam size and decrease in divergence [13]. This publication reports on direct measurements of the emittance of LWFA electrons that are both energy resolved and that include the beam transport of the density downramp at the accelerator exit. This is achieved by analyzing their beam size around a focus using a quadrupole lens scan method [19].The transverse phase space of an electron beam is often specified using the Twiss parameters , , , and the natural emittance ". These parameters describe the volume and orientation of the particle distribution in phase space. The beam size at a particular position ðs 1 Þ is related to the Twiss parameters at s 0 by [20] ðs 1 Þ 2 ¼ M 2 11 ðs 0 Þ À2M 11 M 12 ðs 0 Þþ M 2 12 ðs 0 Þ: (1)Here M ij refers to the ij eleme...

Demonstration Scheme for a Laser-Plasma-Driven Free-Electron Laser

“…However, as recent diagnostic experiments [10][11][12][13][14][15] agree with simulations and general theoretical predictions [2], the properties measured in these experiments do set a parameter range and allow us to define a typical electron bunch that can be expected from a laserplasma accelerator, operating at a few 10 18 cm À3 plasma density and using a few tens of terawatt laser power. In order to base our studies on experimentally verified data [10,13,14], we assume an electron bunch with a normalized transverse emittance of x;y ¼ 0:2 mm mrad at a moderate normalized beam energy of ¼ 600. Since different experiments report similar bunch lengths at different bunch charges [13,14], the bunch charge Q is chosen as a free parameter in our studies and we assume a Gaussian current profile of z ¼ 0:5 m, which is consistent with [13,14].…”

“…Based on recent novel diagnostic experiments, which characterized the typical LWFA emittance [9][10][11][12], bunch length [13,14], and slice-energy spread [15], we demonstrate the application of an optimized undulator and longitudinal bunch decompression to currently available laser-plasma-generated beams. We conclude with a start-to-end simulation of a proof-ofprinciple experiment, which solely focuses on demonstrating detectable FEL gain well above the spontaneous emission background using a laboratory-size, 2-m-long undulator.…”

Plasma Undulator Based on Laser Excitation of Wakefields in a Plasma Channel

“…This plasma undulator configuration is in a strongly focused regime, and for typical LPA beam parameters [18] with ultra-low emittance [29,30], the beam transverse size will be smaller than the radiation mode size. Consider the radiation produced by an LPA-generated electron beam propagating through a plasma undulator with the laser-plasma parameters n 0 = 10 18 cm −3 , λ L = 1 µm, w 0 = 7 µm, a 0 = 0.28, and with the laser matched to the plasma channel with centroid oscillation amplitude x ci = 2.5 µm.…”

An ultracompact X-ray source based on a laser-plasma undulator

“…In order to validate the proposed scheme and to predict more precisely the radiation features, the laser-plasma undulator was modelled using particle-in-cell simulations (see Methods). We considered a quasi-monoenergetic electron beam having the properties given in Table 1, which corresponds to state-of-the-art laser-plasma accelerators [19][20][21] . Moreover, shown in Table 1 are the laser parameters at the entrance of the undulator.…”