Timon Mehrling

High-quality electron beams from beam-driven plasma accelerators by wakefield-induced ionization injection

We propose a new and simple strategy for controlled ionization-induced trapping of electrons in a beam-driven plasma accelerator. The presented method directly exploits electric wakefields to ionize electrons from a dopant gas and capture them into a well-defined volume of the accelerating and focusing wake phase, leading to high-quality witness bunches. This injection principle is explained by example of three-dimensional particle-in-cell calculations using the code OSIRIS. In these simulations a high-current-density electron-beam driver excites plasma waves in the blowout regime inside a fully ionized hydrogen plasma of density 5×10(17)cm-3. Within an embedded 100  μm long plasma column contaminated with neutral helium gas, the wakefields trigger ionization, trapping of a defined fraction of the released electrons, and subsequent acceleration. The hereby generated electron beam features a 1.5 kA peak current, 1.5  μm transverse normalized emittance, an uncorrelated energy spread of 0.3% on a GeV-energy scale, and few femtosecond bunch length.

The FLASHForward facility at DESY

The FLASHForward project at DESY is a pioneering plasma-wakefield acceleration experiment that aims to produce, in a few centimetres of ionised hydrogen, beams with energy of order GeV that are of quality sufficient to be used in a free-electron laser. The plasma is created by ionising a gas in a gas cell with a multi-TW laser system. The plasma wave will be driven by high-current-density electron beams from the FLASH linear accelerator. The laser system can also be used to provide optical diagnostics of the plasma and electron beams due to the <30 fs synchronisation between the laser and the driving electron beam. The project will explore both external and internal witness-beam injection techniques. The operation parameters of the experiment are discussed, as well as the scientific programme.

Mitigation of the Hose Instability in Plasma-Wakefield Accelerators

Current models predict the hose instability to crucially limit the applicability of plasma-wakefield accelerators. By developing an analytical model which incorporates the evolution of the hose instability over long propagation distances, this work demonstrates that the inherent drive-beam energy loss, along with an initial beam-energy spread, detunes the betatron oscillations of beam electrons and thereby mitigates the instability. It is also shown that tapered plasma profiles can strongly reduce initial hosing seeds. Hence, we demonstrate that the propagation of a drive beam can be stabilized over long propagation distances, paving the way for the acceleration of high-quality electron beams in plasma-wakefield accelerators. We find excellent agreement between our models and particle-in-cell simulations.

Wakefield-Induced Ionization injection in beam-driven plasma accelerators

We present a detailed analysis of the features and capabilities of Wakefield-Induced Ionization (WII) injection in the blowout regime of beam driven plasma accelerators. This mechanism exploits the electric wakefields to ionize electrons from a dopant gas and trap them in a well-defined region of the accelerating and focusing wake phase, leading to the formation of high-quality witness-bunches [Martinez de la Ossa et al., Phys. Rev. Lett. 111, 245003 (2013)]. The electron-beam drivers must feature high-peak currents ( Ib0≳8.5 kA) and a duration comparable to the plasma wavelength to excite plasma waves in the blowout regime and enable WII injection. In this regime, the disparity of the magnitude of the electric field in the driver region and the electric field in the rear of the ion cavity allows for the selective ionization and subsequent trapping from a narrow phase interval. The witness bunches generated in this manner feature a short duration and small values of the normalized transverse emittance ( k...

Chirp Mitigation of Plasma-Accelerated Beams by a Modulated Plasma Density

Plasma-based accelerators offer the possibility to drive future compact light sources and high-energy physics applications. Achieving good beam quality, especially a small beam energy spread, is still one of the major challenges. Here, we propose to use a periodically modulated plasma density to shape the longitudinal fields acting on an electron bunch in the linear wakefield regime. With simulations, we demonstrate an on-average flat accelerating field that maintains a small beam energy spread.

Observation of the Self-Modulation Instability via Time-Resolved Measurements

Self-modulation of an electron beam in a plasma has been observed. The propagation of a long (several plasma wavelengths) electron bunch in an overdense plasma resulted in the production of multiple bunches via the self-modulation instability. Using a combination of a radio-frequency deflector and a dipole spectrometer, the time and energy structure of the self-modulated beam was measured. The longitudinal phase space measurement showed the modulation of a long electron bunch into three bunches with an approximately 200  keV/c amplitude momentum modulation. Demonstrating this effect is a breakthrough for proton-driven plasma accelerator schemes aiming to utilize the same physical effect.

Simulations of laser-wakefield acceleration with external electron-bunch injection for REGAE experiments at DESY

We present particle-in-cell simulations for future laser-plasma wakefield experiments with external bunch injection at the REGAE accelerator facility at DESY, Hamburg, Germany. Two effects have been studied in detail: emittance evolution of electron bunches externally injected into a wake, and longitudinal bunch compression inside the wakefield. Results show significant transverse emittance growth during the injection process, if the electron bunch is not matched to its intrinsic betatron motion inside the wakefield. This might introduce the necessity to include beam-matching sections upstream of each plasma-accelerator section with fundamental implications on the design of staged laser wakefield accelerators. When externally injected at the zero-field crossing of the laser-driven wake, the electron bunch may undergo significant compression in longitudinal direction and be accelerated simultaneously due to the gradient in the acting force. The mechanism would allow for production of single high-energy, ultra-short (on the order of one femtosecond) bunches at REGAE. The optimal conditions for maximal bunch compression are discussed in the presented studies.

Intrinsic Stabilization of the Drive Beam in Plasma-Wakefield Accelerators

The hose instability of the drive beam constitutes a major challenge for the stable operation of plasma-wakefield accelerators. In this Letter, we show that drive beams with a transverse size comparable to the plasma blowout radius generate a wake with a varying focusing along the beam, which leads to a rapid detuning of the slice-betatron oscillations and suppresses the instability. This intrinsic stabilization principle provides an applicable and effective method for the suppression of the hosing of the drive beam and allows for a stable acceleration process.

Analytical model for the uncorrelated emittance evolution of externally injected beams in plasma-based accelerators

Author(s): Aschikhin, A; Mehrling, TJ; Martinez de la Ossa, A; Osterhoff, J | Abstract:

Characterization of Self-Modulated Electron Bunches in an Argon Plasma

The self-modulation instability is fundamental for the plasma wakefield acceleration experiment of the AWAKE (Advanced Wakefield Experiment) collaboration at CERN where this effect is used to generate proton bunches for the resonant excitation of high acceleration fields. Utilizing the availability of flexible electron beam shaping together with excellent diagnostics including an RF deflector, a supporting experiment was set up at the electron accelerator PITZ (Photo Injector Test facility at DESY, Zeuthen site), given that the underlying physics is the same. After demonstrating the effect [1] the next goal is to investigate in detail the self-modulation of long (with respect to the plasma wavelength) electron beams. In this contribution we describe parameter studies on self-modulation of a long electron bunch in an argon plasma. The plasma was generated with a discharge cell with densities in the 10 cm to 10 cm range. The plasma density was deduced from the plasma wavelength as indicated by the self-modulation period. Parameter scans were conducted with variable plasma density and electron bunch focusing. INTRODUCTION Motivated by the ongoing experiments of the AWAKE collaboration [2] the self-modulation instability [3] is investigated at the electron accelerator PITZ. This effect was demonstrated for the first time by utilizing a lithium heat pipe oven plasma cell [1]. Flat top electron bunches with a FWHM length of about 20 ps and with rise/fall times of <2 ps were generated by impinging similarly shaped photocathode laser pulses [4] onto a Cs2Te photocathode. The bunches were accelerated with an L-band electron gun and a subsequent booster linac to a momentum of 22.3 MeV/c. A gun solenoid and four quadrupole magnets were used to focus these bunches into a heat pipe oven which provided a lithium plasma with densities up to 10 cm. The sharp transition of charge density at the head of the bunch triggers a plasma wake which is seeding the self-modulation instability along the electron bunch. Since the bunch is several plasma wavelengths long this results in a periodical bunch diameter and energy modulation. These modulations were observed on Ce:YAG and LYSO scintillation screens by resolving the temporal charge distribution with an RF deflector and the energy distribution with a dipole spectrometer. Here we describe a follow-up experiment using the same setup with the only difference that the lithium heat pipe oven was replaced with a discharge plasma cell [5]. EXPERIMENTS The setup used for these experiments is depicted in Fig. 1. Argon plasma was generated with a 2.4 kV, 250 A discharge pulse of 2 s length. The timing of the discharge pulse is adjustable with respect to the electron bunch arrival at the plasma cell. Since the plasma is recombining after the discharge pulse has ended, this variable delay translates into a scan of the plasma density which the electron bunch is experiencing. The bunch charge is adjustable by tuning the pulse energy of the photocathode laser, while the focusing of the bunch into the plasma cell can be scanned by changing the drive current of the gun solenoid. Figure 1: Experimental setup. Streaked Bunch For the first set of experiments a removable Ce:YAG screen was inserted to observe the electron bunches which are vertically streaked with an RF deflector [6]. Results of a timing scan are shown in Fig. 2. The bunch charge was 600 pC and the main solenoid current 390 A. The horizontal axis shows the horizontal size of the bunch while the vertical axis is the axis of RF streaking, which is ____________________________________________ * matthias.gross@desy.de Th is is a pr ep ri nt — th e fin al ve rs io n is pu bl ish ed w ith IO P 9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW Publishing ISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-TUPML046 03 Novel Particle Sources and Acceleration Technologies A22 Plasma Wakefield Acceleration TUPML046 1645 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 18 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.