As discussed in section VII.I, an electron-pulse energy spread of considerably less than 1 % is key to allow for LWFA-driven FEL operation. Hence, a reduction of the longitudinal emittance of laser-accelerated electron bunches is a high priority goal on various research agendas. Cur- rently, three main strategies exist that may lead to a solution of this issue. de Loos et al.
[2006] are pursuing the idea of feeding pre-accelerated narrow energy spread 6 MeV-electrons created by a conventional radio-frequency photoinjector into a laser-plasma wakefield. A main problem of this approach arises from the rather long initial electron-pulse duration of ∼100 fs, which is limited by RF-technology and Coulomb repulsion. That kind of spatially dispersed charge distribution captured into a wakefield structure will experience varying accelerating field strengths at different positions along the bunch. Therefore it will be difficult to maintain ex- cellent longitudinal emittance. Such an external-injection technique may prove succesful only in an experimental configuration, in which the original electron-bunch length is considerably shorter than the plasma-oscillation wavelength. In addition, the synchronization of the wake- driving light burst to the RF-cavity timing has proven problematic on a femtosecond-time scale. Hence, only laser-triggered injection methods might yield applicable answers. In general, mi- nimizing the time-window during which electron trapping occurs, entails a reduction in energy spread, since for short trapping periods every injected particle may be accelerated over a similar distance with a similar electric field profile. Moreover, if the maximum acceleration distance, which ends at the electron-dephasing point in momentum-phase space, is slightly exceeded, then an accompanying minimization of the bunch-energy spread can be achieved by longitudi- nal momentum compression (confer figure 1.4.5). As a matter of course, such a scheme requires exact control over the trapping process, which may be acquired by employing a two-laser beam setup [Faure et al. 2005]. Varying the intensity and focal position of the injection pulse allows
for altering the trapping-time window and the acceleration length, respectively. Preliminary experiments have demonstrated the feasibility of this method yielding RMS spreads of 1 % around ∼ 200 MeV of electron energy [Rechatin 2008]. A different way to manage electron injection with high accuracy represents controlled Langmuir wave-breaking at steep plasma- density gradients [Bulanov et al. 1998; Suk et al. 2001; Hemker et al. 2002]. Recently, Geddeset al. [2008] have adopted this technique and created stable, low longitudinal momen-
tum spread (∼ 170 keV/c) electron bunches, albeit at sub-MeV/c average momenta. Further
acceleration of these beams in a second energy-spread maintaining LWFA-stage of lower density may allow for driving a free-electron laser in an all-optical scheme in the future.
VII.III
Staged accelerator concepts
The scalability of laser-driven wakefields to support electron-beam energies beyond the 10 GeV frontier [Katsouleas 2006] potentially permits their application in an advanced particle-
collider concept. Indeed, the construction of such a machine is an ambitious task, which requires an implementation of acceleration units working in series and thus motivates the research to- wards the realization of staging. Modern petawatt-class laser facilities are expected to enable LWFA in a single stage to access electron energies of several GeV [Lu et al. 2007]. However,
a single laser system allowing for peak energies that may compete with today’s modern parti- cle colliders would have to feature light-peak powers several orders of magnitude higher than currently available. Therefore, tens of optically synchronized petawatt-laser beams could drive tens of successive wakefield-acceleration units, each one operating with longitudinal electric fields in excess of 10 GVm−1. Such an assembly would enable the creation of TeV-class electron
pulses in a machine of some hundred meters length, which compared with conventional collider technology constitutes a reduction in accelerator size and costs by a factor of∼100.
Nevertheless, peak-particle energy does not represent the only challenge for an application of laser-accelerated electrons in high-energy physics. A principal attribute governing collider- design considerations is the event rate for the physical process of interest. This event rate is proportional to the beam luminosity L, given by (see e.g.Musiol et al. [1988]):
L= frepN
2 part
4πσxσy
Here, frep is the bunch repetition rate and Npart the number of particles contained in each
bunch, σx andσy describe the spot size at the collision point in transverse dimensions. It is no
exaggeration to state that the requirements for a next-generation particle collider allowing for measurable access to novel particle physics are immense regardless of the acceleration technique used, e.g. electron collisions at 250 GeV demand thatL≈1034cm−2s−1, which calls for particle
beams focusable to nanometer-size spots [Katsouleas2006]. Hence, laser-driven colliders at
the forefront of particle physics will remain a visionary concept for the next decades.
VII.IV
Temporal electron-bunch characterization
A unique feature of laser-accelerated electron pulses is their ultra-short nature. While particle- in-cell codes and theoretical considerations have predicted bunch durations of just a few fem- toseconds [Pukhov and Meyer-ter-Vehn 2002; Geissler et al. 2006], an experimental
proof is still missing. Indeed, this property is of vital interest for the feasibility of the all- optically-driven FEL concept (sec. VII.I) and accounts for the potential of applying this source of coherent radiation e.g. for time-resolved single molecular imaging [Neutze et al. 2000].
According to simulations, the few-cycle laser sources planned or operated at MPQ such as the upcoming Petawatt Field Syntheziser (PFS) [Karsch et al. 2008] or the LWS-10 system
[Tavella et al. 2006] are able to create these very short electron bursts when focused into
high plasma-density environments of ne >1019cm−3. The detection of their duration however
is difficult. Electro-optic sampling, a default technique often used at conventional particle ac- celerators, is limited to pulse durations well above 50 fs FWHM and therefore unsuitable [van Tilborg et al. 2006]. An applicable alternative is the spectroscopy of the coherent cut-off
region of infrared to terahertz transition radiation, which is emitted when electrons pass a re- fractive index boundary [Jackson 1975]. The radiated photon spectrum from such a process
is directly related to electron-bunch shape and duration [van Tilborg et al. 2004; Delsim- Hashemi 2008]. However, this correlation is not unique, with the consequence that a retrieval
of pulse duration must assume a model for the electro-magnetic phase of different radiation components, since the measured spectrum does not contain phase information, and hence can- not deliver a fail-safe bunch-length proof. Several other ideas exists, which promise to provide unambiguous results but are more complicated to realize, such as the exploitation of the laser- assisted Auger-decay effect [Schins et al. 1994], attosecond streaking [Reckenthaeler et al.
2008], inverse free-electron laser electron-bunch slicing [Sears et al. 2005, 2008] or the optical
replica syntheziser [Angelova et al. 2008]. Nevertheless, the true duration of laser-wakefield
accelerated electron pulses lacks confirmation and thus remains their last major undetermined property.