Realizando una acción

The presented experiments show that with a relatively simple tool, the variable-length gas cell, different laser-wakefield regimes can be accessed with the available laser parameters. These regimes can be well characterized and the acceleration process can be optimized for maximum electron energies. The gas cell length scan and the pressure scan demonstrate that in our case at low pressures acceleration is limited by insufficient self-focusing. In order to reach higher electron energies an external guiding channel is necessary. For our laser parameters this is the case below≈ 100 mbar≡4.8·1018cm−3(fig. VI.12). At high pressures self-guiding can be sustained over longer distances (> Ldeph) with lower laser

powers, but the energy depletion rate of the driver pulse increases. Still a wakefield can be driven over more than the dephasing length (see paragraph III.2.4.4). The electrons are even decelerated again in the wakefield. This and also the evolving shape of the energy spectrum highlight the necessity of a well adapted gas cell length.

The maximum accelerating field in the bubble could be estimated to ≈ 160 GV/m for a plasma density ofne = 6.42·1018 cm−3 (130 mbar) and the corresponding dephasing

length to 4.9 mm, which is in good agreement with theoretical predictions.

While plasma-length scans have been performed by other groups, e.g. [1], to the best of the author’s knowledge, this is the first comprehensive scan that covers a wide range of lengths, even beyond the dephasing length. In combination with the meaningful statistics of this measurement this allows for a reliable determination of acceleration parameters. Especially in combination with further sophisticated diagnostics the variable length gas cell will contribute to a better understanding of the acceleration dynamics. Two follow- up experiments were already mentioned. In a measurement with the transition radiation diagnostics described in section V.6, a decrease of the electron bunch durations with the acceleration length was observed [4, 138]. Possible interpretations of this result have yet to be evaluated. For a gas cell length beyond≈10 mm on the interference signal of two or even three electron bunches can be observed. In the second experiment it was shown that the emittance of the electron bunch during the acceleration process is conserved [104]. In further experiments it will be equally interesting to analyze the evolution of the trans- mitted laser pulse. It should be possible to directly observe e.g. the self-focusing length or the progress of self-compression. Unfortunately, the longitudinal resolution will be restricted by the large Rayleigh length of the laser beam.

The pressure scan at fixed gas cell length supports the findings of the length scan con- cerning depletion and diffraction of the laser. For high pressures and therefore short de- phasing lengths the spectral and spatial shape of the electron bunch changes significantly. This might be explained by interaction of the pre-accelerated electron bunch with the laser pulse and direct laser acceleration of a small part of the electrons. The confirmation of this interpretation will be the subject of further investigations.

One problem of the gas cell design is that a gas ”plume” at the entrance/exit holes is unavoidable, as can be seen in figure V.6(d). If there is a long density up-ramp in a re- gion before the geometrical focus the laser will start self-focusing and, if the intensity is too high, potentially break up into several filaments [64, 65] before the bubble regime is reached, thus reducing the stability and reproducibility of the acceleration process. The ”soft” focusing used in the experiment probably inhibits strong filamentation in the density up-ramp since the gas plume was shorter than the Rayleigh length, preventing a strong change in beam shape over the density ramp6. But at the same time, with these laser parameters self-focusing occurs until the blowout-regime is reached. The simula- tions discussed in the next section suggest that injection into a wakefield already sets in in the rising edge of the density profile and while the laser spot size is still decreasing. Both continuously varying plasma wavelength and laser beam size change the wakefield shape and injection dynamics.

For optimized electron bunch characteristics it might be advantageous to work with a pa- rameter set that is matched from the beginning. A 1 m focal length setup will lead to to focal spot size of w0 = 12.44µm, an a0 of roughly 3.5 (depending on the exact energy

available on target). With a backup pressure around 60 mbar≡3·1018cm−3, the require- ments for complete blowout, self-injection and sufficiently long self-guiding according to

run ne z1 z2 z3 z4 (1018cm−3) (mm) (mm) (mm) (mm) A100 5 0 1 2.83 3.73 A100(100mbar) 4.83 0 4.54 10.3 10.8 A100(120mbar) 5.80 0 4.54 10.3 10.8 A100(150mbar) 7.25 0 4.54 10.3 10.8

Table VI.3.: Simulation parameters. z₁−4 are the supporting points of the density profile. The

laser focus is at z2. Between z2and z3the density is constant as given in the second column.

[58] are approximately met. In this case no prior self-focusing should occur.