Experimental Setup

2^nd jet

1^st jet razor blade

(a) Photograph of the double-jet and shock-front injection setup.

d= 0-14 mm Laser

Razor blade Shock front

10 µm Al foil

Electrons

Scintillating screen 30 MeV 78 MeV

1.4 m

Electron spectrometer

He jets Hydrogen

cells

Steel tapeElectrons

(b) Sketch of double-jet and double-gas-cell setup. Here only the spectrometer used in the double-jet experiment is shown.

The spectrometer of the double-gas-cell experiment is de-scribed in [Popp, 2011].

Figure 5.3: Experimental setup of collective deceleration.

The overview of experimental setup including both of the jet and double-gas-cell experiment is shown in Fig. 5.3. The setup as well as the electron spec-trometer was inside a vacuum chamber which was located in a radiation shielded bunker on the ground floor. During experiments, all devices were automatized and controlled remotely from a control room outside of the bunker. The chamber was connected through a 12 m long vacuum beamline to the ATLAS laser system on the first floor. In order to maximize the transmission, all mirrors inside beam-line had high damage threshold dielectric coating. The ideal energy transmission through the beamline is > 80%. However, during the double-jet experiment, the beamline had a much lower transmission than the designed value.

Double gas jets

In the double-jet experiment, the laser with 28 fs FWHM pulse duration was focused onto the target by an off-axis parabolic mirror with f/13 optical geometry to a 15.9 µm FWHM spot. Due to energy loss and wave-front distortion inside the beamline, we only reached 550 mJ within the first Airy ring of the focus from 870 mJ. The RMS energy fluctuation was≤5% and pointing instability<20µrad.

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5. LWFA Driven by ATLAS System and Observation of Collective Deceleration

Figure 5.4: On-target focus of ATLAS during double-gas-jet experiment. The colorbar shows the relative intensity.

The energy inside FWHM of the focus contained 32.2% of the total energy.

-40 -20 0 20 40

-40 -20 0 20 40

X position (µm)

Yposition(µm)

50 100 150 200

A typical focus is shown in Fig. 5.4. Beside the main beam to drive wakefield, a small part of the laser was used as a probe beam. It was coupled out by a wedge and sent perpendicularly with respect to the main beam and spatially and temporally overlapped on the target to look at the shadowgraph of the plasma channel. The probe beam path was equipped with a delay stage to adjust the delay between main and probe beams with sub-fs resolution.

The target was composed of two helium supersonic jets and well characterized by interferometry [Schmid, 2009] and Rayleigh scattering [Dorchies et al., 2003]

(more details are described in chapter 4).

The distance between laser and jets was measured by shadowgraphy using the probe beam with an uncertainty below 10 µm. This information was used to determine density from interferometry result. The backing pressures of the jets were adjusted separately by two closed-loop electropneumatic pressure controllers (TESCOM ER3000), and the pressures were measured by thin film strain pres-sure transducers which offered an accuracy of 0.125%. A careful choice of PID parameters [Bennett, 1993] was critical to achieve fast and accurate adjustment of backing pressure during experiment. In practice, these parameters depended on the pressure, shooting frequency and pumping speed. An improper setting causes large oscillation in pressure and increase shot-to-shot uncertainty of density. The resulted error bar of the determined electron density was better than 10% in this measurement.

In order to prevent ambiguous results caused by unstable electron sources based on self-injection, we used the shock-front injection scheme. The electron bunches were injected from a shock front generated by the edge of a razor blade inside the supersonic flow in the first jet (see section 4.4),which had a 300 µm exit aperture and 1002 µm FWHM Gaussian-like density profile along the laser axis.

5.2 Study of Collective Deceleration 93

The electron bunch propagated into a second jet with 1500 µm exit aperture and 1503 µm flap-top profile. The distances between the laser and the first and the second jet were 1.6 mm and 0.88 mm respectively. The electron density of the first jet was fixed at 1.7× 10¹⁸ cm⁻³ while the density in the second jet was varied between 1.2−4.8× 10¹⁸ cm⁻³. The two jets were oriented on the top of each other in order to overcome the geometrical limit of jets to reach closer separation. Both jets were mounted on motorized 3-axis translation stages which offered 1 µm resolutions and were controlled remotely from control room. The separation d of two jets were defined as the distance between shock front and the middle position of second jet, which was tunable between d=0 and 14 mm. The electrons were detected by a spatially resolved electron energy spectrometer, as described in section 3.2, and the accompanied direct laser light was blocked by a 10 µm aluminum foil at the spectrometer entrance. The calculated collisional and radiative energy loss from the foil was less than 15 keV and the increase of divergence was negligible. The electrons were generated at 1 Hz repetition rate.

The only limitation factor of repetition rate was the pumping speed of the vacuum system.

Double gas cells

Although, as it will be shown in the next section, the double-jet experiment gave a clear indication of the electron-driven wakefield, the laser was not blocked between two jets which might change the electron energies in the second jet. In order to completely exclude the influence of the laser, an additional experiment was conducted. In this experiment, two hydrogen-filled gas cells were put adjacent to each other with holes in the middle of the gas cell surfaces to let laser pass through it. Similarly, the gas densities in each cell were controlled separately by two closed-loop electropneumatic pressure controllers. The stability of the pressure was better than 1%. The two cells were separated by a 10 µm stainless steel tape which blocked the laser completely before being destroyed and was shifted/refreshed after every shot.

This automatic renewal of tape was also important for the density distribution because the holes on the gas cell were gradually damaged by the laser and got enlarged from 300µm up to 2 mm which changed the density distribution between the cells. The length of plume at the exit of the first cell would also change significantly, as noticed in fluid simulation. The laser pulse of 1.7 J and 26 fs was focused by f/19 parabolic mirror to a 22 µm spot on the target.

The geometry of each cell is the same as what was used in section 5.1. The 1st gas cell was operated in the self-injection regime. Due to small divergence (∼ 1.7 mrad) and < 1 µm source size [Weingartner et al., 2012] of the electron bunches, the hydrogen gas in the second cell was field-ionized [Martinez de la

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5. LWFA Driven by ATLAS System and Observation of Collective Deceleration

Ossa et al., 2013] by electron bunches in our configuration. The experiment was operated at one shot every 2 minutes because of the large volume of the gas cells.