Proton acceleration: experimental setup

The laser source

The Salle Jaune laser is a Titanium-Sapphire (Ti:Sa) solid state laser and belongs to the category of T 3 lasers (Table Top Terawatt). It is a multi-stage CPA chain composed by a pre-amplified front-end and three multipass amplifiers, capable of delivering up to 2J per pulse (before compression) at a repetition rate of 10Hz. More in detail (see scheme in Fig.6.1):Oscillator: a Ti:Sapphire, Kerr-lens mode-locked, oscillator produces a train of 8.5fs pulses, with a per-pulse energy of 5nJ at a repetition rate of 88MHz. The central wavelength is λ = 810nm and the spectrum is 150nm FWHM1 . Preamplification and XPW: The repetition frequency from the oscillator is cut down to 10Hz by Pockels cells. The selected pulse is stretched, pre-amplified to 1mJ and compressed back to 23fs, in order to be injected in the XPW system (see 6.1.1). The preamplifier is charged by a diode-pumped CW laser, which noticeably improves the stability. Multipass amplification: the pulse is stretched to 600ps and injected in a three stages (4-pass, 5-pass and 3-pass) chain of Ti:Sa amplifiers pumped by doubled Nd:YAG lasers. The beam undergoes spatial filtering after each amplifier. The complete amplification process brings 100nJ (after XPW) to ≈ 2J. The contrast worsen from 1012 to 109 ÷ 1010 . Separation of the beams: Before the compression, a partially reflecting mirror (in our experiment was 10%) is used to create a second beam (probe) with lower energy. A λ/2 filter and a polarizer cube are used to introduce a variable loss. Finally, a motorized delay line is provided to change the delay between the two beams. Compression: The two beams of 400ps pulses are compressed by two separate grating compressors to a duration of approximately 30fs with an efficiency that ranges between 45% and ≈ 55%. Phase front cleaning: the wave front of the pump beam is corrected by a cylindrical symmetry deformable mirror (see 6.2.1). 

Enhancement of the Contrast

 The contrast ratio (Fig.6.2) of an UHI pulse is a parameter of great importance when speaking of interaction with matter (Ch.3, 3.4). Many features, many of them not completely exploited, of the interaction process are correlated with the total energy and the over-threshold duration of the pedestal that precedes the main peak. From a CPA chain, three types of effects are in general observed.Prepulses Replica of the fs pulse can be produced, by various reasons, before and after the main pulse. Examples are (i) insufficient extinction power of the Pockels cell that selects the pulse from the oscillator, (ii) the presence of shortcuts in the optical path or iii cuts in the pulse spectrum. Background light In the oscillator cavity, the production of fs pulses (with a repetition rate of 88MHz and a per-pulse energy of some nJ) is a process which is in competition with the continuous wave (CW) production, which results in a continuous background around the fs pulses. The contrast in the oscillator pulse train can however be as high as 109 . Amplified Spontaneous Emission The ASE comes from the spontaneous emission in charged crystals, which is produced before the passage of the main pulse. The ASE is amplified in the forthcoming crystals, worsening the contrast. The importance of the spontaneous emission is directly bound to the gain in the amplifiers and to the timing between the pump laser pulses (used to charge the mediums) and the seed laser pulse (to be amplified). The path of the pulse in the optical system controlled by optical shutters (Pockels cells). Their synchrony to the pulse sets the mark on the prepulse structure, cutting the sum of the three effects to a well defined light front. In Salle Jaune, the Pockels cells cut the pedestal between 2.5ns and 3ns before the fs pulse. In the original chain, before any active contrast enhancement technique was adopted, the peak to pedestal ratio was, Imax/IASE = 107 from 3ns before, and Imax/Ipre−p = 105 for prepulses 100ps before (Fig.6.3-(upper),[96]). 

XPW 

The use of XPW (acronym for Crossed Polarized Wave generation) to improve the contrast in a laser chain has been introduced at the Laboratoire d’Optique Appliqu´ee [45, 47, 48] after previous studies [69, 70, 46] on the χ (3) anisotropy of BaF2 glasses. The generation of a crossed-polarized wave is a degenerate, ω → ω, four wave mixing process, where a rotation of the polarization is produced between the input and the output wave. From [47] the equations for the non-linear propagation are written as dA dz = iγ1|A| 2A − iγ2

Adaptive correction of the phase front 

The extent the beam can be focus to, meaning the ability to concentrate the biggest part of energy into a spot as small as possible, depends on the quality of the beam’s transverse profile. On a beam from a CPA is often present a certain level of distortion, due to the number of refractive optics on its path and the non perfectly uniform effect of crystals, including thermal lensing effects and non completely isotropic pumping. On the Salle Jaune laser system, a deformable mirror is used to partially correct the Figure 6.5: Deformable mirror: the mirror surface is on the focal plane of a lens, where the spatial frequencies of the phase surface are mapped. beam wave front, so the quality of the focused spot. It is composed by 52 bi-morph actuators in cylindrical symmetry, which can correct up to the 6th order of aberration in Zernike polynomials. The correction procedure is accomplished in two steps. In the first step a self-learning loop builds the matrix of correlation between the movement of actuators and the projection of the phase surface – after the mirror– on the space of Zernike polynomials2 . The correction matrix depends on the beam characteristics, so that the procedure have to be repeated for every important realignment of the laser system. In the second step a genetic algorithm finds the best actuators’ configuration. 2The phase surface is measured by a 16 × 16 Shack-Hartmann analyzer. 

The experimental installation 

The plane on the beam path that is conjugated to the Shack-Hartmann (SH) and the deformable mirror is situated right after the beam compressor. There are some 10m of free propagation between this plane and the point where beam is focused on the target. This fact limits the efficacy of the active correction by the DM and does not consider any of the forthcoming optics. 6.2 The experimental installation The experimental setup provides (i) the optics to transport and focus the big, high power, beam, (ii) the optical diagnostics for the positioning of the target and (iii) the devices related to the ion beam. In Fig.6.6 the general scheme of the installation is presented. 1. Pump beam 2. Probe beam 3. Parabola 4. Target Holder 5. Microscopy of the focal spot (removable) 6. High magnification reference of target focus 7. Motorized delay line 8. (a-c) Thomson parabola and MCP 9. Transparency microscopy of the target (removable) 10. Lead screen and motorized image plate .