A.1 Laboratory for Extreme Photonics - Closer Look
A.1.2 Optimization of the Laser Contrast
The laser contrast has been one of the greatest challenges for laser-driven ion accel-eration. The laser contrast is typically measured with third order auto-correlators [148, 149] (see Section B.4.1). In LEX Photonics we observed that the temporal laser contrast has limited the acceleration of ions. In this section we outline differ-ent optimization stages of the laser contrast inLEX Photonics. A typical trace of the
ASE level Coherent contrast
Prepulses
Peak
Postpulses
Figure A.1 |Laser contrast. This figure shows a typical laser contrast curve, recorded with an autocorrelator (see SectionB.4.1). It shows the different segments of the temporal laser contrast. The level ofASE, the coherent contrast and the prepulses.
laser contrast is shaped by different processes and can be seen in Fig. A.1. Those are Amplified Spontaneous Emission, short prepulses and the so called coherent contrast.
1. ASE: Is created by spontaneous emission in any of the laser amplification media (e.g. Ti:sapphire crystals) that is further amplified in the laser chain.
Its duration or influence can be minimized by pumping the crystals as short as possible prior to the laser pulse. It can be further suppressed by using an OpticalParametricAmplification (OPA) [238] orOpticalParametricChirped PulseAmplification (OPCPA) systems [239,240], or implementing a cross (X) PolarizedWave generation (XPW) [241] or Pockels cells with fast raise times.
2. Short prepulses: Short prepulses are replicas of the main pulse. There are two possibilities how they can be created. The most common source is internal double reflection in transmissive optics. The resulting postpulse can further create a temporally mirrored prepulse in theCPA laser systems due to inter-ference of main an prepulse in the stretched configuration and nonlinearities
in the material the laser passes. This effect is theoretically described by Di-denko et al. [242] and is experimentally demonstrated in Fig. A.2. Secondly direct prepulses can be created in the cavity of the REGEN. In particular the surfaces of the Pockels cell, that are close to the cavity end-mirrors, can cause prepulses.
3. Coherent contrast: Coherent contrast is typically associated with the raising part of the laser pulse that starts a few tens of picoseconds prior to the peak of the laser pulse [243].
9 ⋅ 10−4
2 ⋅ 10−4 3 ⋅ 10−7 4 ⋅ 10−6
4 ⋅ 10−8
Figure A.2 |Generation of prepulses by postpulses. This graph shows that during CPA a postpulse can be transformed to a prepulse [242]. The contrast was measured once with a glass plate just before the measurement creating a postpulse. The measured prepulse is an artifact of the measurement (see Section B.4.1) and quadratically lower than the postpulse. In the second situation the glass plate was installed shortly after the oscillator and thus before the CPA stage and the amplification of the laser pulse. The measurement reveals the additional postpulse that has been broadened and slightly shifted in time. The postpulse is smaller since part of its energy is transformed to the prepulse and the artificial prepulse is still visible. However, the hereby created postpulse is smaller than the prepulse.
Identifying Contrast Limitations
The evaluation of the experimental data, especially the fraction of light transmitted through the target [244] and measured proton energies as a function of target thick-ness, are an indicator for the target density during interaction and thus also the laser contrast. Our experiments showed that we were not able to accelerate ions from foils thinner than about 100 nm and further the ion energies did not exceed 5 MeV at this stage. Since the measurement of the temporal contrast revealed some prepulses
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while the ASE level seemed small, the effect of prepulses was studied in more de-tail. Zhou studied how short prepulses affect the target conditions, in particular the target density, during the arrival of the laser peak [245]. The target thickness and the arrival time of the prepulse were considered and simulations supported our experimental findings.
-350 -300 -250 -200 -150 -100 -50 0 50 100 150 200 Time [ps]
10-10 10-5 100
Intensity [a.u]
Before Adjustments After Adjustments
Figure A.3 | Optimization of the REGEN. This graph shows the optimization of theREGEN performed in January 2016. The exchange of the Pockels cells, in particular the extraction, suppressed the postpulses and consequently the prepulses at 150 and 170 ps.
After removing or at least suppressing prepulses around 170 ps prior to the main pulse (by exchanging Pockels cells in theREGEN) the contrast curve was improved (Fig.A.3). But to our disappointment the achieved proton energies and particle yield did not improve significantly. The implementation of a double plasma mirror, using an integrated plasma-mirror-target, resulted in proton energies of about 12 MeV corresponding to a doubling of the proton energy [43]. This observation proved that it was still the temporal contrast that limited the optimization of ion acceleration.
One relevant cause for the limiting contrast was revealed when the contrast was measured with another third-order-auto-correlator (Tundra) capable of measuring up to two ns prior to the main pulse. Two significant prepulses (at ≈ 500 ps and
≈ 660 ps) were revealed in the trace. As illustrated in Fig. A.4, they originated from reflections of the surfaces of the Pockels cell within the cavity of the REGEN and were thus direct prepulses and not created from former postpulses. This is significant since the prepulse is typically less intense than its corresponding postpulse
Time -500ps -660ps 0 ps
Pockels Cell
Cavity Mirror Regen
Main Pulse Direct Prepulses
a b
c
Figure A.4 | Generation of direct prepulses. The source of the 500 and 660 ps prepulses is illustrated. Reflections from the front surface (a) and the backside (b) of the cavity Pockels cells of the REGEN cause direct prepulses. The distance of those to the main pulse is dependent on the distance of the Pockels cell and the end cavity mirror.
(see Fig. A.2) and thus the direct prepulses have higher intensities. A small tilt of the Pockels cell lowered the intensity of the prepulses, but could also cause a reduced amplification and an increased ASE level of the REGEN. With this findings and modification the measured proton energies increased significantly (as can be seen in June and July 2016 Fig. 3.10). Nonetheless, it was no solution to the problem, since the prepulses were only attenuated and realignment and optimization of the REGEN impaired the contrast again.
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Figure A.5 | Discovery of more prepulses. Comparison of a short- (Sequoia) and long-range (Tundra) contrast measurement. Two significant prepulses at 500 and 660 ps prior to the main pulse have been identified. With the previously used short-range device they have not been detectable. In contrast to previous prepulses those are origin from direct reflections in theREGEN and not transformed from postpulses.