Outlook: CALA

is a milestone achievement. It is therefore fair to state LEX Photonics has served its purpose as a prototype facility for CALA. Weak spots (see ”lessons learned”

Section3.3.1) have been identified and various key components (such as the target wheel, WASP, PMQ) have been tested and optimized and are now implemented in CALA.

Ion Optic Detector

Target

Plasma

Laser

b a

c g

i

d e

f j h

Application

Figure 6.1 | Successful experimental campaigns at LEX Photonics. The exper-imental campaign at LEX Photonics is summarized showing the variety of results. Each pillar ofILDIAS(a) has been addressed and various topics have been published or submit-ted. The spatial laser contrast (b),TRIC(c), target wheel (d), integrated plasma mirror (e), WASP (f), I-BEAT (g), TOF (h), PMQ (i) and the irradiation of the Zebrafish embryos (j).

6.2 Outlook: CALA

Since the completion of the building forCALAin September 2016, the installations have begun. The experiences and ”lessons learned” from the experimental cam-paigns inLEX Photonicswere thoroughly examined and various modifications have been carried out to overcome previous limitations. In Fig. 6.2 the thirteen ”lessons learned” and their respective solutions are summarized.

Modification at CALA Name

A new REGEN in a ring configuration prevents the generation of direct prepulses and thus prepulses are significantly reduced REGEN prepulse

1

The configuration in CALA enables an operation under non-zero degree such that the back-reflected light is suppressed significantly.

Back-reflection 2

A new large Pockels cell is prepared for the ATLAS-3000 Pockels cell

3

A 3 cm, 1 W cw diode is installed to align through the beamline and each mirror is equiped with reference switches

Laser beam delivery 4

The leakage through Turning Mirror 1 (TM1) is taken to analyze it Incoming laser light

5

The back reflected light is also analyzed via TM1, which has a leakage of about 1% for fundamental wavelength

Back-scattered light 6

An adaptive optic including analyzis of near field and wavefront is installed

Spatial Intensity 7

Since there is no solution yet, copper parabolas are used that are less expensive and can more easily be reworked

Debris 8

The foil based plasma mirror worked but was challenging to align. A novel foil based plasma mirror is currently developed by M. Speicher.

Plasma mirror 9

The development of a Pablone would allow an intrinsic calibration of the WASP

No solution yet WASP calibration

10

No solution yet Shielding QP

11

The angle of the QP is now motorized and can be adjusted during operation

Orientation QP 12

New motorized and detection tools are built in order to be operable with a Tango system

Data monitoring 13

Figure 6.2 | Lessons learned for CALA. This figure shows the lessens learned of Chapter 3 and the modifications that have been carried out to solve the problem for CALA. Solved issues are marked with green while red and orange indicate that no real solution has been found yet.

Even though the majority of the known issues had been solved, there are still open issues that need to be addressed. Increased challenges connected to EMP, debris or the destruction of neighboring targets due to higher laser energy and intensity, will be among the biggest issues. Another challenge is the intensity of the transmitted laser light (lesson 11) that is often discarded as negligible. But with thin foils, due to induced transparency, or simply because isolated micro-targets are used, dumping the non-converted laser energy becomes important (as in the case of laser-wakefield acceleration). The intensity will be about 10¹⁴W cm⁼² at a distance of one sixth of the focal length of the OAP (behind TCC). This becomes in particular relevant for long focal length parabolas. The laser intensity (without attenuation) would be high enough to ionize matter up to 30 cm distance to TCC, when using the 1.50 m focal length OAP. This would lead to destruction and more debris in the chamber.

6.2 Outlook: CALA 119

In June 2019 CALA has started its operation with reduced laser energy, which will now be steadily increased. A first successful acceleration of about 15 MeV protons emerging from 50 nm thick Formvar targets with roughly 4 J on target have been reported² on August 1st, 2019. A picture of the prepared experimental chamber ofLIONis shown in Fig.6.3. More details about CALAand some relevant key components therein can be found in Section A.2.

Figure 6.3 | Picture of the LION chamber. This picture shows the experimental chamber ofLION. A more detailed description of the setup can be found in SectionA.2.

Considering the improvements and the upgrade of the laser system,CALAis well poised for laser-driven ion acceleration in the next years. It is worth considering some immediate ideas and follow up tasks that can arise over the next years.

1. With the increased laser energy and meliorated contrast of the laser, proton energies of about 30-40 MeV should be achievable on a regular basis. Such ion energies will enable applications such as proton imaging and irradiation of more complex three dimensional biological models.

2. A second set ofQPsallows for using the first set for collimation and the second set for refocusing (shorter focal length), thus generating higher peak dose rates in even smaller ion foci. Assuming that the focus of the PMQ would have a radius of 5µm at 10 MeV with 1 % energy spread and a proton spectrum

2Private communication with Jens Hartmann.

of 10⁸/(1%E·msr), we could get about (assuming an acceptance of 10 msr) 1·10⁹ protons within 78µm² and thus about 10¹⁵ protons cm².

3. Besides the divergence, the energy spread of the ion bunch leads to increasing temporal dispersion with increasing distance from the source. A temporal re-compression of the ion bunch, typically referred to as re-bunching, could further increase the ion peak flux at application site.

4. The detector ofI-BEATis planned to be advanced to measure the 3D heating map and thus enable reconstructing the 3D spatio-energy distribution of the ion bunch.

5. With respect to advancing the target (ion source), it is planned to implement the Paul trap [237] into LION. Isolated micro-plasmas have shown great po-tential for laser-driven ion acceleration [54]. The temporal resolved probing technique could be used to study this interaction and acceleration process.

6. With improved contrast condition, the use of newly developed foil based plasma mirrors, higher laser intensities, and target optimizations, one long-term goals is to demonstrate an acceleration of ions beyond 100 MeV per nucleon.