04 Mar TW-class table top laser system for proton/ion generation with respect to cancer research
1 The Laser system
In Monocrom we have tackled the challenges of generating high power ultra-short laser pulses by building a CPA laser system that is pumped by several MOPA systems. We used titanium doped sapphire (Ti:Sa) as the active material for both the oscillation and amplification stages. Based on the our expertise with laser diode-based pumping heads (PH) applied to materials like Nd:YAG [1064nm] or Nd:YLF [1053nm] all pumping systems used in this setup rely on this technology . Compared to other well-established techniques (flash lamp-based pumping) our PHs allow much higher repetition rates (up to the kHz regime) and higher long-term stability.
Figure 1: Schematic overview of a chirped pulse amplifier laser system.
The CPA laser system is comprised of four main sub-systems. The first one consists of an oscillator that generates low energy femtosecond pulses at a base rep rate of several MHz. A pulse picker selects 1 out of 106 of all generated pulses to be injected into a stretcher, which uses a diffracting grating with 1200 grooves per mm and is designed in an on-axis Oeffner configuration. After the stretcher the amplification chain begins, consisting of three stages, one regenerative and two multi-pass amplifiers (Figure 1). All of them are pumped by diode pumped frequency converted solid state laser. Once the pulse has reached proper energy, a compressor using identical diffraction gratings as in the stretcher reverse the pulse length to its original temporal state, giving rise to a TW peak power. To avoid air-breakdown, the compressor is placed in a vacuum chamber with a Treacy design. The pulse is then finally focused into the target for proton/ion generation (Figure 2).
Figure 2: Schematic overview of a dual output MOPA pump laser setup used to pump the last amplifier stage in the CPA.
Figure 3: A strong and ultra-short laser pulse is directed onto a thin target and the generated particles inside the plasma are accelerated by means of its high electric field. Caused by the mass difference between electron and protons (mp/me≈ 2000) both particle species are well separated after a short distance in vacuum.
2 Proton/ion generation
The power densities achieved are so high (>1018 W/cm2) that the laser basically acts as a particle accelerator when it reaches the target surface. After striking, electrons are directly accelerated and penetrate the target. Most of them spread and dissipate energy inside of it, but the most energetic ones escape, leaving behind an electrostatic potential which generates an electric field that ionizes and accelerates surface ions in a process called Target Normal Sheath Acceleration (TNSA).
As a consequence, an intense collimated beam of high-energy protons is emitted normal to the rear surface of the irradiated targets. This beam presents a spectrum of energies, typically with an exponential profile, and present three main features (see Figure 4): a large energy spread, a smooth decrease of particle number with energy and, finally, an abrupt “cutoff” that delimits the maximum proton energy achievable and determines the experimental scaling laws for the acceleration process. This spectrum is highly dependent on the target properties, especially considering their thickness. After different experiments, it has been proved that the maximum proton energy accelerated via TNSA can be enhanced by using thinner targets.
Despite the promising results, proton accelerated energies are still low to be applied in cancer treatments. Nonetheless, the advantages over existing techniques like cyclotron accelerators (reduced radiation shielding requirements, compactness, energy consumption or economic viability from institutions with constrained means) makes further developments to be worth the effort. That is why advances are being made to increase the laser peak power up to the sub-petawatt level, all relying on Monocrom current and improved pumping modules.
Figure 4: The graph visualizes the particle yield per narrow solid angle over particle energy for different target thicknesses. The maximum particle energy reached is clearly depended on the target thickness used. Spectral distribution is measured by a time of flight detector.