Maria Nicolai

Establishment of technical prerequisites for cell irradiation experiments with laser-accelerated electrons.

PURPOSE In recent years, laser-based acceleration of charged particles has rapidly progressed and medical applications, e.g., in radiotherapy, might become feasible in the coming decade. Requirements are monoenergetic particle beams with long-term stable and reproducible properties as well as sufficient particle intensities and a controlled delivery of prescribed doses at the treatment site. Although conventional and laser-based particle accelerators will administer the same dose to the patient, their different time structures could result in different radiobiological properties. Therefore, the biological response to the ultrashort pulse durations and the resulting high peak dose rates of these particle beams have to be investigated. The technical prerequisites, i.e., a suitable cell irradiation setup and the precise dosimetric characterization of a laser-based particle accelerator, have to be realized in order to prepare systematic cell irradiation experiments. METHODS The Jena titanium:sapphire laser system (JETI) was customized in preparation for cell irradiation experiments with laser-accelerated electrons. The delivered electron beam was optimized with regard to its spectrum, diameter, dose rate, and dose homogeneity. A custom-designed beam and dose monitoring system, consisting of a Roos ionization chamber, a Faraday cup, and EBT-1 dosimetry films, enables real-time monitoring of irradiation experiments and precise determination of the dose delivered to the cells. Finally, as proof-of-principle experiment cell samples were irradiated using this setup. RESULTS Laser-accelerated electron beams, appropriate for in vitro radiobiological experiments, were generated with a laser shot frequency of 2.5 Hz and a pulse length of 80 fs. After laser acceleration in the helium gas jet, the electrons were filtered by a magnet, released from the vacuum target chamber, and propagated in air for a distance of 220 mm. Within this distance a lead collimator (aperture of 35 mm) was introduced, leading, along with the optimized setup, to a beam diameter of 35 mm, sufficient for the irradiation of common cell culture vessels. The corresponding maximum dose inhomogeneity over the beam spot was less than 10% for all irradiated samples. At cell position, the electrons posses a mean kinetic energy of 13.6 MeV, a bunch length of about 5 ps (FWHM), and a mean pulse dose of 1.6 mGy/bunch. Cross correlations show clear linear dependencies for the online recorded accumulated bunch charges, pulse doses, and pulse numbers on absolute doses determined with EBT-1 films. Hence, the established monitoring system is suitable for beam control and a dedicated dose delivery. Additionally, reasonable day-to-day stable and reproducible properties of the electron beam were achieved. CONCLUSIONS Basic technical prerequisites for future cell irradiation experiments with ultrashort pulsed laser-accelerated electrons were established at the JETI laser system. The implemented online control system is suitable to compensate beam intensity fluctuations and the achieved accuracy of dose delivery to the cells is sufficient for radiobiological cell experiments. Hence, systematic in vitro cell irradiation experiments can be performed, being the first step toward clinical application of laser-accelerated particles. Further steps, including the transfer of the established methods to experiments on higher biological systems or to other laser-based particle accelerators, will be prepared.

Optical control of hard X-ray polarization by electron injection in a laser wakefield accelerator

Laser-plasma particle accelerators could provide more compact sources of high-energy radiation than conventional accelerators. Moreover, because they deliver radiation in femtosecond pulses, they could improve the time resolution of X-ray absorption techniques. Here we show that we can measure and control the polarization of ultra-short, broad-band keV photon pulses emitted from a laser-plasma-based betatron source. The electron trajectories and hence the polarization of the emitted X-rays are experimentally controlled by the pulse-front tilt of the driving laser pulses. Particle-in-cell simulations show that an asymmetric plasma wave can be driven by a tilted pulse front and a non-symmetric intensity distribution of the focal spot. Both lead to a notable off-axis electron injection followed by collective electron–betatron oscillations. We expect that our method for an all-optical steering is not only useful for plasma-based X-ray sources but also has significance for future laser-based particle accelerators.

Development of a Superconducting Transverse-Gradient Undulator for Laser-Wakefield Accelerators

Relativistic electrons with small energy spread propagating through undulators produce monochromatic radiation with high spectral intensity. The working principle of undulators requires a small energy spread of the electron beam in the order of ΔE/E ~ 0.1%. Laser-wakefield accelerators can produce electron bunches with an energy of several 100 MeV within a few millimeters acceleration length, but with a relatively large energy spread (ΔE/E ~ 1-10%). In order to produce monochromatic undulator radiation with these electrons, a novel scheme involving transverse-gradient superconducting undulators was proposed in an earlier work. This paper reports on the design-optimization and construction of an iron-free cylindrical superconducting undulator tailored to the particular beam properties of the laser-wakefield electron accelerator at the University of Jena, Germany.

Laser Particle Acceleration for Radiotherapy: A first radiobiological characterization of laser accelerated electrons

In recent years, the technology of laser-based particle acceleration has developed at such a rate that compact and potentially more cost-effective accelerators are promised for medical application, e.g. for high precision hadron radiotherapy. Necessary requirements are the supply of stable and reliable particle beams with reproducible properties, sufficient particle intensities and monoenergetic spectra. Additionally, a precise dose delivery in an appropriate time and the exposure of a desired irradiation field are needed. Beside these physical demands, the consequences on detection and dosimetry as well as the radiobiological effect on living cells have to be investigated for the ultra-short pulsed laser-based particle beams.

Betatron radiation based measurement of the electron-beam size in a wakefield accelerator

We present a spatial and spectral characterization of a laser-plasma based betatron source which allows us to determine the betatron oscillation amplitude of the electrons which decreases with increasing electron energies. Due to the observed oscillation amplitude and the independently measured x-ray source size of (1.8±0.3)μm we are able to estimate the electron bunch diameter to be (1.6±0.3)μm.

SU‐GG‐T‐459: Laser‐Based Particle Acceleration for Future Ion Therapy: Current Status of the Joint Project OnCOOPtics with Special Focus on Beam Delivery and Dosimetry

Purpose: Before the novel technology of laser‐based particle acceleration can be used for clinical applications, several requirements have to be fulfilled. These are the supply of stable and reliable particle beams with reproducible properties, sufficient particle intensity and useable energy spectra. Additionally, a precise dosedelivery in an appropriate time and the exposure of a desired irradiation field are needed. Beside these demands, consequences on dosimetry as well as on the radiobiological effect have to be investigated for ultra‐short pulsed laser‐accelerated particle beams.Method and Materials: The joint project onCOOPtics, an interdisciplinary and multicenter institution focusing on the development of a laser particle accelerator for radiation oncology, is introduced. The worldwide first systematic in vitroirradiations with laser‐accelerated electrons performed with the JeTi laser system will be presented focusing on the experimental setup, practical experiences and on dosimetric and radiobiological results. In a next step, cell irradiation experiments with laser‐accelerated protons have been prepared. Therefore, a dedicated dosimetric system was developed. It is integrated into a device that can be installed at different laser and conventional accelerators and serves also as cell or animal irradiation device. Results: A laser accelerator was successfully optimized for systematic radiobiological experiments performed over 3 months. No significant differences between laser‐accelerated and conventional 6 MeV electron beams were found. An integrated dosimetry and cell irradiation device for systematic in vitro and in vivo experiments with laser‐accelerated protons was developed, characterized, calibrated and successfully tested with both continuous and pulsed protonbeams. Cell irradiations with protons have been started. Conclusion: Laser accelerators can be used for radiobiological experiments, meeting all necessary requirements like homogeneity, stability and precise dosedelivery. Nevertheless, before fulfilling the much higher requirements for clinical application, several improvements concerning i.e. proton energy, spectral shaping and patient safety are necessary. Supported by BMBF (03ZIK445).

Diagnostics of electron beams and plasma wave in laser-plasma accelerators

Relevant techniques for temporal characterization of laser-driven electron bunches as well as accelerating plasma waves are discussed. Emphasis is placed on a combination of two state-of-the-art approaches providing unique temporal information about laser plasma acceleration process and on its applicability to conventional lasers.

Dynamics of electron acceleration in plasmas

We report the first direct temporal observation [1] of self-injected and shock-front injected [2] laser-driven electron acceleration. The dynamics of the plasma wave and an electron bunch duration of 6 fs is obtained.

Complete characterization of laser wakefield acceleration

We report on measurement techniques of the charge, spectrum, divergence, transverse emittance and the first real-time observation of the accelerated electron pulse and the accelerating plasma wave. Our time-resolved study allows a singleshot measurement of the electron bunch duration providing a value of 5.8 +1.9 -2.1 fs full-width at half maximum (2.5+0.8 -0.9 fs root mean square) as well as the plasma wave with a density-dependent period of 12-22 fs. It reveals the evolution of the bunch, its position in the surrounding plasma wave and the wake dynamics. The results afford promise for brilliant, sub-angstrom-wavelength ultrafast electron and photon sources for diffraction imaging with atomic resolution in space and time.

Optical Characterization of Laser-Driven Electron Acceleration

We present the first well-resolved experimental observation of the non-linear formation of a laser-driven plasma wave, its breaking leading to self-injection and acceleration of electrons in the waves electric field in the regime of “Bubble-acceleration”.