Dear colleagues and friends of the HI Jena,

welcome to the November issue of our institute newsletter.
Below you find informations and news about recent activities of the HI Jena.

Kind Regards,
Helmholtz Institute Jena

Momentum resolved study of the saturation intensity in multiple ionization

Strong-field ionization of atoms is of fundamental interest for many phenomena like laser-based electron or ion acceleration by ultra-intense laser pulses. When atoms are exposed to super-intensive or relativistic laser intensities, they will be ionized to high charge states. In the optical regime, the ionization probability depends highly nonlinear on the field strength. Therefore, for a pulsed field, ionization is concentrated in a narrow intensity and a correspondingly narrow time interval for each ionization step. This intensity where ionization peaks is known as saturation intensity. An accurate modeling of the ionization dynamics over a large range of charge states plays an important role in strong-field laser physics and plasma physics [1].

Figure 1: a) Measured ion momentum distribution of Ne³⁺, b) Simulated momentum distribution for Double ionization from fit method, Simulated momentum distribution for first (c) and second (d) single ionization from fit method.

In this contribution we report on the recent momentum resolved study of strong field multiple ionization of ionic targets [2]. A beam of Ne⁺ ions is ionized up to charge state 5 by an elliptically polarized laser field with peak intensities of up to about 10¹⁷ W/cm². The three-dimensional momentum distributions are reconstructed from the time and position information recorded for each ion by a delay-line detector [3].
In order to quantify the observations and to subsequently enable the determination of photoelectrons’ momenta for each ionization step, a method to deconvolve the measured momentum distribution of multiply ionized ions was developed (see Fig.1). This technique allows to reconstruct the electron momenta from the ion momentum distributions after multiple ionization up to four sequential ionization steps and to extract the saturation intensities as well as of the electron release times during the laser pulse [4].
Thus, the typically large experimental uncertainties in the intensity determination are removed. This allows the retrieval of averaged ionization times of all charge states created in the laser focus (see Fig.2). The measured results are compared with predictions of frequently used models of strong field ionization in the quasistatic tunneling and over-the-barrier regime. Therefore, the method can be used to verify the modeling of the ionization dynamics, which is the basis of all strong-feld laser matter interaction, for example in plasma physics.

Figure 2: a) The radial electron momenta are depicted in vertical direction. The time of ionization can be obtained by projection on the envelope of the vector potential in the respective focal volume. b) electron momenta as function of the charge state n of the respective individual ionization event for the different final charge states k. The solid circles show measured values. Open circles and rectangles are the results of calculations based on tunneling ionization rates.

References:
[1] M. Chen, E. Cormier-Michel, C. G. R. Geddes, D., L. Bruhwiler, L. L. Yu, E. Esarey, C. B. Schroeder, and W. P. Leemans, J. Comput. Phys. 236, 220 (2013).
[2] P. Wustelt, M. Möller, T. Rathje, A. M. Sayler, T. Stöhlker, and G. G. Paulus, Phys. Rev. A 91, 031401(R) (2015).
[3] T. Rathje, A. M. Sayler, S. Zeng, P. Wustelt, H. Figger, B. D. Esry, and G. G. Paulus, Phys. Rev. Lett. 111, 093002 (2013).
[4] A. N. Pfeiffer, C. Cirelli, M. Smolarski, R. Dörner, and U. Keller, Nat. Phys. 7, 428 (2011).

News and Announcements

Johannes Ullmann awarded with “Giersch Excellence Award”

Johannes Ullmann received the Giersch Excellence Grant on October 13, 2015 for his works on the determination of the hyperfine splitting energy of the ground state of hydrogen- and lithiumlike Bismuth by laser spectroscopy.
The Experiment was conducted at the experimental storage ring of the Helmholtz center for heavy ion research in Darmstadt and allows a precise test of quantum electrodynamics in extreme electric and magnetic fields, which are experienced by the ground state electron in a highly charged, heavy ion. Uncertainties in the theoretical prediction of nuclear effects on the hyperfine splitting were eliminated by measuring two charge states of the same isotope.
Although the hyperfine splitting energy in hydrogen-like Bismuth is known since 1994, the resonance in the lithium-like charge state was not found until 2011. Due to too large uncertainties in the determination of the ions' velocity, no accurate value could be determined then.
An in-situ high voltage measurement was therefore installed in collaboration with Physikalisch-Technische Bundesanstalt, the national metrology institute of Germany. In combination with extensive improvements of detection systems and data acquisition, the uncertainty could be reduced by one order of magnitude. An improved value of the hyperfine splitting energy in hydrogen-like bismuth has been published recently in Journal of Physics B. The upcoming finalization of analysis of the lithium-like Bismuth data in connection with the published value promises the first accurate test of quantum electrodynamics in strong fields.

Opening of the Marburg Ion Beam Therapy Center — cancer treatment using a process developed at GSI

The Marburg Ion Beam Therapy Center was ceremonially opened on November 11, 2015. Treatment of the first patients at the center had started in October. Heavy ion therapy was developed at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt. The accelerator facility was successfully used for the treatment of tumor patients between 1997 and 2008. A new accelerator facility of this kind is now going into operation in Marburg. It is the second such facility in Germany that is attached to a clinic and is capable of treating large numbers of patients.

The MIT will offer an efficient form of cancer therapy, with minimal side effects, to as many as 750 patients per year. Following the example of the Heidelberg Ion Beam Therapy Center, the Marburg facility uses a process of irradiation with ions that is based on research and development work done by GSI, the Heidelberg University Hospital, the German Cancer Research Center (DKFZ), and the Helmholtz Zentrum Dresden-Rossendorf.

“We are delighted that the Marburg facility is now completed and that from now on more patients will be able to benefit from the extremely effective and gentle process of ion beam therapy we developed at GSI,” said Gerhard Kraft, the former head of the Biophysics department at GSI. “It’s an outstanding example of how basic research can benefit society and individuals thanks to successful technology transfer.” Karlheinz Langanke, the Scientific Director of GSI, said, “This is also a great personal success for Gerhard Kraft, the founder of ion therapy at GSI and a pioneer in Europe.”

The first promising biological experiments and technical developments related to an innovative technology for irradiating tumors with heavy ions were already being conducted at GSI during the 1980s. Biophysicists worked closely with accelerator physicists, technicians, and physicians to further develop the accelerator facility for cancer therapy. The same accelerator that was used for studying supernovae and neutron stars was to be applied to medical treatment for human beings. The first clinical study was conducted jointly with the Heidelberg University Hospital, the DKFZ, and the Helmholtz Zentrum Dresden-Rossendorf from 1997 to 2008. A total of 444 patients, most of whom suffered from basal skull tumors, were treated using beams of carbon ions from the GSI accelerator facility with great success.

This process is especially effective and gentle, because the ion beams penetrate into the body and have a particularly strong effect in the tumor tissue, where they are absorbed. In addition, the ion beams’ effect can be directed with millimeter precision to individual points within the malignant tumor by means of the raster scan process developed at GSI, so that the healthy surrounding tissue is spared. The experience gained from the GSI pilot project flowed directly into the design of an accelerator facility that is intended specifically for therapeutic use and designed to make routine clinical procedures possible. The Heidelberg Ion Beam Therapy Center HIT was then constructed on this basis. A significantly smaller accelerator facility was developed by GSI for this center. The Marburg facility has also been constructed according to this model.