Ernst E. Fill

Power scaling of a high-repetition-rate enhancement cavity

A passive optical resonator is used to enhance the power of a pulsed 78 MHz repetition rate Yb laser providing 200 fs pulses. We find limitations relating to the achievable time-averaged and peak power, which we distinguish by varying the duration of the input pulses. An intracavity average power of 18 kW is generated with close to Fourier-limited pulses of 10 W average power. Beyond this power level, intensity-related effects lead to resonator instabilities, which can be removed by chirping the seed laser pulses. By extending the pulse duration in this way to 2 ps, we could obtain 72 kW of intracavity circulating power with 50 W of input power.

Sub-fs electron pulses for ultrafast electron diffraction

We present a new concept for an electron gun generating subrelativistic electron pulses with a duration down to the attosecond range. It is based on a cylindrical RF cavity (a so-called pill-box cavity) oscillating in its TM010 eigenmode with a photocathode triggered by a fs-laser pulse. Injecting electrons at an appropriate phase of the RF cycle compensates for their initial velocities and time delays and makes the electrons arrive at a target in a sub-fs temporal window. Such electron pulses will allow nuclear motion and electronic dynamics to be studied on an attosecond time scale.

Hybrid dc–ac electron gun for fs-electron pulse generation

We present a new concept of an electron gun for generating subrelativistic few-femtosecond (fs) electron pulses. The basic idea is to utilize a dc acceleration stage combined with an RF cavity, the ac field of which generates an electron energy chirp for bunching at the target. To reduce space charge (SC) broadening the number of electrons in the bunch is reduced and the gun is operated at a megahertz (MHz) repetition rate for providing a high average number of electrons at the target. Simulations of the electron gun were carried out under the condition of no SC and with SC assuming various numbers of electrons in the bunch. Transversal effects such as defocusing after the dc extraction hole were also taken into account. A detailed analysis of the sensitivity of the pulse duration to various parameters was performed to test the realizability of the concept. Such electron pulses will allow significant advances in the field of ultrafast electron diffraction.

Megawatt-scale average-power ultrashort pulses in an enhancement cavity

We investigate power scaling of ultrashort-pulse enhancement cavities. We propose a model for the sensitivity of a cavity design to thermal deformations of the mirrors due to the high circulating powers. Using this model and optimized cavity mirrors, we demonstrate 400 kW of average power with 250 fs pulses and 670 kW with 10 ps pulses at a central wavelength of 1040 nm and a repetition rate of 250 MHz. These results represent an average power improvement of one order of magnitude compared to state-of-the-art systems with similar pulse durations and will thus benefit numerous applications such as the further scaling of tabletop sources of hard x rays (via Thomson scattering of relativistic electrons) and of soft x rays (via high harmonic generation).

The asterix III pulsed high-power iodine laser

A system description and first results of the Asterix III high-power iodine laser built at IPP Garching are given. This laser is designed to yield an output energy of 1 kJ in about 1 ns. Until now pulses with output energies up to 300 J and pulse lengths ranging from 1 to 3 ns have been obtained.

Large-mode enhancement cavities

In passive enhancement cavities the achievable power level is limited by mirror damage. Here, we address the design of robust optical resonators with large spot sizes on all mirrors, a measure that promises to mitigate this limitation by decreasing both the intensity and the thermal gradient on the mirror surfaces. We introduce a misalignment sensitivity metric to evaluate the robustness of resonator designs. We identify the standard bow-tie resonator operated close to the inner stability edge as the most robust large-mode cavity and implement this cavity with two spherical mirrors with 600 mm radius of curvature, two plane mirrors and a round trip length of 1.2 m, demonstrating a stable power enhancement of near-infrared laser light by a factor of 2000. Beam radii of 5.7 mm × 2.6 mm (sagittal × tangential 1/e(2) intensity radius) on all mirrors are obtained. We propose a simple all-reflective ellipticity compensation scheme. This will enable a significant increase of the attainable power and intensity levels in enhancement cavities.

A novel tape target for use with repetitively pulsed lasers

A compact tape drive applicable in experiments with repetitively pulsed lasers is described. By exposing fresh material to the laser pulse at each shot, it is possible to obtain up to 104 shots in a single run. A particular tape can be used many times. The tape target provides stable output, is easy to adjust, and is low cost in operation. The possibility of illuminating samples with the rear-side emission is a further advantage for many applications. Experiments are described in which a copper tape, 10 m long, 12 mm wide, and 25 μm thick, is used to generate Cu Kα radiation. Applying 2 TW titanium–sapphire laser pulses, yields 2.7×109 Kα photons/Sr per shot. Bragg and powder spectra generated with this radiation are shown.

Sensitivity of lasing in neonlike zinc at 21.2 nm to the use of the prepulse technique.

We report strong lasing in zinc on the neonlike 3p → 3s, J = 0 → 1 transition at 21.2 nm when the prepulse technique of using a low-intensity prepulse before the main optical drive pulse is used with the 1.315-μm Asterix laser to illuminate the zinc target. Without the use of the prepulse, this line is not visible. The usual J = 2 → 1 lines at 26.2 and 26.7 nm are observed weakly both with and without the use of the prepulse. By variation of the prepulse energy, we demonstrate that even a very weak prepulse with less than 0.1% of the main pulse energy can enable the zinc to lase at 21.2 nm.

Ultrabroadband efficient intracavity XUV output coupler

We report an efficient intracavity XUV output coupler based on an anti-reflection-coated grazing incidence plate (GIP). Conceptually, GIP is an extension of a Brewster plate, affording low loss of the circulating fundamental light and serving as a highly efficient, extremely broadband output coupler for XUV. Due to the grazing incidence geometry, the short wavelength reflectivity can be extended to the keV range. The first GIP realized shows parameters close to the design. We discuss both the limitations of the GIP in comparison with other XUV output couplers and the applicability of the GIP extension at longer wavelengths, down to the MIR.

Study of Ne- and Ni-like x-ray lasers using the prepulse technique

Recent studies of lasing in Ne- and Ni-like ions on the Asterix IV iodine laser [H. Baumhacker et al. Appl. Phys. B 61, 325 (1995)] using the prepulse technique are reviewed. Experimental evidence shows that beam refraction is the main factor for the lack of lasing in low-Z elements, as well as the J=0−1 vs J=2−1 anomaly in Ne-like ion lasers when there is no prepulse. It is shown that the role of the prepulse in enhancing the J=0−1 lasing line in Ne-like ion is to produce a larger and more homogeneous plasma. The measurement of lasing on the J=0−1, 3p−3s transition in Ne-like Mn, V, Sc, Ca, K, Cl, S, and Si using the prepulse technique is reviewed. Wavelengths of these lasers range from 22 to 87 nm with gain lengths between 7 and 12. The drive energy for S was scaled down to 20 J. The experiment demonstrating the 12 nm lasing on the J=0−1, 4d−4p transition in Ni-like Sn is also reviewed.