Michal Nevrkla

Time-resolved XUV radiation diagnostics from nitrogen discharge Z-pinch plasma

XUV radiation from nitrogen filled capillary discharge plasma was diagnosed using a 104 grooves/mm SiNx free-standing transmission grating. The resolution bandwidth of 0.3 nm was achieved. Time dependence of 13.4 nm line emitted power was recorded by photomultiplier in order to verify inherence of resonant radiation emission corresponding to NVII 2-3 laser transition. An increase of emitted power is expected during the pinch decay caused by recombination processes. We report here results obtained with 90 mm long capillary discharge supplied by a current pulse with maximum amplitude of 50kA and quarter-period of 80 ns. This high-current pulse was generated by a 1.5 ohm water line high-voltage generator which is used for underwater wire explosion experiments and which was adjusted for capillary discharge design using results of PSPICE simulations. Initial nitrogen pressures were varied in the range of 20 ÷ 500 Pa. MHD and kinetic simulations of the discharge plasma were performed and compared with experimental data. Simulations were performed with presumption of wall ablation. The capillary wall and electrodes material emission lines were also identified in measured spectra.

Table-top water-window soft X-ray microscope using a Z-pinching capillary discharge source

The development and demonstration of a table-top transmission soft X-ray (SXR) microscope, using a laboratory incoherent capillary discharge source has been carried out. This Z-pinching capillary discharge water-window SXR source, is a first of its kind to be used for high spatial resolution microscopy at λ = 2.88 nm (430 eV) . A grazing incidence ellipsoidal condenser mirror is used for focusing of the SXR radiation at the sample plane. The Fresnel zone plate objective lens is used for imaging of the sample onto a back-illuminated (BI) CCD camera. The achieved half-pitch spatial resolution of the microscope approaches 100 nm, as demonstrated by the knife-edge test. Details about the source, and the construction of the microscope are presented and discussed. Additionally, the SXR images of various samples, proving applicability of such microscope for observation of objects in the nanoscale, are shown.

Focusing and photon flux measurements of the 2.88-nm radiation at the sample plane of the soft x-ray microscope, based on capillary discharge source

Feasibility measurements leading to the development of a Soft X-ray (SXR) microscopy setup, based on capillary discharge XUV source is presented. Here the Z-pinching plasma is acting as a source of XUV radiation, emitting incoherent radiation in the “water-window” (λ = 2.3 – 4.4 nm) region of interest (natural contrast between the carbon and oxygen edges).This soft X-ray microscopy setup will realize imaging of the biological objects with high spatial resolution. The 2.88 nm radiation line is filtered out from the water-window band, and is focused by an axi-symmetric ellipsoidal mirror, coated with nickle. The focussed spot size is measured and reported. Flux measurements for the available number of photons (photons/pulse) at the sample plane has been carried out with AXUV PIN diode at the sample plane (slightly out of focus). For imaging, a fresnel zone plate lens will be used as an objective. The overall compact transmission SXR microscopy setup design is presented.

Discharge driver for 13.4 nm XUV laser

During adiabatic expansion of Z-pinching plasma column can arise conditions suitable for amplified spontaneous emission (ASE) pumped by 3-body recombination. MHD and kinetic simulations had shown1, that capillary discharge with current amplitude I > 60 kA and slope dI/dt > 2 × 1012 As-1 can produce plasma condition for ASE of H-like nitrogen Balmer-alpha line at 13.4 nm. Discharge driver capable to deliver up to 90 kA with rise-time ~25 ns into 22 cm long capillary was designed. Driver is based on water slab capacitor pulse compression stages switched by spark-gap. Capillary is charged symmetrically with voltage up to ±160 kV. -100 A high-frequency current pulse is used to pre-ionize gas in the capillary. The driver will be used primarily as an experimental device to study capillary plasma for ASE by 3-body recombination, and secondarily as a source of high intensity incoherent XUV radiation in water-window region.

Development and demonstration of a water-window soft x-ray microscope using a Z-pinching capillary discharge source

The development and demonstration of a soft X-ray (SXR) microscope, based on a Z-pinching capillary discharge source has been realized. The Z-pinching plasma acts as a source of SXR radiation. A ceramic capacitor bank is pulsed charged up to 80 kV, and discharged through a pre- ionized nitrogen filled ceramic capillary. The discharge current has an amplitude of ~25 kA. Working within the water-window spectral region (λ = 2.88 nm), corresponding to the 1s2-1s2p quantum transition of helium-like nitrogen (N5+), the microscope has a potential in exploiting the natural contrast existing between the K-absorption edges of carbon and oxygen as the main constituents of biological materials, and hence imaging them with high spatial resolution. The SXR microscope uses the grazing incidence ellipsoidal condenser mirror for the illumination, and the Fresnel zone plate optics for the imaging of samples onto a BI-CCD camera. The half- pitch spatial resolution of 100 nm [1] was achieved, as demonstrated by the knife-edge test. In order to enhance the photon-flux at the sample plane, a new scheme for focusing the radiation, from multiple capillary sources has been investigated. Details about the source, and the construction of the microscope are presented and discussed.

Time-resolved EUV spectra from nitrogen Z-pinching capillary discharge

Time-integrated spectra and time-resolved spectra (20 ns resolution) of nitrogen discharge plasma radiation were recorded and analyzed. Plasma was created by a 70 kA, 29 ns rise-time current pulse flowing through a 5 mm inner diameter, 224 mm long capillary filled with nitrogen to initial pressure ∼0.1 ÷ 1 kPa. Spectra were captured in the wavelength range 8.3 ÷ 14 nm. This spectral region contains nitrogen Balmer series lines including potentially lasing NVII 2 – 3 transition1. Spectral lines were identified using the NIST database and the FLY kinetic code. Together with spectra the capillary current was measured. Due to the low inductance design of the driver, the pinch is observable directly from the measured current. 13.38 nm NVII 2 – 3 line was observed in gated, and also in time-integrated spectra for currents <60 kA. For higher gas-filling pressure also other Balmer series lines were observed.

Dynamics of pre-ionized fast capillary discharge

The goal of this work is to determine the best conditions for pre-ionization of the nitrogen filled capillary plasma column applying an external exponentially damped or high-frequency alternating current. As we supposed, optimal pre-ionization conditions are achieved when the plasma is quiescent, motionless and isothermal, near the local thermodynamical equilibrium. At the time of optimal conditions for the pre-ionization plasma column, the main pulse is applied. This approach enables us to estimate the influence of such prepared plasma on the value of emitted energy during the main current pulse. For modeling of plasma during the pre-pulse and main pulse, the magneto-hydro-dynamics (MHD) NPINCH code [1] and the radiative-MHD Z* code [2] were used. The computer results are used for further improvement of x-ray–ultraviolet-capillary sources designed in IPP ASCR and CTU FNSPE laboratories in Prague.

Measurement of characteristics of a XUV capillary laser

This work concerns in measurement of characteristics of a XUV argon capillary laser, which was developed at the Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering. This laser generates at 46.9 nm in Ne - like Ar. Youngs double pinhole experiment was realized to estimate spatial coherence of this laser system and to detect the beam profile. We used double pinholes drilled by Ti:saphire laser with four different pinhole separations. pinhole separations were 50 μm, 60 μ m, 100 μm and 150 μm. Apertures had oval shape with diameter 20 - 25 μ m. Interference structure was detected by XUV CCD camera with resolution 512x512 pixels. This work contains also short overview about sources of XUV radiation.

XUV Radiation Emitted by Capillary Pinching Discharge

Fast capillary discharge in nitrogen is studied as a source of incoherent monochromatic radiation in “water window” wavelength range. Discharge system with current amplitude 13.5 kA and half period 140 ns was designed, realized and modeled. Strong spectral line at 2.88 nm corresponding to the quantum transition 1s2p→1s2 of helium -like nitrogen ions was detected. The initial pressure nitrogen in the alumina capillary was varied from 10 to 220 Pa. Peak value of the line intensity at about 50 Pa was found and proved by the computer modeling.

Design and realisation of apparatus to study capillary discharge in gas

Summary form only given. A compact low-inductance capillary discharge system designed for the study of a capillary plasma is described. The system is able to produce a plasma Z-pinch in a gas by the switching of a 30 nF capacitor bank into a gas loaded in a 21 cm long alumina capillary at a voltage up to 80 kV. The capillary current pulse is measured by a Rogowski coil and at 40 kV charging voltage its maximum is 10 kA, the pulse width is 170 ns and rise time is 60 ns. The system is intended to be optimized further to obtain a Ar8+ laser.