Hans-Joachim Kunze

Lasing mechanism in a capillary discharge

Abstract The possibility is analyzed that charge exchange between plasma ions preceded by an m =0 instability is responsible for amplified spontaneous emission observed on the CVI Balmer-alpha line emitted from a capillary discharge.

Charge-exchange collisions in interpenetrating laser-produced magnesium plasmas

Charge-exchange collisions are one of the effective pumping methods for soft X-ray lasers. Experiments are performed to investigate charge-exchange collisions between highly charged Mg ions in colliding laser-produced magnesium plasmas. Pinhole photography and XUV spectroscopy are used as diagnostic tools. Spectroscopic studies show selective population of n = 3 levels of Mg IX ions, which results in enhancement of respective line intensities. Theoretical calculations also give a large cross section as high as 10 -15 cm2 for these charge-exchange collisions when the relative velocities of the colliding ions are of the order of 10 7 cm s -1 . XUV pinhole pictures are taken in early stages, which give more insight into the expansion dynamics of the colliding magnesium plasmas.

In-band EUV radiation of ablative capillary discharges in PVC

A study of the performance of an ablative capillary discharge through a PVC capillary is reported. It is shown that the PVC capillary is capable of producing strong in-band 13.5 nm EUV radiation. This band of radiation is produced by transition arrays of multiply ionised heavy metal ions (Sn, Zn, etc.) that are constituents of various chemical additives in commercial PVC grades. A low concentration of radiating ions, good thermalisation of the capillary plasma and plasma homogeneity along the capillary axis produce well-defined bands with little line radiation outside the band of interest. Two interesting cases are discussed, both producing band radiation with a full-width at half-maximum of around 1 nm and maxima near 10, 13.5 and 15.5 nm, respectively. The plasma spectra are not very sensitive to input discharge voltage, but in some cases the amount of energy in the band of interest can be controlled by it. The most serious problem of such discharges still remains the amount of macroscopic debris produced by the capillary ablation.

Aluminum plasma produced by a nitrogen laser

13 mJ laser pulses from a nitrogen laser were focused onto an aluminum target in air. The target surface was perpendicular to the axis of the laser beam. A peak energy density of 1.3 J/cm2 and a power density of 80 MW/cm2 were achieved with a laser pulse duration of 16ns. This high power density produced a transient plasma cloud that expanded explosively into the surrounding atmosphere. An initial electron density of about 1 × 1019 cm−3 and an electron temperature of about 2 eV were determined by optical spectroscopy. The line of sight was parallel to the surface and perpendicular to the laser beam axis. The height of the line of sight above the target surface was varied in order to gather data about the whole plasma cloud. In about 500 ns the plasma cloud expands to about 0.5 mm above the target surface, cools down to about 1.2 eV and is tenfold reduced in electron density. The initial expansion velocity was determined to be about 2 km/s.

Experiments and simulations on the possibility of radiative contraction/collapse in the PF-1000 plasma focus

Abstract Experimental studies of discharges in the plasma focus facility with neon filling and respective numerical simulations employing the radiative Lee code are reported. The pinch currents exceed the Pease-Braginskii current, which indicates that radiative losses are larger than heating and that contraction of the formed plasma should occur. Both of these effects were indeed observed. Parallel numerical simulations were crucial for the identification of such an effect.

Radiative Processes in Plasmas

All plasmas emit and absorb electromagnetic radiation and Fig. 6.1 shows, as an example, the characteristics of the emission spectrum of a dense laboratory hydrogen plasma with some impurities at the electron temperature of k B T e = 10 eV. The dashed line is the blackbody limit drawn for comparison. We identify the following contributions: – Bremsstrahlung (dot–dashed contribution), a continuum radiation, is emitted when the electrons experience deflection in the electric field of the ions. At long wavelengths the optical depth becomes large and the bremsstrahlung approaches the blackbody limit (Planck function). – Recombination radiation, also a continuum but characterized by edges. It is emitted when electrons recombine with ions. – Line radiation corresponds to transitions of electrons between levels in atoms and ions, and at low temperatures also in molecules. Lines may become optically thick especially at long wavelengths and then they also reach the blackbody limit.

A SIMPLE XUV SOURCE AT 13.5 NM BASED ON ABLATIVE CAPILLARY DISCHARGE

A XUV source that produces a strong emission band at the wavelength of 13.5 nm with a FWHM of 0.6 nm and a duration of about 100 ns is described. In particular this wavelength has attracted the attention of many scientists working in the field by being a good candidate for the development of XUV lithography. The source was generated by using an ablative capillary discharge where the capillary was made of PVC (polyvinyl chloride). A remarkable burst of radiation at the above wavelength was recorded, the intensity of the radiation being higher by a factor of 10 in the spectral region of interest, as compared to usually used capillaries made of POM (polyacetal), or to recently developed capillary discharges in noble gases. Total XUV radiation energy of up to 50 mJ per pulse seems to be possible with such a device. Due to its simplicity, the described capillary discharge is a good candidate for a simple incoherent XUV source at 13.5 nm.

PF1000 High-Energy Plasma Focus Device Operated With Neon as a Copious Soft X-Ray Source

The measured current waveforms of PF1000 plasma focus operating in neon are first fitted to the computed currents using the Lee model for the pressures of 0.5 and 0.8 torr. The fitting fixes the model parameters. The code is then run to study and to optimize the soft X-ray yield from (~352 kJ) PF1000 device. The maximum characteristic soft X-ray (H-like and He-like lines) yield of 4.2 kJ is found to occur at the pressure of 0.85 torr, with the pinch duration of 207 ns and with an all-line yield of 4.8 kJ. Maximum compression (corresponding to smallest pinch radius) of 0.585 cm with the duration of 224 ns is obtained at 1 torr with the greatest all-line yield of 7.1 kJ but a lower characteristic soft X-ray yield of 3.2 kJ. Detailed computation results indicate that the maximum compression (minimum pinch radius) at 1 torr is attributed to two mechanisms: thermodynamics specific heat ratio effects and radiative losses.

Evidence of lasing on the Balmer-α line of OVIII in an ablative capillary discharge

In a low-inductance ablative discharge through a capillary made of polyacetal (POM), lasing on the Balmer-α line of OVIII at 10.24 nm is identified. In line with previous studies of lasing on CVI ions, it is argued to be the consequence of charge exchange collisions after a m=0 instability. Lasing in both cases occurred at about the same time after beginning of the discharge, although lasing on the Balmer-α line of OVIII was less frequently observed, i.e., in approximately one out of ten discharges. Lasing on the CVI ion was seen in one out of three discharges. This is probably due to the need of reaching higher electron temperatures to completely strip oxygen ions simultaneously in the hot constrictions (necks) of the plasma instability.

Quantities of Spectroscopy

The total energy emitted by a plasma as electromagnetic radiation per unit time is called its radiative loss and plays a crucial role in all power balance considerations. As a physical quantity it is a radiant flux Ф (through the surface of the plasma), and its unit is watt (W). It is also called radiant power. Radiant flux density φ refers to the flux per unit area φ = dΦ/dA with the unit Wm−2, irrespective of whether the radiation is emitted from an area, crosses an imaginary surface in space, or falls onto an area A. In this latter case, it is customary to call this flux density at the surface irradiance E. The energy deposited per unit area during a given time is the fluence H = ∫ Edt, with the unit Jm−2.