OPTICAL AND X-RAY EMISSION SPECTROSCOPY OF HIGH-CURRENT PULSE DISCHARGES OF THE PLASMA-FOCUS TYPE

1. IPPLM OPTICAL AND X-RAY EMISSION SPECTROSCOPY OF HIGH-CURRENT PULSE DISCHARGES OF THE PLASMA-FOCUS TYPE M.J. Sadowski1-2) 1) National Centre for Nuclear Research (NCBJ), 05-400 Otwock, Poland 2) Institute of Plasma Physics and Laser Microfusion (IFPiLM), 01-497 Warsaw, Poland e-mail: marek.sadowski@ncbj.gov.pl ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 01

2. IPPLM Outline 1. Introduction 2. Optical emission spectroscopy of free-propagating plasma streams from Plasma-Focus discharges 3. Optical emission spectroscopy of plasma produced by interactions of intense plasma streams with solid targets 4. X-ray spectroscopy of Plasma-Focus discharges 5. Summary and conclusions ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 02

3. 1. Introduction Plasma studies in Swierk (initially at IBJ, since 1989 at IPJ, and since 2011 at NCBJ) have been concentrated on Multi-Rod Plasma Injectors (RPI) and Plasma-Focus (PF) facilities. In PF-type discharges, a current-sheath layer is accelerated in the axial direction and when it arrives to the coaxial-electrode ends - its part undergoes the radial compression. Therefore, there is formed a dense magnetized plasma column (so-called pinch column). The main phases of PF discharges are well defined, but the PF phenomena still require more detailed experimental and theoretical studies M.J. Sadowski, M. Scholz: Plasma Sources Sci. & Technol. 17 (2008) 024001. High-speed camera pictures of plasma compression in a PF-type discharge. Important phases of a typical PF-type discharge. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 03

4. Plasma-Focus (PF) experiments at NCBJ in Swierk Experimental chamber of the MAJA-PF facility with some diagnostic equipment. Experimental chamber of the PF-360 facility which was a test module for the large PF-1000 machine. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 04

5. PF-1000 experiment at IPPLM in Warsaw Experimental chamber of the PF-1000 facility, which was designed at the IPJ in Swierk and built at the IPPLM in Warsaw. It has been so far the largest Mather-type machine all over the world. The picture was taken during first shots at energy > 1.1 MJ in September 2000, but the facility is still investigated at the International Centre for Dense Magnetized Plasma. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 05

6. PF-1000 experiment -50 ns -30 ns -10 ns 0 ns 10 ns 30 ns 50 ns Hard X-rays emission Fusion-neutrons emission -30 ns 0 ns -20 ns -10 ns 30 ns Correlation of high-speed camera pictures with the X-ray and neutron emission from discharges within the PF-1000 facility. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 06

7. ends of the outer electrode 10 cm 2. Optical emission spectroscopy of free-propagating plasma streams PF-type discharges generate pulsed streams of dense plasma which emits electromagnetic radiation in a wide range of wavelengths, including very intense visible radiation (VR). Time-integrated pictures of the visible radiation from plasma stream generated in PF-1000U, which were taken side-on by means of a CCD camera without (left) and with a red filter (right). The time-integrated pictures do not deliver information how the optical spectrum changes during the discharge.Hence, it is necessary to perform spectroscopic measurements with appropriate exposition and in different instants of the investigated discharge. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 07

8. OES of free-propagating plasma streams in PF-1000 Scheme of the experimental setup for spectroscopic measurements within the PF-1000 facility. Picture of the PF-1000 chamber during positioning the optical collimator for OES measurements. OES measurements were performed at different angles to the symmetry axis of the PF-1000 facility, and in particular at the angle of 670 (i.e. from the centre of the tungsten-insert) and perpendicular side-on (i.e. at a distance of 30 cm from the electrode outlets). ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 08

9. Optical emission spectroscopy (OES) Mechelle900 spectrograph with a CCD camera Wavelength range: 200 nm –1000 nm Total length of spectrum about 41 mm Entrance aperture: 20x150m Gate time (time of exposition): 100 ns – 50ms Delay time setting: 100 ns - 1 ms Resolution tests Source: Hg low-pressure discharge Line 404.7 nm, FWHM = 0.46 nm, / = 900 435.8nm, FWHM = 0.42nm, / =1050 1014.0 nm, FWHM = 0.95 nm, / = 1070 Correlation of OES measurements with hard X-ray and neutron signals, as well as the current waveform. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 09

10. OES of free-propagating plasma streams in PF-1000 Optical spectra of a pulsed plasma-ion stream emitted from the PF-1000 facility, which was equipped with a W-insert in the inner electrode. The measurements were performed from this insert region at different instants after the current dip. For a comparison there is shown a spectrum obtained from a W-target in the laser experiment (red trace). Thin vertical lines show intense WI and WII lines taken from the Kurucz database. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 10

11. OES of free-propagating plasma streams in PF-1000 Comparison of spectra (from 390 nm to 440 nm) obtained from a W-target in the laser experiment (red trace) and those from three PF-1000 discharges (blue, grey and green traces) recorded at different instants after the current dip. E. Skladnik-Sadowska et al., Proc. ICPPCF Alushta-2012, Oral 9-04. Thin lines show the most intense WI and WII lines taken from the Kurucz database. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 11

12. OES of free-propagating plasma streams in PF-1000 D D D D D and D profiles recorded in the PF-1000 experiment, with the exposition of 100 ns, at various instants after the current dip (from 0 to 10 s), and at different distances from electrodes ends. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 12

13. OES of free-propagating plasma streams in PF-1000; determination of electron density from D and D lines Db Da Db ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 13

14. OES of free-propagating plasma streams in PF-1000 Temporal changes of the optical spectra in the wavelength region from 350 nm to 750 nm for different instants after the current peculiarity (dip). ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 14

15. OES of free-propagating plasma streams in PF-1000 Optical emission spectra from plasma produced with the PF-1000 facility equipped with at the W-insert, as recorded perpendicular to the z-axis, at different distances from the electrode outlets. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 15

16. 3. OES of plasma-streams interactions with solid targets Research on behaviour of different materials during interactions of intense plasma streams is of primary importance for fusion technology, because materials to be used in large fusion facilities must be resistant enough to intense electromagnetic and corpuscular radiation. Basing on experience gained during numerous fusion experiments particular attention is paid to special stain-less steels, graphite, CFC and tungsten. To study erosion of these materials the use is made of plasma streams generated by various plasma accelerators (RPI, QSPA) and Plasma-Focus (PF) facilities. In some cases the use can also be made of intense laser pulses. 5 cm Interaction of a pulsed plasma stream from the RPI-IBIS facility with a solid target. Optical spectra recorded from the irradiated W-target. E. Skladnik-Sadowska, et al., Nukleonika 57 (2012) 193-196. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 16

17. Some OES measurements of the visible radiation from plasma, which was produced during interactions of intense deuterium-plasma streams with solid targets made of tungsten or CFC, were carried out within the PF-360 device and a modified PF-1000U facility. E. Skladnik-Sadowska, R. Kwiatkowski, K. Malinowski, et al., VANT No.1(83) (2013) 279-283. Optical spectra recorded in the PF-360 facility with a CFC target. It was impossible to estimate the plasma electron density from an analysis of deuterium Balmer lines in early phases of the plasma expansion because of strong re-absorption effects. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 17

18. Other experiments with irradiation of tungsten and CFC targets by intense deuterium plasma streams have been performed within the modified PF-1000U facility. M. Kubkowska, et al., Phys. ScriptaT161(2014) 014038. Optical spectra obtained for plasma from the CFC target irradiated by PF-1000U discharges. One can identify many lines of carbon ions which appear 1 us after the current dip. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski – 18

19. Recently, particular attention has been paid to interactions of intense deuterium plasmastreams (1-5 MW/cm2) with targets made of a silicon carbide (SiC). The use has been made of high-current plasma discharges in PF-1000U and PF-360U facilities and optical emission spectroscopy (OES). E. Skladnik-Sadowska et al., Proc. ICPPCF, Kharkov, 15-18 Sept. 2014, Oral 6-03; to be published in VANT (2014). Experimental set-up for OES studies within the PF-1000U facility. Exemplary spectrarecorded at different instants of PF-1000U discharges with a SiC-target. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 19

20. Energy Dispersive X-ray Spectroscopy (EDS) of irradiated SiC samples Virgin surface Sample No. 8 Sample No. 9 carbon 100 mm silicon silicon and carbon Distribution of carbon and silicon (mapping) upon surfaces of the virgin and irradiated samples. E. Skladnik-Sadowska et al., to be published in VANT (2014). After the irradiation the silicon distribution became almost uniform and the total quantity of the carbon was decreased. Some other elements (Cu and O) could also bee observed. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 20

21. Energy Dispersive X-ray Spectroscopy (EDS) of irradiated SiC samples Characteristic lines of elements identified by means of the EDS for the SiC sample No. 9 which that was irradiated by 6 successive plasma discharges in the PF-1000U facility. For the irradiated samples, in addition to the C- and Si-lines from the irradiated target one can also identify lines corresponding to other elements (such as Cu, Fe, Al and W). ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 21

22. 4. X-ray spectroscopy of Plasma-Focus discharges In aPF-pinch column there are often formed „current filaments”, which due to various mhd instabilities (mostly of m = 0 type) are broken into high-density micro-regions (hot spots).The hot spots are sources of intense X-ray emissions, as well as sources of pulsed ion- and electron-beams emitted on the opposite directions. The X-ray emission has been investigated within the MAJA-PF device, particularly for discharges with the deuterium filling and a small argon admixture. The X-ray spectra in the wavelength range of 3.8 – 4.2 Å were measured by means of a crystal spectrometers of the Johann type. There were recorded intense lines of highly ionized argon, and particularly Ar-XVII – 1s2p(1P1) – 1s2(1S0) - resonance line, 1s2p(3P1)- 1s2(1S0) – inter-combination line, as well as Li-like (Ar-XVI) satellite lines. L. Jakubowski, M.J. Sadowski et al., AIP CP 812 (2006) 260-263. X-ray lines intensities (A), corresponding X-ray spectra (B), and an X-ray pinhole picture (C), which shows the spatial distribution of hot spots. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 22

23. X-ray pinhole camera measurements Recent studies within the PF-1000U facility concerned the structure of pinch. Examples of X-ray pinhole images of PF-1000U discharges showing „current filaments”. Examples of X-ray pinhole images of PF-1000U discharges showing distinct „hot-spots”. W. Surala, M.J. Sadowski, et al., submitted for publication in Nukleonika (2014). ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 23

24. Previous X-ray studies, as perfromed within the MAJA-PF facility, concerned polarization effects. X-ray spectral lines, as emitted from different parts of the pinch, can have various polarization, which can change considerably when the X-ray spectra are measured at different directions. Scheme of an experimental arrangement. Polarization effects can be measured by means of two identical crystal spectrometers with different orientation of their dispersion planes: A – parallel to the pinch axis, and B – perpendicular to that. MAJA-PF facility equipped with two identical crystal spectrometers (for X-ray studies) and other diagnostic tools. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 24

25. In a real experiment it is not easy to understand which hot spot gives the contribution to the considered X-ray spectral line. Field of view for resonance and inter-combination lines. The resonance line can be observed from hot spots 1, 2 and 3, while the inter-combination line can be recorded from hot spots 2, 3 and 4. Comparison of X-ray spectra recorded at the same experimental conditions, but with stochastic jitter in the formation of hot spots along the pinch axis, as shown on X-ray pinhole pictures (left column). The spectrum obtained from a shot SX65-4 shows mainly the ArXVI satellite and K-alpha lines of Li and Be-like ions, while in the spectrum recorded for the SX65-6 shot the most intense appears to be a resonance line 1s2p-1s2 1P. The spectrum obtained from the SX65-7 shot shows the intense emission within the whole spectral range. Spectra in SX65-7 shot contains five lines (from the left to the right): resonance line 1s2p-1s2 1P, inter-combination line 1s2p – 1s2 3P, satellites and weak K-alpha lines of Li and Be-like ions. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 25

26. The interpretation of the X-ray spectra is much easier when a single distinct hot-spot is formed only. Such experiments were performed especially within the MAJA-PF facility operated in Swierk. X-ray pinhole picture of the pinch column with a single hot-spot and X-ray spectra recorded by means of 2 crystal spectrometers with mutually perpendicular dispersion planes). L. Jakubowski, M.J. Sadowski, E.O. Baronova: Nuclear Fusion 44 (2004) 1-5. The measured ratio of the resonance and inter-combination lines (Ir/Ii), as determined on the basis of the recorded spectra, was equal to 4.09 in the case (a), and 1.39 in the case (b). Those values show that the polarization of the analyzed X-ray lines differed considerably. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 26

27. IPPLM 5. Summary and conclusions • The optical emission spectroscopy enabled to investigate dynamics of Plasma-Focus discharges, to identify ion species in free plasma streams and to estimate some basic plasma parameters. • The optical emission spectroscopy appeared to be useful also for studies of plasma produced by interactions of intense plasma streams with solid targets (e.g. tungsten, graphite and CFC), and in particular to identify ion species in such plasmas. • The energy dispersive X-ray spectroscopy of plasma-irradiated samples enabled to investigate characteristic lines of different elements and to determine surface distribution of the investigated species. • The reported X-ray spectroscopic studies of PF discharges enabled also to discover differences in polarization of the recorded spectral lines, which deliver additional information about directional motions of electron streams. • The spectroscopic studies should be accompanied and verified by the simultaneous use of • other diagnostic techniques, e.g., laser interferometry for measurements of electron density. ADAS Workshop 2014, Warsaw, 28-30 September 2014 M.J. Sadowski - 27