Electron density and plasma dynamics of a colliding plasma experiment

Electron density and plasma dynamics of a colliding plasma experiment Electron density and plasma dynamics of a colliding plasma experiment J Wiechula, A Schönlein, M Iberler, C Hock, T Manegold, B Bo[.]

Electron density and plasma dynamics of a colliding plasma experiment J Wiechula, A Schönlein, M Iberler, C Hock, T Manegold, B Bohlender, and J Jacoby Citation: AIP Advances 6, 075313 (2016); doi: 10.1063/1.4959590 View online: http://dx.doi.org/10.1063/1.4959590 View Table of Contents: http://aip.scitation.org/toc/adv/6/7 Published by the American Institute of Physics AIP ADVANCES 6, 075313 (2016) Electron density and plasma dynamics of a colliding plasma experiment J Wiechula,a A Schönlein, M Iberler, C Hock, T Manegold, B Bohlender, and J Jacoby Plasma Physics Group, Institute of Applied Physics, Goethe University, 60438 Frankfurt am Main, Germany (Received 15 February 2016; accepted 12 July 2016; published online 19 July 2016) We present experimental results of two head-on colliding plasma sheaths accelerated by pulsed-power-driven coaxial plasma accelerators The measurements have been performed in a small vacuum chamber with a neutral-gas prefill of ArH2 at gas pressures between 17 Pa and 400 Pa and load voltages between kV and kV As the plasma sheaths collide, the electron density is significantly increased The electron density reaches maximum values of ≈8 · 1015 cm−3 for a single accelerated plasma and a maximum value of ≈2.6 · 1016 cm−3 for the plasma collision Overall a raise of the plasma density by a factor of 1.3 to 3.8 has been achieved A scaling behavior has been derived from the values of the electron density which shows a disproportionately high increase of the electron density of the collisional case for higher applied voltages in comparison to a single accelerated plasma Sequences of the plasma collision have been taken, using a fast framing camera to study the plasma dynamics These sequences indicate a maximum collision velocity of 34 km/s C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4959590] I INTRODUCTION In a prior publication1 the experimental characterization of a coaxial plasma accelerator for plasma collision experiments was presented It was mentioned that the used coaxial plasma accelerators are a compromise between a Marshall gun2 and a dense plasma focus (DPF)3 with the goal to seize the advantages of both technologies in order to generate an optimized ambient for the examination of the colliding plasmas In addition the prefilled gas environment should allow for a more precise and synchronized ignition of the plasma guns than the implementation of gas puff valves.4 A second benefit of this device is a larger pressure range for operation in opposite to a common DPF that usually operates in a smaller pressure range.5 This publication reports first results of a colliding plasma experiment with the described pulsed-power-driven coaxial plasma accelerators.1 A major focus in the experiments lay on the behavior of the electron density and on the comparison for single accelerated plasmas compared with the collision zone of the plasma sheaths The measurements for the electron density are supplemented by visual images taken with a fast gated camera in order to study the plasma dynamics of the propagation of the plasma sheaths, beginning with their ejection until their collision in the middle of a small vacuum chamber The main attempt of the setup is to perform basic research of the collision process from the accelerated plasma sheaths in regard to use it as a plasma target6–8 for the investigation of ion beam plasma interaction and to study the UV and VUV radiation from the collision zone In order to develop the colliding plasma as an UV/VUV radiation source, a suitable ionization degree is required since only a few neutral argon appear in the VUV region Thus the the experiment should show both an enhanced electron density and augmented electron temperature a wiechula@physik.uni-frankfurt.de 2158-3226/2016/6(7)/075313/12 6, 075313-1 © Author(s) 2016 075313-2 Wiechula et al AIP Advances 6, 075313 (2016) There exist a long history of experiments with coaxial plasma accelerators for a variety of different applications Some examples for coaxial plasma accelerators are plasma thruster, dense plasma focus (DPF), plasma refueling and high current switches.3,9–12 While there is also a long history of experiments with colliding plasmas there are still new approaches to study colliding plasmas Some examples for colliding plasmas based on pulsed power driven accelerators (e.g coaxial accelerators and plasma railguns) are studies for high energy density physics, astrophysical applications and magneto-inertial fusion (MIF).13–15 Until now mainly colliding laser produced plasmas are investigated as XUV sources, but also in indirect drive laser nuclear fusion systems and for astrophysical applications.16–18 The paper is organized as follows: Sec II gives an overview of the experimental setup; Sec III presents the experimental results and a brief discussion on the spectroscopic measurements and measurements for the plasma dynamics; Sec IV gives a conclusion and outlook II EXPERIMENTAL SETUP The experiment uses a six-way flange cross as vacuum chamber, connecting the two coaxial plasma accelerators in an opposite direction on a collision axis In addition to the accelerators, the vacuum and gas flow system is connected to this cross The vacuum system consists of a turbo-molecular pump (230 l/min Varian Turbo-V 301 Navigator) and a dry scroll pump (210 l/sec Varian TriSroll 300) leading to a base pressure of typical 10−4 Pa As working gas serves a mixture of ArH2 with a ratio of 97.2 % Ar and 2.8 % H2 Though this main experimental setup has already been described in a preceding publication,1 some improvements have been made, which are addressed in the following For the experiments discussed here a new capacitor bank and a new transmission line have been installed As capacitor bank six in parallel connected capacitors of 4.5 µF, leading to a total capacitance of 27 µF, are used This storage device is installed twice, one for each plasma accelerator The former transmission line consisting of aluminum has been replaced by a transmission line made of copper and an improved design Thus, a lower inductance has been achieved after the replacement, what should lead to a higher current rise rate and better performance.19 The complete overview of the experiment can be seen in Fig The ignition of the discharge is forced by a thyratron, TDI1-200k/25H A stored energy of about 1.1 kJ with voltages of kV and currents of up to 130 kA are switched in this experiment The total distance between the two plasma accelerators is about 60 mm The CPAs are electrode systems consisting of two massive copper electrodes building the anode and the cathode with PEEK as insulator between them Regarding an easy replacement of the electrodes due to service and the variation of the geometric factors, screw threads are used on both sides of the electrodes For the short circuit measurements, the anode has been grounded above the cathode, connecting both electrodes with a copper cup that can be screwed onto the electrodes ending The effective discharge FIG Experimental setup 075313-3 Wiechula et al AIP Advances 6, 075313 (2016) TABLE I Summary of the technical parameters Radius of the outer electrode (cathode) r a Radius of the inner electrode (anode) r i Effective axial length of the CPA Distance of both CPA Complete inductance of the System L Complete resistance of the System R Capacitance of the bank C Resonant frequency ν mm 4.5 mm ≈ 90 mm ≈ 60 mm ≈ 124 nH - 130 nH ≈ mΩ - 15 mΩ 27 µF ≈ 86 kHz length of the plasma accelerators is set to 90 mm with a gap distance of 2.5 mm The inner electrode has a diameter of 4.5 mm, the diameter of the outer electrode is mm The LCR-circuit indicates a resonant frequency of about 86 kHz with an inductance of about 127 nH Both the electrical and geometrical parameters for the entire setup are shown in “Table I” The used diagnostics include electrical and optical observing techniques like high voltage probes, current transformer, fast framing camera (Princeton instruments PI-MAX2/Camera in Fig 1), a 0.5 m spectrometer (Princeton Instruments Acton series SP-2500i) and a small spectrometer (Ocean Optics HR 4000) to give an overview from the emission spectrum in the wavelength region between 300 nm and 900 nm A CCD camera (Princeton Instruments PIXIS 256E/Camera in Fig 1) serves as detector for the 0.5 m spectrograph The light was coupled to the 0.5 m spectrometer and the HR 4000 via an optical fiber that was placed perpendicular (sideon) to the axis A glass window made of BK7 is used to seal the vacuum chamber The transmission for BK7 begins at around 300 nm and reaches its maximum at around 350 nm In accordance with the numerical aperture of the fiber, the investigated area was about cm in diameter The complete investigated volume was about 50 cm3 The used fast framing camera was also positioned perpendicular (side-on) to the axis but at an opposite place to the optical fiber The object lens of the fast framing camera was adjusted to record a picture from a size of about cm × cm (see Fig 13) A Shematic view from the optical set up is shown in Fig III EXPERIMENTAL RESULTS AND DISCUSSION A Spectroscopic measurements The electron density ne is determined using the stark broadening due to equation (1)20 and tables from Griem:21  3/2   −3 12 ∆λ 1/2 · 10 cm ne = 8.02 · 10 α1/2 (1)   3/2   ∆λ 1/2 = 2.53 · 1014 cm−3 , α1/2 FIG Shematic view from the optical set up 075313-4 Wiechula et al AIP Advances 6, 075313 (2016) where ∆λ 1/2 is the line width (FWHM) of the observed H β emission line in nm and α1/2 is the reduced half-width The electron density is then estimated in the unit [cm−3] Values of α1/2 depend on the temperature and electron density, α = α(ne,Te), and were tabulated for hydrogen and helium at temperatures between (0.5–4) eV and densities in the range of (1014 –1018) cm−3 by Griem.21 The electron temperature was estimated by comparing the observed spectrum with the atomic database NIST (http://www.nist.gov) as well as with non–LTE, steady–state calculations using FLYCHK (http://nlte.nist.gov/FLY/) for an argon spectrum.22 The main aspect here is the absence of Ar IV–lines in the measured line spectra and a major contribution of Ar II Also Ar I and Ar III–lines were found Nevertheless, only a few Ar III–lines, that only could be found with the better resolution from the 0.5 m spectrometer, were observed in the range of 300 nm to 900 nm This circumstance is shown in the following two overview spectra given by Fig The spectra was taken with the Ocean Optics HR 4000 for an applied voltage of kV and a gas pressure of about 117 Pa To give an overview of the spectra the following lines are addressed in the drawing: Ar II: 358.8 nm, 434.8 nm, 442.6 nm, 488.0 nm, 611.5 nm, 664.4 nm, Hα : 656,3 nm, Ar I: 706.7 nm, 738.4 nm, 750.4 nm, 763.5 nm, 811.5 nm It should be mentioned that mainly all the lines on the left side of the Hα –line are Ar II–lines The overview spectra indicates that the spectra are dominated by Ar II with a certain fraction of Ar I The line with the highest intensity is given by the Hα –line The spectra form the single accelerated plasmas show that both spectra agree with only small deviations in the intensity from each other In the case of the colliding plasma the intensity is enhanced In addition it looks as if there appears some kind of continuum emission in the spectrum A better explanation is the fact that the resolution of the used HR 4000 spectrometer is only about 1.8 nm together with the fact that in the case of the plasma collision all line widths are enhanced As a result the spectrometer can not longer distinguish between the different lines The detector in the used HR 4000 spectrometer is also limited to a maximum intensity of 170 [arb.unit] This becomes obvious especially in the case of the colliding plasma Here the cutoff from the Hα –line can be seen The found ion distributions are a good indicator to give a qualitative conclusion for the electron temperature As a result of FLYCHK the ion distribution for a given electron density and electron temperature indicates that the desired electron temperature should be in a region between 1.5 eV and eV Therefor Fig shows the relative ion population in dependence of the charge state for argon ions It can easily be seen that for an electron temperature of Te = eV only neutral argon emission lines appear in the spectrum Because the spectra (compare Fig and Fig 4) the spactra are dominated by a major contribution of Ar II only a minor contribution of Ar I and even a lower contribution of Ar III appears Now FLYCHK Fig indicates that the electron temperature for FIG Overview spectrum taken with an ocean optics HR 4000 measured for the incoming left and right plasma sheath in a wavelength range between 300 nm and 900 nm 075313-5 Wiechula et al AIP Advances 6, 075313 (2016) FIG Overview spectrum taken with an Ocean Optics HR 4000 measured for the incoming left and right plasma sheath in a wavelength range between 300 nm and 900 nm the argon–plasma lies between 1.5 eV and eV At higher temperatures above eV the data from FLYCHK show no neutral argon–lines in the spectra and there should be a higher contribution of Ar III–lines in opposite to Ar II–lines Fig shows the behavior of the charge states in dependence of the electron density Only at densities around ne ≈ 1017cm−3 the relative ion populations start to change a little bit These high densities have not been reached so far As a conclusion in our case the density does not influence the relative ion population in a higher order In the following it was stated that the electron temperature has a value of about eV The spectroscopic measurements have been performed at gas pressures between 17 Pa and 400 Pa, using a 0.5 m spectrometer (Czerny Turner Acton series SP-2500i) with a grating of 2400 grooves/mm, and an adjustable entrance slit set to 100 µm (Gaussian instrumental function with a FWHM of 0.063 nm1) With the detector, a high resolution camera (PIXIS 256E), only a time averaged spectroscopy is possible The spectrograph has been corrected for the wavelength range between 250 nm and 1000 nm using a Hg-calibration Lamp All the spectroscopic measurements are made in side-on view i e perpendicular to the axis FIG Relative Ion populations in dependence of the Ion Charge States for Argon at a fixed electron density of n e = · 1015cm−3 and different electron temperatures 075313-6 Wiechula et al AIP Advances 6, 075313 (2016) FIG Relative Ion populations in dependence of the Ion Charge States for Argon at three different electron densities and electron temperatures A typical experimental spectrum in the region of the H β –line from 480 nm to 490 nm obtained at a gas pressure of about 117 Pa for a single accelerated and a colliding plasma is shown in Fig for an applied voltage of kV It can be seen that the intensity as well as the width of the lines are enhanced in the case of the plasma collision The width of the H β –line is so high that it overlaps several other lines To estimate the quantity of the electron density, the convolution of the H β –line due to natural, Doppler, instrumental and Stark broadening is fitted A Gaussian function of the order of about 0.076 nm is taken into account.1 Fig shows the Spectrum of the colliding plasma sheath together with the Voigt–Fit and in addition with two argon–lines It should be mentioned that for the estimation of the Voigt–Fit of the H β –line, every line in the spectrum was taken into account That was necessary because the width of the H β –line is so high that several other lines can be found in the wings of the H β –line Without taking the other lines into account it would have come to an overestimation of the line width In this sense the presented Voigt–Fit is the best reached fit for this line The resulting time averaged electron densities achieved for single accelerated plasmas compared to colliding plasmas for three different applied voltages depending on gas pressures between 17 Pa and 400 Pa are shown in Fig 9, Fig 10 and Fig 11 FIG Typical spectrum of the accelerated and colliding plasma in the H β –region from 480 nm to 490 nm taken with a grating of 2400 grooves/mm in a Czerny Turner spectrograph 075313-7 Wiechula et al AIP Advances 6, 075313 (2016) FIG Spectrum of the colliding plasma with best Voigt–Fit It can be seen that the plasmas from both accelerators (left and right) show the same values of electron density with small deviations from each other The electron density for a single accelerated plasma takes values between ne ≈ 1.3 · 1015 [cm−3] for an applied voltage of kV and ne ≈ 8.5 · 1015 [cm−3] for an applied voltage of kV For the colliding plasma, the electron density has average values between ne ≈ 2.6 · 1015 [cm−3] for an applied voltage of kV and ne ≈ 2.62 · 1016 [cm−3] for an applied voltage of kV The enhancement of the electron density of a factor of 1.3 to 3.8 in case of collision indicates a further interaction of the two plasma sheaths Fig 12 shows the estimated electron density in dependence of the applied voltage for a gas pressure of 400 Pa It shows that the electron density scales with the applied voltage For a colliding plasma it is drastically enhanced compared to a single accelerated plasma, especially for higher applied voltages This behavior should be studied using higher applied voltages in order to approve this behavior and to get a more precise scaling law B Plasma Dynamics A fast framing camera (Princeton instruments PI-MAX2) is used to study the plasma sheaths’ movement and their dynamic interaction with each other The two accelerated plasma sheaths have been detected before, during and after their collision to give an overview of their temporal evolution Fig 13 shows a sequence of the plasma sheaths accelerated out of the plasma accelerators FIG Electron densities for single accelerated plasmas and the colliding plasma with an applied voltage of kV 075313-8 Wiechula et al AIP Advances 6, 075313 (2016) FIG 10 Electron densities for single accelerated plasmas and the colliding plasma with an applied voltage of kV and colliding in the middle of the vacuum chamber The exposure time has been set to 15 ns The delay has been set relatively to the beginning of the voltage breakdown and has been increased successively The delay times are shown at the bottom of each picture After the ejection of the plasma sheaths out of the accelerator section, the sheaths form a conical shape This behavior can be seen in Fig 13 at the pictures with delay times µs and 5.5 µs At the picture with delay time µs the length of the left conical plasma sheath is about 1.83 cm and the length of the left plasma sheath is about 1.5 cm The heights in both cases are about 3.5 cm The distance of the plasma sheaths from each other is about 0.9 cm just 0.5 µs prior to the collision This indicates for a velocity of about 18 km/s just before the collision starts The collision of the plasma sheaths starts at the picture with delay time 5.5 µs It indicates a high increase in intensity with a zone of stagnation of only a thickness of a few mm Otherwise the length of the collision zone ranges over the full height of the vacuum chamber (≈ cm) In addition the picture shows two brighter regions with a dark region between them This suggests for either a degree of initial plasma interpenetration or reflected shocks that travel away from each other The state of the collision is stable for about µs until the plasma starts to deform Beginning at µs and beyond, the interaction appears quite complex, reminiscent of helical magnetic structures However, an estimation of the magnetic field inside the vacuum chamber has not been measured and still need further investigations The estimated density of the left plasma sheath is ne ≈ 1.19 · 1015 [cm−3], of the right sheath it is about ne ≈ 1.24 · 1015 [cm−3] and the density of the colliding plasma is about ne ≈ 3.6 · 1015 [cm−3] The propagation velocities of FIG 11 Electron densities for single accelerated plasmas and the colliding plasma with an applied voltage of kV 075313-9 Wiechula et al AIP Advances 6, 075313 (2016) FIG 12 Comparison of the mean electron densities in dependence of the applied voltage the plasma sheaths in the vacuum chamber before the collision have been estimated to mean values of 17.8 km/s for the left plasma and to 17.3 km/s for the right one Fig 14 shows vertically averaged intensities of the fast–camera images vs the horizontal position of the plasma sheaths Position is the middle of the vacuum vessel where the plasma sheaths collide To show the temporal evolution of the intensity during the collision the vertically averaged intensities are shown for four different time delays comparing to those in Fig 13 According to the vertically averaged intensities during the collision the intensity is, at its maximum, a factor of about FIG 13 Collision sequence for a stored Energy of 486 J (6 kV, 27 µF) and a gas pressure of 17 Pa in false-color 075313-10 Wiechula et al AIP Advances 6, 075313 (2016) FIG 14 Fast-camera image vertically averaged intensity vs horizontal position seven times higher than after the ejection of the plasma out of the accelerator In order to achieve the velocity of the plasma sheath, the position of the maximum of the plasmafront in Fig 14 was taken and compared to the delay times of the camera Thus a path–time diagram is found A linear fit of the first three points can be studied to figure out the velocities of both plasma sheaths The path–time diagram is shown in Fig 15 Many different stopping mechanisms have been introduced to describe the collisions of plasmas, but the basic process has not been understood clearly.23 Most recent publications deal with a formation of a shock front inside the collision zone.14,24,25 It is not yet totally clear if both plasma sheaths stagnates fully in each other Thus the points for the collision zone only show the positions of the maximum values and does not give any remarks if the sheaths fully stagnate C Further calculations By estimating the velocity of the plasma sheaths, the sonic Mach  number can be estimated γ ·R S ·T M = V/CS , where V is the velocity of the plasma sheath and CS = is the speed of sound M FIG 15 Path–time diagram of both plasma sheaths and the collision zone 075313-11 Wiechula et al AIP Advances 6, 075313 (2016) for an ideal gas With γ = 35 the adiabatic constant for a monatomic gas, RS the specific gas conJ stant, that for argon is about 208.122 [ Kg·K ] and T the plasma temperature, that in our case can be estimated by the electron temperature Te ≈ eV a value of about CS ≈ 2835 can be estimated Therefor with a velocity of 17 km/s the sonic Mach number is about in the case of an accelerated argon gas The sonic Mach number indicates for the formation of a shockwave According to26 the interaction between two moving plasma depends on the ion-ion meen–free path, l ii , relative to the gradient scale length L, of the colliding front For l ii ≪ L the interaction should take place only at a narrow interface between the plasmas Stagnation occurs at this interface, usually resulting in a large temperature increase and formation of shock waves For l ii ≫ L the moving plasmas penetrates each other For the case that l ii ≈ L the interpenetration should only occur over a finite distance and a “soft” stagnation may occur IV CONCLUSION AND OUTLOOK Measurements to compare single accelerated plasmas with colliding plasmas have been made The collision show an interaction between the plasma sheaths with a high increase in the density An increase in the temperature could not be measured so far This still needs further investigations The CCD images have been analyzed for the velocity of the plasma sheath and the collision zone It indicates a high increase in intensity The zone of stagnation has only a thickness of a few mm The stagnation zone gives a hint for the appearance of either a small penetration or a reflected shock This circumstance must be clarified for further investigations by calculating the slowing length from the particles The collision zone stays in the middle of the vacuum chamber for µs before it starts to deform The deformation may occur because of helical magnetic structures In order to clarify this behavior magnetic measurements are about to be made ACKNOWLEDGMENT The authors would like to thank the BMBF (Bundesministerium für Bildung und Forschung) and HICforFAIR for the support J Wiechula, C Hock, M Iberler, T Manegold, A Schönlein, and J Jacoby, “Experimental characterization of a coaxial plasma accelerator for a colliding plasma experiment,” Physics 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Manegold, A Schönlein, and J Jacoby, “Experimental characterization of a coaxial plasma accelerator for a colliding plasma experiment, ” Physics of Plasmas 22(4), (2015) J Marshall, “Performance of a. .. In a prior publication1 the experimental characterization of a coaxial plasma accelerator for plasma collision experiments was presented It was mentioned that the used coaxial plasma accelerators...AIP ADVANCES 6, 075313 (2016) Electron density and plasma dynamics of a colliding plasma experiment J Wiechula ,a A Schönlein, M Iberler, C Hock, T Manegold, B Bohlender, and J Jacoby Plasma