On phenomena in ionized gases
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- Analysis of the K-radiation structure for the determination of HED-plasma parameters and their spatial variations along the line of view
- Ecton processes in the generation of picosecond runaway electron beams
- 1. References
- Experimental and numerical study of electrical arc movement
- 4. Preliminary results perspectives
- 5. References
- Optimizing the CO 2 conversion efficiency in a low-pressure pulsed microwave plasma source
- 4. References
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1. Introduction
Understanding turbulent plasma transport in magnetized plasma is a subject matter of great significance from the perspective of understanding plasma loss in fusion devices. Although, significant progress has been made in understanding physics of ion thermal transport over the past decade but various aspects of turbulent transport in electron and particle channel remains to be elucidated[1-2]. In this background, Large Volume Plasma Device (LVPD) ( ) has successfully demonstrated unambiguous excitation of ETG turbulence, where plasma profiles characterized as
and
by introducing an Electron Energy Filter (EEF)[3]. Radial profiles of turbulent particle flux
and density - potential cross phase,(
) has been measured. It is observed that the net electrostatic flux is negative (
and is directed radially inward. Turbulent particle flux is predominantly electrostatic in spite of nature of excited turbulence is electromagnetic ( ). The particle flux maximizes when EEF is ON suggesting that the flux is due to ETG driven turbulence. Theoretically, net particle flux results when phase difference is, this agrees well with our observation. Turbulence intensity maximizes roughly at the location where particle flux maximizes. The experimental cross phase angle and flux has been compared radially with theoretical counterparts resulting due to the non-adiabatic ion response because of the resonant interaction of the ions with the ETG mode,
agrees well within 20%. Theoretical standpoint suggests that thermo- diffusive turbulent flux radially inward in the background of ETG
[4]. Comparison of experimental results with
theoretical model
suggesting it as a thermo diffusive turbulent particle flux will be present in this conference.
[1] Coppi B and Spight C, Phys. Rev. Lett. 41 551(1978). [2] Tang W et al. Phys. Fluids 29 3715 (1986). [3] S. K. Mattoo, S.K. Singh, L.M. Awasthi, Phys. Rev. Lett. 108 255007(2012) [4] R Singh, Hogun Jhang and P.H. Diamond, Phys. Plasmas 20 112506 (2013)
Topic number-1 134 XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
V. Bernshtam, E. Kroupp, A. Starobinets, O. Nedostup, Yu. Zarnitsky, Yu. Kuzminykh, and Y. Maron
P Faculty of Physics, Weizmann institute of Science, Rehovot, Israel .
The spectral structures of the Heα and Lyα groups of lines are strongly dependent on the electron temperature in the plasma, on the plasma charge state composition, and the presence of fast electrons. We analyze the effect of the various parameters on the features of the spectral structures, and demonstrate methods to determine electron-temperature gradients in the plasma, and to infer bounds on the electron density. The analysis includes fits to the satellites emitted from plasmas of rather-different electron temperature, and discrimination of satellites due to inner shell excitations and ionizations. The analysis is applied to K-emission data obtained in our neon-puff Z-pinch experiment [1, 2]. As a result, we obtain quantitative estimates of the simultaneous contributions of plasmas of various densities and temperatures to the spectrogram.
13.4
13.5 13.6
13.7 13.8
0.0 0.2
0.4 0.6
0.8 1.0
Plasma 3 T e = 69 eV N i = 3e20 cm -3 T i = 69 eV
R = 0.05 mm Plasma 2
T e = 190 eV N i = 5e18 cm -3 T i = 1520 eV R = 1 mm Intensity, arb.u
. Wavelength, Å CR - model (sum) Experimental data CR - model of Ne He a group with the experimental data Three plasma model Plasma 1
T e = 190 eV N i = 5e17 cm -3 T i = 9120 eV R = 1 mm
[1] Eyal Kroupp, Thesis for the degree of Ph.D., April 2004, Faculty of Physics, Weizmann institute of Science, Rehovot, Israel. [2] E. Kroupp, D. Osin, A. Starobinets, V. Fisher, V. Bernshtam, I. Uschmann, E. Forster, A. Fisher, C. Deeney and Y. Maron, PRL 98, 115001 (2007) 6 135 XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Ecton processes in the generation of picosecond runaway electron beams
G. A. Mesyats UP 1 P
P 1 P
The mechanism of the generation of runaway electrons and of the cutoff of their current in a gas discharge is considered. It is shown that the field emission current from the cathode microprotrusions in the discharge is enhanced due to ionization processes occurring in the cathode region. This hastens explosive electron emission, which lasts tens of picoseconds. Thus, the runaway electrons current pulse is similar in nature to the ecton process in a vacuum discharge.
It is well known from the physics of nanosecond pulsed electrical discharges in gases that if the energy acquired by electrons in the electric field is greater than the energy lost by them in collisions, the electrons become running away. In this case, the generation of runaway electrons (REs) is a pulsed process. However, the nature of these RE pulses still remains obscure [1–5]. As shown experimentally, the duration of the RE current pulse in a discharge between stainless steel electrodes in atmospheric air is 2.4·10 –11 s. A nearly triangular voltage pulse of rise time t 0 = 1.5·10 –10 s
and amplitude 160 kV was used. The RE current was equal to several amperes. The leading and trailing edges of the RE pulse, each lasting no more than 2·10 –11 s, are of different nature. We believe that the leading edge is due to the field emission (FE) current from cathode microprotrusions (CMPs) [6]. This current is enhanced due to ionization of the gas until explosive electron emission is initiated as a result of the Joule heating of CMPs during a time determined by the relation j 2 t 1 = h, where h is the specific current action for an electrical explosion of the cathode metal and j is the density of the electron current from the CMPs. For copper we have h=4.1·10 9 (A·s) /cm 4 [7]; hence, using the Fowler– Nordheim formula, we estimate the FE current density at t 1 =2·10
-11 s as j=1.4·10 10 A/cm
2 . This
corresponds to the electric field at the tip of an CMP E=1.5·10 8 V/cm. Note that in Ref. 6, the time t 1 is
estimated as t 1 =0.11t 0 =1.6·10
-11 s. In vacuum, these extreme values of the parameters t 1 , j, and E cannot be attained because of the electronic space charge effect [8]. In gases, this effect is not essential or even absent due to that the space charge is neutralized by the gas ions. The electron emission mechanism changes after EEE: a cathode spot (CS) arises, and the intensity of electron emission from the spot quickly decreases as a result of energy loss. Assuming that a CS cools only due to heat conduction, we have t 2 =i 2 /64π
2 a 2
where a is the thermal diffusivity of the cathode metal [7]. For copper we have a≈1.2 cm 2 /s; thus, for the RE current i=1A, we obtain t 2 =1.8·10 -11 s.
The formation of RE pulses is similar to the formation of an electron butch (ecton) during a cycle of the CS operation in a vacuum arc [9]. The ecton processes in a vacuum arc take 10 –9 –10
–8 s.
Note that in our experiment, we have obtained historically high rates of rise of the electric field at CMPs: ~10 19 V/(cm·s). Previously, it was supposed that the mechanism of the RE current cutoff is related to the plasma processes occurring in the electrode gap [3, 10]. In our opinion, this mechanism is governed by the emission processes taking place at the cathode. This concept seems to be more realistic in view of the very short times of the generation of runaway electrons and of the cutoff of their current. 1. References [1] G.A. Mesyats, Yu.I. Bychkov, and V.V. Kremnev, Sov. Fiz. Usp. 15 (1972) 282. [2] L.P. Babich, T.V. Loiko, and V.A. Tsukerman, Sov. Fiz. Usp. 33 (1990) 521. [3] V.F. Tarasenko and S.I. Yakovlenko, Phys. Usp. 47 (2004) 887. [4] G.A. Mesyats, M.I. Yalandin, A.G. Reutova, et al., Plasma Physics Reports 38 (2012) 29. [5] S. Yatom, A. Shlapakovski, L. Beilin, et al., Plasma Sources Sci. Technol. 25 (2016) 064001. [6] G.A. Mesyats, JETP Letters 85 (2007) 109. [7] G.A. Mesyats, Pulsed Power, Kluwer/Plenum (2004).
[8] J.P. Barbour, W.W. Dolan, J.K. Trolan, et al., Phys. Rev. 92 (1953) 45. [9] G.A. Mesyats, Phys. Usp. 38 (1995) 567. [10] S.Y. Belomyttsev, I.V. Romanchenko, V.V. Ryzhov, and V.A. Shklyaev, Technical Physics Letters 34 (2008) 367. Topic number 3 136
XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Experimental and numerical study of electrical arc movement
J. Quéméneur P 1 P , J-J. Gonzalez UP 1 P , P. Freton UP 1
, P. Joyeux P 2 P
P 1 P
118 route de Narbonne, F-31062 Toulouse cedex 9, France P
P
The movement of an electrical arc between two parallel arc runners is studied by numerical and experimental approaches. The measurement setup and two methods to model the arc roots motion are presented alongside with a tool to determine the position of the arc using its light emission. The experiment consists in a simplified low-voltage circuit breaker (LVCB) chamber where voltage, current, pressure measurements and high-speed imaging are performed. 3-D computational fluid dynamic model based on the Fluent software is developed for the simulation of the arc in the same configuration as the experiment. Comparison between tests and modelling are presented.
Electrical arc motion is a significant issue for several industrial applications such as breaking arcs, plasma torches, welding or in-flight lightning strike. In LVCB the arc must be moved quickly from the opening contact to the extinction chamber. This improvement is mainly made by long empirical developments. Hence the industry is calling for predictive models. Here two numerical methods to simulate the arc movement are presented and compared to experimental results.
LVCB have a complex design. For an easier study a rectangular arc chamber with two parallel iron electrodes is used. One of the lateral walls is transparent to allow high-speed imaging. The discharge can be ignited by contact opening or by a fuse wire. In order to get rid of the opening speed parameters the fuse wire ignition is used. The breaking current is a 50Hz sine wave up to 10kA.
A magneto-hydrodynamic model is developed to describe the plasma [1] and two different methods are used to calculate the arc motion: In the first method named “Global Current Resolution Method” (GCRM), current density and heat transfer between the plasma and the metallic electrodes are solved neglecting the sheath mechanisms. This is an easy method to implement but it requires advanced calculation algorithms due to a strong difference of electrical conductivity between the two mediums. Therefore, calculation time is high. In the second method, named “Mean Electrical Conductivity Method” (MECM), classical boundary conditions for anode and cathode roots [2] are used and applied where the electrical neighbouring conductivity of the plasma is the highest. Two arc roots can be specified on the same electrode.
A comparison between numerical and experimental results for the arc movement is presented Fig. 1. The good agreement validates the two methods used.
Fig.1: Experimental and simulated arc motion With such tools we are able to investigate the parameters influencing the arc movement. To improve our model, description of the anodic [3] and cathodic [4] sheaths could be developed in order to allow a better description of arc commutation and calculation of the arc voltage.
[1] A. Gleizes, J-J. Gonzalez, P. Freton, J. Phys. D: Appl. Phys. 38 (2005) R153-183
[2] P. Freton, J-J. Gonzalez, A. Gleizes, J. Phys. D : Appl. Phys. 33 (2000) pp. 2442-2452
[3] F.Lago, J-J. Gonzalez, P. Freton, A. Gleizes, J. Phys. D: Appl. Phys. 37 (2004) 883-897
[4] M.S. Benilov, J. Phys. D : Appl. Phys. 41 (2008) 144001
11 137 XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Optimizing the CO 2 conversion efficiency in a low-pressure pulsed microwave plasma source
N. Britun 1 , T. Godfroid 2 P
3 P , and R. Snyders 1,2
1 P
2 P
3 Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal
The CO 2 decomposition process in a pulsed 2.45 GHz microwave surfaguide discharge has been studied. The CO 2 conversion efficiency is found to be mainly affected by the plasma pulse repetition rate (at fixed applied power), along with the other discharge parameters such as the gas flow rate, residence time, etc. The electron and gas temperatures have been additionally studied using spectroscopic methods. A several time increase in the CO 2 conversion/energy efficiency points out on a crucial role of the pulsed plasma regime for better CO 2 conversion. The found effects are explained by the relevant energy relaxation mechanisms in the discharge, such as the electron-vibrational, vibrational-vibrational, and vibrational-translational ones.
In spite of the numerous works devoted to plasma-based greenhouse gas conversion, related to microwave (MW) plasma [1], dielectric barrier discharge (DBD) [2], gliding arc plasma (GAP) [3], as well as those involving plasma catalysis, the effects of CO 2 conversion in the pulsed discharges are still far from being understood clearly. So far the beneficial role of plasma power modulation has been only demonstrated in the DBD case [4]. The present work studies the power modulation effect in MW surfaguide discharge, as a promising candidate for plasma-assisted CO 2 conversion. 2. Experimental A surfaguide-type pulsed microwave plasma source has been used. The plasma was sustained by the electromagnetic waves with the filling frequency of 2.45 GHz, and modulated by nearly rectangular pulses with the repetition rate ranging from 0.01 to 2.5 kHz. The pulse duty ratio was mainly fixed at 50%. The discharge has been sustained in a quartz tube (14 mm in diameter and 31 cm long) in which the gas flow was regulated by digital mass flow controllers. The quartz tube has been cooled down by 10 C flow of Si oil. More details are available in [1,5]. The optical emission actinometry [1] and two- photon absorption laser induced fluorescence (TALIF) techniques were used for monitoring the CO 2 conversion efficiency in our case [5].
In this work it was shown that the CO 2 conversion efficiency depends dramatically on the plasma pulse repetition rate. Up to a fourfold improvement in the CO production in the post-discharge (and thus in the CO 2 conversion rate) has been detected (see Fig. 1).
Fig. 1. Relative CO density in the post-discharge as a function of plasma pulse repetition rate.
It is also shown that the CO 2 conversion depends on the molecule residence time and the gas pressure. The electron temperature determined based on the Ar line ratio shows rather minor variations in the studied pulse frequency range, leading us to a conclusion that the vibrational energy exchange is the main reason for the observed effects. The estimates made for the electron-vibrational (e-V), vibrational-vibrational (V-V) as well as vibrational- translational (V-T) energy relaxation time point out on a primary role of vibrational excitation for CO 2
rep. rates), as well as its contribution to the gas heating via the V-T process. At the same time at high repetition rates these processes are less efficient due to shorter plasma pulse duration.
[1]. T. Silva, N. Britun, T. Godfroid, R. Snyders, Plasma Sources Sci. Technol. 23, 25009 (2014). [2]. S. Mahammadunnisa, E. L. Reddy, D. Ray, et al., Int. J. Greenh. Gas Control 16, 361 (2013). [3]. T. Nunnally, K. Gutsol, et al., J. Phys. D. Appl. Phys. 44, 274009 (2011). [4]. A. Ozkan, T. Dufour, et al., Plasma Sources Sci. Technol. 25, 55005 (2016). [5]. T. Silva, N. Britun, et al. Plasma Process. Polym. DOI: 10.1002/ppap.201600103 (2016).
Topic 6 138 |
