On phenomena in ionized gases
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- 4. References
- Uniform and strongly magnetized plasma using a Halbach array
- Measurement of reactive species in Plasma Babbled-up Water affecting human cultured cells
- Removal of supersonic ion singularity in radial Langmuir probe models
- 2. References
- Study of variation of hysteresis effects in self–excited amplitudes of a coaxial DC electrode system
- 3. Results and Discussions
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1. Introduction Processes related to radiative transfer with reabsorption play a significant role in non- equilibrium plasmas. Multiple approximate approaches were developed over the last decade, among them so called matrix method (reviewed in [1]) for plasma sources of certain symmetry. The latter allows for accurate treatment of the radiation trapping process within consistent collisional- radiative models [2].
Figure 1: Density profiles of excited Ar atoms, obtained by means of matrix method and using escape factors: (a) free-burning arc; (b) constricted glow discharge.
Radiation transport equation for resonance atoms is solved in coupling with balance equations for other species. As Fig. 1 clarifies, correct description of radiation transport causes notable broadening of radial profiles of excited argon atoms, leading to an excellent agreement with experiment (Fig 1(b)). 3. Matrix method for arbitrary geometry For an arbitrary 3D object the source geometry is discretized on a Cartesian voxel grid. Matrix coefficients, which describe a coupling between unit volumes, are computed using fast ray traversal algorithm [3]. Numerical scheme is efficiently parallelized for running on a graphical processing unit. As an example, solutions of the Holstein- Biberman equation for case of finite cylinder with point excitation source in the center using previously developed matrix approach and a new one are illustrated by Fig.2. Results are in a good agreement.
Figure 2: Solution of the equation with point excitation source in the finite cylinder. Orange – old matrix method, blue – ray tracing method.
[1] Yu. B. Golubovskii et al., Plasma Sources Sci. Technol. 22 (2013) 023001. [2] Yu. B. Golubovskii et al., J. Phys. D. Appl. Phys. 49 (2016) 475202. [3] J. Amanatides, A. Woo, Eurographics 87 (1987) 3–10 Topic #2 244
XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Ground electrode Biased electrode Electrical probe Probe shadow Figure 2: Magnetic flux distribution (B
) along the axis of the cylinder (Z axis).
Figure 3: Plasma discharge inside Halbach array. White boxes with arrows represent the permanent magnets and their magnetic orien- tation.
O. Vasilovici 1,2 , S. Costea 1 , B.S. Schneider 1 , R. Schrittwieser 1 , C. Ionita 1
P 1 P
2 Faculty of Physics, Alexandru-Ioan-Cuza University, Iasi, Romania
For plasma confinement often magnetic fields are used, especially in fusion devices. Magnetic fields can either be produced by coils or by permanent magnets. Coils have the advantage of con- trolling the magnetic field strength by varying the current, but for high currents cooling systems have to be implemented. Permanent magnets can deliver magnetic flux into the airgap of a mag- netic circuit without continuous consumption of energy and nowadays they are fully competitive with electromagnets for fields up to 2 T, and fields as high as 5 T can be produced in small vol- umes [1]. We present a way to produce magnetized plasma using a special magnet assembly, known as the Halbach array, which is able to produce a homogeneous magnetic flux density in a cylindrical volume. Electric probes were used to characterize this highly magnetized plasma.
An ideal Halbach array is a ring magnet where the polarization direction varies continuously along the circumference so that the magnetic flux increases inside and reduces or cancels outside. In practice, typical Halbach cylinders are built using discrete permanent magnets each with its own magnetization direction, approximating the Halbach distribution [2]. Choosing the orientation of each segment properly, the fields will add at the centre. We have simulated the magnets' positions in or- der to obtain a uniform and homogenous magnetic field and the optimum cylinder bore diameter, using Quick Field v6.1 Student Edition software tool (Fig- ure 1). The input parameters for the magnetic mate- rial were set according to the magnet’s technical datasheet.
2. Plasma device To produce the magnetic field, we used 8 identi- cal 50×15×15 mm Nd 2 Fe 14 B cubic bar magnets. The perpendicular magnetic field strength (B
) was
measured along the symmetry axis (Z) using a tes- lameter (Figure 2). The plasma was created using two electrodes placed in such a way that the electrical field lines are parallel with the magnetic field (Figure 3). One elec- trode was grounded and the other was biased with negative voltages through a 2 KΩ resistor. 3. Acknowledgement This work was also supported by the CEEPUS net- work AT-0063.
[1] J.M.D. Coey, J. Magn. Magn. Mater. 248 (2002) 441–456 [2] C.K. Chandrana et al., J. Magn. Magn. Mater. 381 (2015) 396–400 Figure 1: Magnetic flux simulation of exper- imental Halbach array.
8 Poster x y 245 XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Measurement of reactive species in Plasma Babbled-up Water affecting human cultured cells J. Hosoda P 1
, U T. Miyake P 1 P , H. Kawano UP 1 P , M. Shimada P 2
, Y. Matsumura 3 3 P ,
H. Miyahara P 1 , A. Iwasawa P 3 , Y. Matsumoto P 2 , A. Okino P 1
1 FIRST, Tokyo Institute of Technology, Yokohama, 226-8502, Japan 2 Advanced Nuclear Research, Tokyo Institute of Technology, Tokyo, 152-8550, Japan 3
As a method to introduce reactive species generated by plasma into water, plasma babbling method was proposed. In this method, it is possible to introduce reactive species into water effectively, compared with conventional method in which plasma is irradiated from above the liquid surface. By plasma babbling method, we measured ozone and hydrogen peroxide concentration in water with various plasma. Ozone was generated in oxygen and air plasma, and was 8.6 µM and 0.5 µM respectively. Also, hydrogen peroxide was generated at all kinds of plasma. Both ozone and hydrogen peroxide were measured the most at oxygen plasma. As measurement result, it was revealed that the amount and type of reactive species depend on kinds of plasma gas. In recent years, atmospheric low-temperature plasma is being applied for medical fields. In addition, for the purpose of large capacity treatment, research about plasma-treated water in which plasma is introduced attracts a lot of attention[1]. However, to apply plasma for medical application, it is necessary to investigate the influence of plasma on living bodies. This main factor is considered to be reactive species such as ozone (O 3 ) and hydrogen peroxide (H 2 O 2 ) generated by plasma. The amount and type of reactive species depend on kinds of plasma gas[2]. In this study, we investigated reactive species introduced in water by various kinds of gas plasma and the influence of them on human cultured cells. As a method to introduce reactive species into the solution, we proposed a plasma bubbling method. In this method, the multi-gas plasma jet (PCT-DMFJ02, Plasma Concept Tokyo) is placed at the bottom of the container containing liquid. Then plasma is introduced as bubbles into the liquid directly. Therefore, the contact area between water and plasma is much larger than a conventional method in which plasma is irradiated from above the liquid surface. Thus, reactive species generated by plasma can be introduced into the liquid efficiently. In addition, since plasma is not influenced by ambient air, it is possible to specify reactive species derived from its own plasma gas. In this study, the water introduced reactive species by plasma bubbling method is called as Plasma Babbled-up Water (PBW).
After 200 mL pure water was babbled with argon, nitrogen, carbon dioxide, air and oxygen plasma at the plasma gas flow rate of 3 L/min for 5 minutes, ozone and hydrogen peroxide concentration in liquid were measured by absorption spectrophotometry. The results are shown in Fig.1. Ozone was measured in oxygen and air plasma, and the concentration was 8.6 µM, and 0.5 µM respectively. Also hydrogen peroxide was measured at all kinds of plasma. Both ozone and hydrogen peroxide were generated the most at oxygen plasma. In the presentation, we will report the measurement results of reactive species other than ozone and hydrogen peroxide, and of reactive species in solvent other than pure water. In addition, influence of reactive species in PBW on human cultured cells will be reported also.
Fig.1: O
3 and H
2 O 2 concentration at each Plasma Babbled-up Water 3. References [1]
K. Sato, K. Yasuoka, IEEE Trans. Plasma Sci., Vol.36, No. 4, pp. 1144-1145, Aug. 2008 [2] T. Takamatsu, K. Uehara, Y. Sasaki, H. Miyahara, Y. Matsumura, A. Iwasawa, N. Ito, T. Azuma, M. Kohno and A. Okino, RSC Adv., Vol. 4, No. 75, pp. 39901-39905, Apr. 2014 17
246 XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Removal of supersonic ion singularity in radial Langmuir probe models
G.F. Regodón P 1 P , U J.I. Fernandez Palop UP 1 P , A. Tejero-del-Caz P 2 P , J.M. Diaz-Cabrera P 3
, R. Carmona-Cabezas 1 , J. Ballesteros P 1
P
P
P
P
P
P
It is well known that a singularity appears when the ions reach the speed of sound in an electropositive plasma. For cold ions, the singularity is at infinity, and so it poses no problem the numerical integration of radial Langmuir probe models. However, for warm ions the singularity typically occurs between the quasi-neutral plasma and the sheath. We have found that we can continuously join the solution at the plasma with the probe thanks to a careful analysis of the mathematical structure of the problem. The technique can be applied to different geometries and to electronegative plasmas as well. For the case of cylindrical Langmuir probes, we have derived potential profiles, ion population profiles and ion current to probe voltage characteristics. These results are used to refine diagnosis techniques by means of Langmuir probes in laboratory plasmas.
In the interest of obtaining the potential profile φ(r) around a Langmuir probe in electropositive plasmas one should solve Poisson’s equation. In the case of cylindrical geometry, we have ! " d d" ?????? d% d" = − ) * + ?????? - ?????? − ?????? ) ?????? . (1) The electron density n e (r) will be described by the Maxwellian distribution function, whereas the ion density n + (r) depends on the ion motion theory used. When using a radial motion theory, the thermal
1 2 ?????? - ?????? - 3 ?????? + ???????????? ?????? + ?????? ?????? − 1 ?????? 9
- ??????
- " ??????
); <=! =
?????? ?????? − 1 ?????? 9
- ,
(2) where κ is the adiabatic coefficient of the thermal flow. As v
(r) is inversely related to the ion density through continuity equation, i being the ion current per unit length collected by the probe, ?????? = ??????2???????????? - ?????? ?????? - ?????? , (3) we get a polynomic equation in n + (r), with defining parameters r and φ(r), which should be solved in order to introduce its value into Poisson’s equation. We have found that this polynomial has two positive
problem variables r and φ. We further found that one of the roots is valid in the plasma in the limit ?????? → ∞, where the ions are at rest, while the other root is valid in the sheath in the cold ions limit ?????? -
We have proved that the transition between these roots must occur, in the variable space (r, φ), in the curve where the two positive roots of the energy balance polynomial coalesce in a sort of bifurcation line, and that the only possible smooth and continuous crossing through the regular singularity [2] where the ions reach the speed of sound is tangent to that bifurcation curve. In figure 1 we show an example of solution of the potential profile.
Figure 1: Potential profile and bifurcation curve solution. The small inserts are qualitative plots of the polynomic equation for n + (r).
This method is valid for any ion temperature. We indicate with a triangle the point where the speed of sound is reached. To the plasma or to the sheath we use the right energy balance polynomial root, as we mark with a dot in the inserts in the figure.
[1] J.I. Fernández Palop et al 1996 J. Phys. D: Appl. Phys 29 2831. [2] H.B. Valentini 1988 J. Phys. D: Appl. Phys 21 311-321 ▲ 0 10 20 30 40 0.0
0.2 0.4
0.6 0.8
1.0 1.2
1.4 r/λ
D -e ϕ (r) / k B T e i = 10·2πen e0 λ D √ T + = 0.5T
e κ = 2
r c =19.25λ D 2k B T e m+ . . . Topic 6 247
XXXIII ICPIG, July 9-14, 2017, Estoril/Lisbon, Portugal
Study of variation of hysteresis effects in self–excited amplitudes of a coaxial DC electrode system
R. Kumar P 1 P , R. Narayanan P 2 P , R. D. Tarey P 2
, A. Ganguli 2 P
P P 1 School of Basic and Applied Sciences, Shobhit University, Meerut–250110, U.P., India 2 C C
The paper analyzes changes in behaviour of self-excited oscillations resulting from extended plasma exposure in coaxial DC discharge plasma having a central powered anode. The role of the system asymmetry seems to play a role in triggering the oscillations. These oscillations are seen to undergo hysteresis effects with discharge current (I
), showing a characteristic difference in the hysteresis shape as the system ages with plasma exposure. The shape change is from a hysteresis, with negligible amplitude shift post hysteresis (Type H4: bump-shaped), to a more generally observed one having a noticeable amplitude shift (Type H1: S shaped). Analysis tools such as phase maps, return maps, recurrence plots are used to characterize the variation of the observed changes in the oscillations and attempt to unravel the underlying physical mechanism to explain it.
Observations of order-to-chaos-to-order transitions in the self-excited oscillations of plasma experiments have been reported [1, 2] earlier. In a study by Kumar et. al. [1], the associated order-to- chaos-to-order transitions have been correlated to a hysteresis in the amplitude of the floating potential fluctuations (V
).
2. Experimental Setup
Fig 1: Schematic setup of coaxial DC plasma system Experiments were carried out in a coaxial DC electrode (central anode diameter = 1.5 mm; outer cathode inner diameter = 48 mm) discharge system (Fig 1). A Langmuir probe, as seen in Fig 1, is used to measure the self-excited V f oscillations.
Under certain operating conditions, hysteresis is observed in V
amplitudes as a function of the discharge current (I
) [1]. This amplitude hysteresis effect is observed between two Negative Differential Resistance (NDR) regions of the discharge characteristics (discharge current, I
vs discharge voltage, V
), which is also seen to undergo a hysteresis effect. With plasma conditioning of several days one observes a flip in the forward-reversal paths of the
-V d hysteresis. The plasma potential (V p ) is seen to be lower in the conditioned electrode discharges by about 30 V - 40 V. However, the more significant effect observed is modification of the behaviour of the self-excited oscillations also with the hysteresis flip in the I
-V d characteristics. For convenience, the unconditioned electrode is termed UE and the conditioned one is termed CE. In the UE case, the hysteresis in the self-excited oscillations is to trigger higher amplitude oscillations in the forward path over a small range of
, reverting back to lower amplitudes at higher I d
(H1-type [3] or bump-shaped). However, in the CE case, the fluctuations show a characteristic change of state at higher I d , viz., a transition from low amplitude, high frequency oscillations to large amplitude, low frequency oscillations (H4-type [3] or S-shaped). This abstract will present characteristic features of the variations in the two discharge cases using nonlinear dynamical analysis tools.
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