1) Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia

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1 SI Nuclear Energy in Central Europe '98 Terme Catez, September 7 to 10, 1998 PLASMA RESPONSE TO A POSITIVE VOLTAGE STEP APPLIED TO AN ANODE IMMERSED IN A WEAKLY MAGNETIZED DISCHARGE PLASMA COLUMN T. Gyergyek 1 ' 2, M. Cercek 1 and M. Stanojevic 1 1) Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 2) Faculty of Electrical Engineering, University of Ljubljana, Trzaska 25, 1000 Ljubljana, Slovenia Abstract Positive voltage steps are applied to an electrode immersed in a weakly magnetized discharge plasma column. Time development of electron distribution function is measured by a one sided plane Langmuir probe and a standard boxcar technique. When a voltage step is higher than approximatelly 40 V, a firerod is created and it is measured how electron beam from cathode plasma is gradually transformed into anode plasma population. 1 Introduction Phenomena related to positive electrodes immersed in plasmas are a subject of intense research activity [1]. In this work response of a plasma to a positive voltage step applied to a planar anode immersed in a weakly magnetized discharge plasma column with its surface perpendicular to the magnetic field lines is investigated. Temporal development of the electron distribution function is measured using one sided plane Langmuir probe. The surface of the probe is parallel to the surface of the anode and perpendicular to the magnetic field lines. The electron Larmor radius is approximatelly 0.1 mm. so a one dimensional electron distribution function is measured. Let us denote the direction of the magnetic field by z. The corresponding one dimensional electron velocity distribution function is f ez {v z ). The electron probe current l v is given by I p = e 0 A p /,- - f ez (v z )v z dv z, Nuclear Energy in Central Europe '98 75

2 BOXCAR GATE E Figure 1: Experimental set-up. The transistor switch is turned on and off with pulses from the signal generator. These pulses are also used as a master trigger for the experiment. In this way a good time resolution is achieved. where U p is the probe bias, measured with respect to the plasma potential and A p is the probe surface. Differentiation of the above integral yields [2] m e dl p P 2 A All (1) So the one dimensional electron velocity distribution function in the energy scale can be obtained from the first derivative of the electron current collected by a Langmuir probe. Characteristics are taken by a one sided plane probe with 3 or 5 mm diamater. First the nonisolated surface of the probe is turned towards the plasma source - cathode, and then towards the anode by which the double layer is created. In this way the electron distribution function is measured in positive and negative direction of z axis. 2 Experiment Experiments are performed in a linear magnetized discharge plasma machine at the J. Stefan Institute in Ljubljana (figure 1). The experimental vessel is evacuated below 10~ 4 Pa.. Then argon is leaked into the vacuum system. The pressure is kept between 0.02 and 0.03 Pa. Plasma is produced by a discharge from WjTh filaments heated by direct current. Primary electrons are accelerated by a discharge voltage V and plasma is created by 16. Nuclear Energy in Central Europe '98

3 impact ionization. The discharge current is between 120 and 300 ma. Plasma is confined by axial magnetic field, which has the density 0.01 T. The electron temperature is approximatelly 2 ev and ion temperature is approximatelly 0.1 ev. Larmor radius for electrons is approximatelly 0.2 mm and for ions about 2 cm. The plasma density is between 10 8 and 10 9 cm" 3. 2,0 r 1,5 a. E 1,0 CD 0,5 0, time [ microsec. ] 30 Figure 2: Time dependence of anode current I a after the application of a positive voltage pulse. Positive voltage pulses are applied to a 2 cm diameter anode using a fast transistor switch. The switch is turned on and off by rectangular pulses from the signal generator which are also used as a master trigger for the experiment. The switch is 2 ms on and 2 ms off. When the switch isoff, the anode is at the floating potential, which is around -20 V. When the switch is on, the anode bias is U a and is determined by the external power supply. When the switch is turned on, the anode current increases instantaneously (figure 2). A current overshoot can be observed. Time resolved measurements of probe charateristics are performed using the standard boxcar technique. Temporal resolution is 1 ^as. The characteristics are digitized and stored in a computer for further analysis. 3 Experimental results When anode bias U a is varied continuously, the anode exhibits the usual current - voltage characteristics (figure 3). At strongly negative electrode bias the current to the electrode is positive. This is the ion saturation current. When the electrode bias becomes less negative, more and more electrons can reach the electrode and the current in the transition region is negative. At the knee of the characteristics, electrode bias reaches the plasma potential. When Nuclear Energy in Central Europe '98 77

4 biased above the plasma potential, the anode collects almost constant electron saturation current. When anode bias U a exceeds approximately 40 V, there is a sudden increase of the anode current. This is beacause additional discharge occurs in front of the anode and a region of quasineutral plasma with higher potential - called the anode plasma or firerod [3] - is created. The high potential - anode - plasma and a low potential - cathode - plasma are separated by a thin space charge structure called the double layer. 0,0-0,1-0,2 "0.3-0,4-0,5-0, Figure 3: The current - voltage characteristics of the anode. When U a exceeds approximately 40 V the high potential - anode - plasma is created in front of the anode. In figure 4 response of the onesided plane Langmuir probe characteristics to the application of a positive voltage step to the anode is shown. The ion saturation current is already subtracted from the probe characteristics. This is done in the following way. A straight line is fitted to the ion saturation part of the characteristics and then this line is subtracted in the whole characteristics. Then the characteristics is smoothed with a digital low pass filter using FFT and differentiated. The first derivative of the characteristics is proportional to the electron energy distribution function (equation 1). Semilog plots of the characteristics are also shown. The characteristics taken when the collecting surface of the probe is turned towards the cathode are labeled by C. If the collecting surface of the probe is turned towards the anode the characteristics is labeled by A.-In this way the entire electron distribution function in the direction of the magnetic field can be analyzed. The distance between the anode and the probe is z = 3 cm. Before the application of the voltage step, the anode is floating. Only one 78 Nuclear Energy in Central Europe '98

5 Figure 4: Characteristics of the onesided plane Langmuir probe recorded, when the anode is floating and 1 /J,S after the application of different voltage steps, together with corresponding derivatives and semilog plots. electron population - the cathode plasma electrons - can be observed on the derivatives (i. e. electron distribution functions) and semilog plots. From the slope of the line of the linear part of the semilog plot the electron temperature can be prety accuratelly determined. Temperature of the cathode plasma electrons is approximatelly 2 ev. Immediatelly (I/us) after the application of the positive voltage step to the anode, a strong electron beam is accelerated towards the anode (figure 4). The energy of the beam [/& is measured as the distance between the peaks of the derivatives of the probe characteristics, i. e. the electron distribution functions and it increases linearly with the applied voltage step (figure 5a). Also the temperature of the electron beam kt e b can be determined from the semilog plots. This temperature is higher than the cathode plasma electron temperature and it increases, when the voltage step to the anode is increased (figure 5b). In figure 6 time development of the Langmuir probe characteristics after the application of a high (approximatelly 45 V) positive voltage step to the anode is analyzed for first 20 /is after the step. The voltage step is so high, that the firerod is created in front of the anode. The ion current on the probe charateristics is already subtracted as described earlier. The corresponding derivatives and semilog plots are also shown. The distance between the anode Nuclear Energy in Central Europe '98 79

6 (a) (b) > > 9" 0) U a [V] 35 Figure 5: The energy of the electron beam Ub and the electron beam temperature kteb versus the applied voltage step U a. and the probe is z 1 cm. The main result are the derivatives, which are proportional to the electron energy distribution functions. When the anode is floating, only one electron population exists - the cathode plasma electrons. Immediatelly after the voltage step, the electrons are strongly drawn towards the anode. This can be seen in the form of a strong electron beam, detected by the probe turned towards the cathode. This beam thermalizes very quickly. Approximately 3-4 pis after the voltage step the formation of second (anode plasma) electron population can be already very clearly observed. These electrons have their distribution centered around a much higher probe bias U v than the cathode plasma electrons. Also their distribution is wider because of the higher' electron temperature. The cathode plasma electrons are also still present. It can be very well observed from the derivatives, how the cathode plasma electron population gradually vanishes, while the density of the anode plasma electrons increases. Some fj,s after the voltage step the cathode plasma electrons almost disappear. Measurements shown in figure 6 were made at z = 1 cm. At larger distances between the probe and the anode results are similar, only the process of the formation of the anode plasma electron population and disapperance of the cathode plasma electron population is somewhat delayed. This indicates that the formation of the anode plasma starts at the anode sheath, which then expands into the cathode plasma. Such scenario is described in the literature [4]- 80 Nuclear Energy in Central Europe '98

7 Figure 6: Plane Langmuir probe characteristics, the corresponding derivatives (distribution functions) and semilog plots recorded at different times after the application of the voltage step to the anode. Curves labeled by C are recorded with the collecting surface of the probe turned towards the plasma source - cathode. Curves labeled by A are taken with the probe turned towards the anode. Nuclear Energy in Central Europe '98 81

8 4 Conclusions Response of a weakly magnetized discharge plasma column to the application of a positive voltage step to an anode immersed in the plasma has been investigated experimentally. Temporal development of the electron energy distribution function has been measured using a onesided plane Langmuir probe. Immediatelly after the application of the voltage step, a strong electron beam is accelerated towards the anode. The energy of the beam is proportional to the applied voltage step. The temperature of the beam increases, if the voltage step has increased and it is 3-5 times of the temperature of the cathode plasma electrons. If the voltage step is high enough, (above approximately 40 V) a firerod is created. Differences between time developments of the electron distribution function in the case when the firerod is created and in the case when it is not created, will have to be investigated in more detail in the future. 5 References [1] Fourth Symposium on Double Layers and other Nonlinear Potential Structures in Plasmas, Innsbruck, Austria July 6-8, 1992, R. Schrittwieser, (editor), World Scientific, (1993) [2] J. D. Swift and M. J. Schwar, Electrical probes for plasma diagnostics, Iliffe Books Ltd., (1970) [3] Tao An, R. L. Merlino and N. D'Angelo, J. Phys. D: Appl. Phys., 27, (1994), [4] M. Sanduloviciu, C. Borcia, G. Leu, Phys. Letters A, 208, (1995), Nuclear Energy in Central Europe '98