Dense plasma formation on the surface of a ferroelectric cathode

Transcription

1 Vacuum ] (]]]]) ]]] ]]] Dense plasma formation on the surface of a ferroelectric cathode K. Chirko, Ya.E. Krasik, A. Sayapin, J. Felsteiner Physics Department, Technion Israel Institute of Technology, 3 Haifa, Israel Received 1 February ; received in revised form July ; accepted 18 August Abstract An enhanced electron emission mode of the ferroelectric plasma cathode operation is reported. The enhanced emission is achieved due to the generation of dense plasma ( m 3 ). This plasma is formed by a flashover which is initiated by charged particles. These particles are attracted to the ferroelectric surface by a driving electric field and are released during its decay. Generation of an electron beam with current amplitude p. ka is demonstrated in a diode under an accelerating voltage of 1 3 kv and pulse duration of 3 ns. r Elsevier Ltd. All rights reserved. Keywords: Plasma sources; Gaseous discharge; Electron beams; Ferroelectric cathodes 1. Introduction Copious electron emission from ferroelectric cathodes (FC) has been studied widely during the last several years [1,]. Recent investigations [,3] have shown that the application of a driving pulse with an amplitude of several kv (electric field EX1 MV/m) to a ferroelectric sample with a front electrode made of strips and a solid rear electrode leads to plasma formation at the samples front surface. The operation of electron diodes with FC Corresponding author. Tel.: ; fax: address: fnkrasik@physics.technion.ac.il (Ya.E. Krasik). was investigated at accelerating voltages of different amplitudes and pulse durations []. Generation of electron beams with current density p ka/ m was demonstrated in the plasma pre-filled mode. In the case when the plasma prefilling of the accelerating gap was prevented, the extracted electron current density was found to be p1 ka/m []. In this case, the density of the FC plasma limits the extracted electron current density. In this paper, we present experimental studies of two modes of the FC operation in which electron beams are extracted with enhanced current density due to a significant increase of the surface plasma density. The increase of the plasma density was achieved by ionization of neutral flow which -7X/$ - see front matter r Elsevier Ltd. All rights reserved. doi:1.116/j.vacuum..8.18

2 K. Chirko et al. / Vacuum ] (]]]]) ]]] ]]] accompanies the formation of the surface plasma []. It was shown that the application of a driving pulse with either slow rise time but short fall time or with a different time delay between the rise and fall slopes of the driving pulse leads to formation of a much denser plasma (up to m 3 compared with o 1 18 m 3 using a nanosecond rise time driving pulse).. Experimental setup The FC was in the form of a disk made of BaTi solid solution with a diameter of 1 mm and a thickness of 8 mm. It was covered by a strip-like front and solid rear electrode, both made of copper (see Fig. 1). The strips were of 1 mm width and had the same distance between them. The ferroelectric sample was placed in a cylindrical aluminum box with an output window covered by a stainless-steel grid with transparency of 7% positioned at a distance of 3 mm from the front electrode. The experiment was carried out in a chamber evacuated to.7.6 mpa. The cathode was ignited by either a positive or a negative driving pulse with a rise time of ns and its amplitude j dr was varied in the range of 1 kv. A gas spark switch was used to control the duration of the driving pulse. By the use of a delayed trigger pulse, the duration of the driving voltage pulse was varied in the range of s. An additional variable inductor was used in the discharge circuit in order to change the fall time of the driving pulse in the range of ns. A fast framing camera QuikA (Stanford Computer Optics, Inc.) was used to observe the visible light emission from the surface plasma. The same camera was used to observe the light emission in the accelerating gap during the accelerating pulse. The plasma parameters were measured by a spectroscopic method. To study the profiles of spectral lines, we used a Jobin-Yvon 7M spectrometer [6]. Waveforms of the current flowing through the ferroelectric sample and the voltage on the sample were monitored by a Rogowsky coil and a high-voltage divider, respectively. Movable biased collimated Faraday cups (CFC) with and without transverse magnetic field were used to measure the current density of the electron and ion flows at different distances from the front surface of the FC. The energy spectrum of the electrons emitted from the surface plasma was measured by a negatively biased collector. A fast Penning probe was used to measure the density and propagation velocity of the neutrals formed on the surface of the FC. We tested the electron beam generation in a planar diode. An accelerating pulse was supplied by a pulse-forming network (PFN) generator (accelerating voltage amplitude j AC p3 kv; internal impedance ffi8o, pulse duration ffi3 ns). A positive accelerating pulse was applied to the anode that was placed at distances of d AC ¼ cm from the cathode. 3. Experimental results Fig. 1. Experimental set up. FC Faraday cup, FES ferroelectric sample, VD voltage divider, RC Rogowsky coil, SG spark gap. First, we performed framing photography (frame duration of 1 ns) of the visible light emitted from the front surface of the FC during the fall of the driving pulse. We studied the influence of the driving pulse polarity and fall duration on the visible light emission (see Fig. ). One can see that, for a positive driving pulse, a bright light emission begins within 3 ns after switching off the driving voltage (Fig. a). The beginning of the fall time of the driving pulse is

3 K. Chirko et al. / Vacuum ] (]]]]) ]]] ]]] (a) (c) 1 1 Light intensity [arb.units] 3 (b) Driving pulse [kv] Light Intensity [arb.units] (d) Drivingpulse [kv] Time [ns] Time [ns] Fig.. Time dependence of the light emission build-up for positive and negative driving pulses obtained by the fast framing camera (frame duration of 1 ns). Positive (a) and negative (b) driving pulses with fall time of 8 ns; positive (c) and negative (d) driving pulses with fall time of ns. characterized by the formation of intense surface discharges and further, these separate surface discharges transform into a diffuse discharge. The latter can be associated with the ionization of adsorbed molecules and atoms [7,8]. In the case of a negative driving pulse bright light emission was also obtained only after the switching process (Fig. b). However, in this case, the light emission was much less intense and less uniform compared with the case of the positive driving pulse. The obtained data are opposite to those presented in Refs [3,9]. when the most uniform and intense light emission was obtained during a nanosecond rise time of a negative driving pulse applied to the rear electrode. Also, it was found that the sharper the fall of the driving pulse, the more intense was the light emission. A decrease of the duration of the fall from ns to 1 ns leads to an increase of the light emission by almost 1. times (Fig. c and d) A comparison of the light intensity emitted during the rise (first current pulse) and the fall (second current pulse) of the driving pulse is presented in Fig. 3 (frame duration t f ¼ ns). The fall of the driving pulse was delayed with (a) (b) Delayed fall of the driving pulse τ 1 = 1µs τ 1 = µs τ 1 =ms Fig. 3. A set of framing photographs of the cathode surface. (a) Light emission during the fall of a non-delayed driving pulse. (b) Light emission during the delayed fall of the driving pulse. t 1 is the time delay between the beginning of the driving pulse and the start of the fall of the driving pulse. Driving pulse voltage j dr ¼ 1 kv: respect to the beginning of the driving pulse within a time delay range of t 1 ¼ s: One can see bright light emission associated with the plasma formation on the surface of the FC at the beginning of the driving pulse. Further this light emission decays quickly (within 1 ms). An increase of t 1 up to 1 s did not produce any significant difference in the obtained images of the light emission during the second current pulse. However, a further increase of t 1 caused a gradual increase of the intensity of the light emission and, beginning from t 1 X1 s; a

4 K. Chirko et al. / Vacuum ] (]]]]) ]]] ]]] uniform light emission from all the surface of the FC was obtained. The most uniform and bright light emission was observed in the range of t 1 ¼ ð8þ1 s: A further increase of t 1 leads to a decrease in the light emission. Thus, the images indicate that the more intense and uniform plasma formation is caused by the delayed fall of the driving pulse. Next, we performed spectroscopic measurements of the parameters of the plasma that exist during the fast fall of the driving pulse with a slow rise time. We observed spectral lines of ions and neutral atoms of Cu, Pb, Sr, Ba, Ti, and H which appear within the first ns after the beginning of the switching process. Thus, the discharge of the ferroelectric capacitor is accompanied by the formation of surface plasma consisting of the atoms and ions of the ferroelectric material, absorbed gases and the material of the front electrode. The electron plasma density n e and the ion temperature T i were estimated by analyzing the Stark and Doppler broadening of the hydrogen H a and H b spectral lines [1]. This calculation yielded T i ¼ : :1 ev and n e ffi m 3 : This is 1 times larger than the density of the plasma (at a distance of 1 mm from the front surface of the ferroelectric) formed by a fast rising driving pulse []. The electron temperature T e was determined by comparing the observed relative intensities of the hydrogen spectral lines with the calculated population ratio of these energy levels [6]. The best fit of the measured and calculated population ratios for n e ffi m 3 was obtained for T e ffi 7eV: Next, we measured the current density of the charged particle flows by a biased CFC placed at different distances from the front surface of the FC (see Fig. ). It was found that, for a positive driving pulse, during its rise time, the electron flow is negligible but it becomes significantly large during the fall of the driving pulse. For instance, at a distance of cm from the front surface of the FC, the amplitude of the electron current density reaches j e ka=m (a total emitted electron current of I e A). It was found that the faster is the fall of the driving pulse, the larger the j e. For instance, j e increases two-fold with the decrease of the fall time from 1 ns to ns. This agrees with ϕ dr =1kV/div ϕ dr =1kV/div j e =1kA/m /div j e =1kA/m /div Time= ns/div Fig.. Typical waveforms of the electron current density measured by a positively biased CFC and typical waveforms of the driving pulse. (a) Positive driving pulse; (b) negative driving pulse. the increase of the light intensity and the decrease of the fall time of the driving pulse. Using a negatively biased CFC with permanent magnets placed at a distance of 3 cm from the output cathode grid, we observed ion flows emitted from the surface plasma (see Fig. ). The obtained ion saturation current density j ip during the fall of the driving pulse is 1 times larger than j ip obtained during the rise of the driving pulse. Also, it was found that there is an optimal t 1 ms when the total ion charge collected by the negatively biased CFC, Q i ¼ R j ip dt; is maximal (see inset in Fig. ). This indicates a maximum in the density of the plasma formed at this t 1 and agrees qualitatively with the obtained framing images. Also, it was found that the faster the fall of the driving pulse, the larger the j ip. For instance, the decrease of the fall time from 13 ns to 8 ns leads to a (a) (b)

5 K. Chirko et al. / Vacuum ] (]]]]) ]]] ]]] Current density [A/m ] 3 1 τ 1 =µs τ 1 =6µs Charge density [mc/m ] τ 1 =µs. 6 Time delay [µs] Driving rise 6 8 Time [µs] two-fold increase in j ip from to 1 A/m.In order to estimate the plasma ion density we calculated the total ionic charge of the slow plasma flow which was collected by the CFC. Assuming the thickness of the surface layer, where plasma is formed as p1 mm, we obtained a plasma density of X1 19 m 3. This estimate agrees with the data obtained by spectroscopic measurements. We measured fast and slow neutral flows which accompany the plasma formation [6]. The velocity of the flow of heavy atoms (Cu, Ba, Sr) was estimated to be p1m/s. Thus, in order to form a uniform layer of neutrals with a thickness of a few millimeters, one requires t 1 X1 s: This time coincides with the observed framing images which show that uniform light emission begins at t 1 X1 s: A rough estimate of the neutral density at a distance of 1 mm from the FC surface yields a value X1 1 m 3. Finally, we carried out the experiment of electron beam generation in a planar diode with the FC and compared its emission properties at the rise and the fall of the driving pulse. In each generator shot, the light emission from the accelerating gap was monitored in order to assure the absence of the explosive emission. In Fig. 6, we present the dependence of the maximal amplitude of the diode current I d on the time delay Dt 1 and Dt between the beginning of the driving pulse and Fig.. Typical waveforms of the ion saturation current density measured by a biased CFC at a distance of 3 cm from the output cathode grid at different values of t 1 : j dr ¼ 13 kv: The dependence of the ion charge density on the t 1 is shown in the inset. Current [ka] the start of the accelerating pulse. One can see that the plasma formed during the second driving current pulse allows generation of an electron beam with. times larger amplitude. Also, a simple estimate of the space-charge limited diode current [11] yields I sc : ka for d AC ¼ 3 mm; accelerating voltage j AC ffi kv and cathode area S c =.1m. This value of I sc coincides well with the diode current amplitude at t 1 ms and Dt X1 ms: In fact, this plasma allows electron beam generation with significantly larger current amplitude at smaller values of d AC. But because of the 8 O internal impedance of the high-voltage generator, we obtained a smaller j AC in the anodecathode gap at I d :ka:. Conclusions Second current pulse First current pulse Time delay[µs] Fig. 6. Dependence of the diode current on Dt 1 and Dt for the first and second current pulses at t 1 ¼ ms; j dr ¼ 13 kv; j AC kv; and d AC ¼ 3cm: It has been shown that, for FC during the fall of the driving pulse, the fast release of the bound surface charges also causes intense electron avalanching that leads to dense plasma formation. It was shown that the increase either in the discharge rate of the FC or in the time delay between the charging and the discharging current pulses lead to a significant increase of the plasma density. This happens due to the enhancement in the number of charged particles participating simultaneously in the avalanching process when compared to the plasma formation during the rise time of the

6 6 K. Chirko et al. / Vacuum ] (]]]]) ]]] ]]] driving pulse. The maximal obtained plasma density was found to be E m 3. This plasma density allows generation of an electron beam with a current density up to 1 3 ka/m. References [1] Gundel H, Reige H, Handerek J, Zioutas K. Appl Phys Lett 1989;:71 3. [] Rosenman G, Shur D, Krasik YaE, Dunaevsky A. JAppl Phys ;88: [3] Krasik YaE, Dunaevsky A, Felsteiner J. J Appl Phys 1999;8: [] Dunaevsky A, Krasik YaE, Felsteiner J, Sternlieb A. J Appl Phys ;91: [] Dunaevsky A, Krasik YaE, Felsteiner J, Sternlieb A. J Appl Phys 1;9: [6] Dunaevsky A, Chirko K, Krasik YaE, Felsteiner J, Bershtam V. JAppl Phys 1;9:18 1. [7] Dunaevsky, Krasik YaE, Felsteiner J, Dorfman S, Berner A, Sternlieb A. JAppl Phys 1;89:8. [8] Miller HC. IEEE Trans Electr Insul 1989;: [9] Dunaevsky A, Krasik YaE, Felsteiner J, Dorfman S. J Appl Phys 1999;8:86 7. [1] Griem HR. Plasma Spectroscopy. New York: McGraw- Hill; 196. [11] Miller RB. Introduction to the Physics of intense charged particle beams. New York: Plenum; 198.