J. Plasma Fusion Res. SERIES, Vol. 8 (2009) Portable Marx Generator for Miroplasma Appliations T. UENO*, T. SAKUGAWA**, M. AKIYAMA**, T. NAMIHIRA**, S. KATSUKI** and H. AKIYAMA** *Department of Eletrial and Eletronis Engineering, Oita National College of Tehnology, Maki 1666, Oita 870-0152, Japan **Graduate Shool of Siene and Tehnology, Kumamoto University, Kurokami 1-39-1, Kumamoto 860-8555, Japan (Reeived: 31 August 2008 / Aepted: 20 Marh 2009) There are many appliations using pulsed power suh as the triggering of high speed ameras and spetrometers, the proessing or hemial analysis of minute material, and the stimulation of ells by eletrial pulses. The pulse width and voltage required for eah appliation are different in the range of ns and kv. The energy required is usually in the mj regime. Here, a novel portable pulse power generator with 35 ns pulse width and 2.5 kv output voltage has been developed, and applied to the miroplasma prodution. The solid-state swithes are used for the high speed losing. The advantages of solid-state swithes are ompatness, reliability, stability of the breakdown voltage, and lifetime in omparison with those using disharges. The key point to ahieve the fast risetime is to use the avalanhe breakdown of the bipolar juntion transistors (BJTs). The behavior of the basi swithing iruit using BJTs in avalanhe mode is desribed, and also the developed pulsed power generator is applied to produe the miroplasma. Keywords: Marx generator, Avalanhe breakdown, Miroplasma, Pulsed Power 1. Introdution The miroplasma jet has been used in various fields suh as materials proessing, hemial analysis, short-wavelength light soure, and biomaterials treatment [1, 2]. The miniaturized eletri power generator has been requested to produe these plasmas in the miro-sale area, beause the present large size generator limits the use of these small plasmas [3]. Under suh bakground, a miniaturized Marx generator whih has Bipolar Juntion Transistors (BJTs) as losing swithes has been developed to generate miroplasma jet [4]. In the miniaturized Marx generator, BJTs were operated in avalanhe mode to obtain a faster swithing speed with nanoseonds regime. In this paper, three kinds of BJTs whih have the different olletor urrents are evaluated as losing swith, and the miroplasma jet is generated by the miniaturized Marx generator. The generator is able to output -2.5kV, 38ns pulse-width by series BJTs for utting load urrent. It has been found that the length of miroplasma jet was depended on load urrent. 2. Marx generator In the onventional Marx generator, the pulse rise time is mainly limited due to the speed of the swithes and the stray indutane. After the swithes turn on, the iruit ueno@oita-t.a.jp 1339 is modeled as a simple RLC iruit, where a harged apaitor is disharged through a RL series onnetion [5]. The effetive apaitane C is C = C stage / N, where C stage and N are the stage apaitane and the number of stages, respetively. Indutane L and resistane R are lumped from all soures and inlude the load indutane and resistane. In the ase that the iruit resistane R is smaller than R<2 (L/C) -0.5 the rise time is dominated by the osillatory frequeny and may be approximated by t ru =1.2 (L C) 0.5. In the ritially damped ase where R=2 (L/C) -0.5 the rise time is given by t r =2 L/R, and in the over damped ase where R>2 (L/C) -0.5 the rise time is dominated by the indutane and an be approximated with t ro =2.3 L/R. In the ase that rise time is ritial the over damped ase is preferable, sine t ro <t r <t ru. In any ases, the indutane of the system should be minimized to redue the rise time. The disussion above neglets the swithing transition and assumes that the swith works instantaneously, and frequently the transition itself would determine the urrent rise time into the load. 3. Avalanhe breakdown The avalanhe breakdown ours when the olletor to emitter voltage is over a breakdown voltage. It is diffiult to haraterize the breakdown voltage from the typial datasheets. In this hapter, the BJTs as swithing 2009 by The Japan Soiety of Plasma Siene and Nulear Fusion Researh
devie of the miniaturized Marx generator are evaluated. Figure 1 shows the evaluation iruit of BJT swithing. The iruit is onsisted of a harging resistor (R h =20 k ), a BJT, a apaitor and TTL signal generator. In this work, three BJTs, named 2SC2655, 2SC5076 and 2SC5000 (NPN type Bipolar Juntion Transistor, Toshiba, Japan), are evaluated as a losing swith. The maximum voltage between olletor and emitter, V CEO of three BJTs on the data sheets, is 50V. The maximum olletor urrents, I o of 2SC2655, 2SC5076 and 2SC5000 BJTs, are 2A, 5A and 10 A, respetively. For the evaluation of BJTs, the apaitane is hanged from 0.015 F to 1.0 F. The apaitor is harged till V h through the harging resistor. The TTL signal from generator (33220A, Agilent) is applied between the base and the emitter of BJT at 3 pps (pulses per seond). The voltage between olletor and emitter, V, and the olletor urrent, I, are measured by the high voltage probe (PHV641, PMK) and the urrent transformer (Model 2877, Pearson Eletronis) whih is loated on the emitter of BJT, respetively. The signals are reorded by the digital storage osillosope (TDS3034B, Tektronix). 3-2.Swithing voltage and urrent waveforms Figure 3 shows the typial V and I waveforms in the ase of using 2SC2655 BJT and 0.1 F of the apaitor. Figure 3 (a) shows the waveforms in the operation with 50 V of V h. On the other hand, Figure 3 (b) indiates the waveforms in the operation with 140 V of V h. It is observed from Figure 3 (a) that the V falls down from 50 to 0 within 500 ns. The peak of I is limited lower than 6 A, whih is 3 times of speified urrent. Here, it is noted that the swithing time is defined as the time duration of the voltage fall time from 90 % to 10 % of V h. From Figure 3 (a), the swithing time of 2SC2655 BJT is 500 ns. From Figure 3 (b), the V falls down from 140 to 0 within 150 ns. The peak of I is 90 A whih is 45 times of speified urrent. The swithing time of 2SC2655 BJT is 120 ns. Figure 4 shows the waveforms of I in the ase of using 2SC5000 BJT and 0.1 F of the apaitor for three different V h. It is shown from Figure 4 that the operation mode of BJT shifts from the normal mode to the avalanhe mode when I exeeds the threshold urrent. This threshold urrent is about 3 times of speified urrent. This threshold urrent is different depending on the V h. As V h inreases, the rise time of I beomes faster, therefore I exeeds the threshold at the earlier time. Fig.1 Evaluation iruit of BJT. 3-1. Leakage urrent in avalanhe breakdown Figure 2 is the dependene of V on V h in the ase of using 2SC2655 BJT. The V inreases linearly with V h in the range of 0 to 160 V. Over 170 V of V h, V is less than V h. The leakage urrent in the evaluation iruit appears when V h reahes 170V. In the ase of other BJTs (2SC5076 and 2SC5000), the onset voltage of the leakage urrent is 200V. From these results, the further evaluations of BJTs are arried out in the range from 0 to the onset voltage of leakage urrent. (a) When BJT is operated in normal mode. (b) When BJT is operated in avalanhe mode. Fig.3 Waveforms of V and I. Fig.2 Dependene of V on V h (2SC2655). Fig.4 Colletor urrent waveforms for three different V h. (2SC5000, C=0.100uF). 1340
3-3.Current and voltage harateristis in avalanhe breakdown Figure 5 shows the urrent and voltage harateristis in the ase of using 2SC2655 BJT. In normal mode operation, I is limited by the speifiation urrent. In the avalanhe mode operation, I inreases faster and reahes the value, whih is given by I = V C L, where C and L are the apaitane and iruit indutane, respetively. This relational expression is lear from Figure 5, sine I is proportional to V or square root C in the avalanhe mode operation. Figure 6 shows the omparison of the urrent and voltage harateristis for different BJT. When BJT is operated in normal mode operation, I of eah BJT has the 5 times differene. However, in avalanhe mode operation, I of eah BJT is almost same. Hene, the speifiation urrent is not important to use BJT as swithing devie of the Marx generator. (1) Figure 8 shows the dependene of the swithing time on V for different BJT. The swithing time of 2SC2655 is faster than other BJTs, sine the speifiation urrent is smaller than other BJTs. Hene, the 2SC2655 shifts into avalanhe mode faster. In other words, the swithing time is faster. The 2SC2655 is suitable as the swithing devie of the Marx generator. Fig.7 Dependene of swithing time on V for different apaitane. Fig.5 Current and voltage harateristis for different apaitane. Fig.6 Current and voltage harateristis for different BJTs. 3-4.Swithing time Figure 7 shows the dependene of swithing time on V in the ase of using 2SC2655 BJT. In normal mode operation, as V or C inrease, the swithing time inreases. In this ase, I is limited by the speifiation urrent. Therefore, when the quantity of eletri harge, Q, is large, the swithing time inreases. In the avalanhe mode operation, as apaitane inreases, the swithing time inreases. However, the swithing time inreases as V dereases. As disussed previous, I exeeds the threshold urrent in earlier time as the voltage is higher. Fig.8 Dependene of swithing time on V for different BJTs. 3-5. Miniaturized Marx generator The miniaturized Marx generator has been developed using BJT 2SC2655 as swithing devie. Figure 9 shows the iruit diagram. The iruit is omposed of 14 stages, and the apaity of eah stage is 0.1 uf. The 14 BJTs are onneted in series. The other BJTs plaed at the output work in the avalanhe mode after a time. Therefore, the waveform of output voltage, that is, the pulse width is onstant even though the load is hanged. Figure 10 shows the output waveform at the open iruit load. The harging voltage is 180 V, and the peak output voltage is about -2.5 kv. The FWHM of pulse voltage is about 35 ns. Even though the BJTs are operated at the region over the rating voltage and the rating urrent, the BJTs work with a long operation of hours for the pulsed mode. Fig.9 Shemati iruit diagram of Miniaturized Marx generator. 1341
Fig.10 Output voltage of Miniaturized Marx generator. 3-6. Miroplasma jet The miroplasma jet ould be generated by a DC power soure and CW sinusoidal power supplies. Our experiene shows the problem of heating. Therefore, the pulsed miroplasma jet is generated with the developed miniaturized Marx generator. Figure 11 shows the miroplasma jet reator. The reator eletrodes are a tungsten needle and a opper plate. The needle eletrode plaed in a erami tube is the tungsten wire of 0.025mm in diameter. The air of 2.5L/min is flowing through the erami tube. The atmospheri plasma is extended from the hole of 1mm in the diameter on opper plate. The amera (D40, Nikon) with 30 seonds of shutter time is used for taking the miroplasma jet. In Figure 12(a), the luminesene from the eletrial disharges in the tube and outside of the opper eletrode appears. The length of the plasma jet is about 0.5mm when the BJTs plaed at the output are operated as a losing swith. On the other hand, the further extension of plasma jet is observed without the series BJTs at the output as shown in Figure 12(b). The length of the plasma jet is about 3mm. Figures 13 and 14 show the voltage and urrent waveforms with and without the BJTs at the output, respetively. The pulse widths of voltage are different. The relation between the plasma extension and the input energy into the plasma has been reported in Ref. [6]. Briefly the torh length was losely related to the total volume of plasma. When the urrent is supplied into the load ontinuously, the further extension of plasma jet is observed. The extension of the plasma jet might depend on the input energy into plasma. In addition to the plasma extension, some interesting differenes of disharges in the left and right side in Fig. 12 are observed. The disharge olors inside the tube are blue and purple in the left and right side disharges, if Fig.12 is olor photographs. Moreover, the disharge between the tube and the eletrode is observed only in the right photograph in Fig. 12. The harateristis and the mehanisms of these disharge modes are now unlear and in our future subjets. Fig.11 Photograph of miroplasma jet reator. (a) (b) Fig.12 Photograph of miroplasma jet. Fig.13 Voltage and urrent waveforms of miroplasma jet reator with BJTs at output. Fig.14 Voltage and urrent waveforms of miroplasma jet reator without BJTs at output. 4. Conlusion The harateristis of BJTs on the avalanhe breakdown was evaluated. The suitable BJT was seleted as the swithing devie for the miniaturized Marx generator. The developed pulsed power generator was applied to produe the miroplasma jet. When the olletor urrent exeeded a threshold 1342
urrent, the operation mode of BJT shifted from the normal mode to the avalanhe mode. It was shown that 2SC2655 was suitable as the swithing devie of the miniaturized Marx generator from the swithing time harateristis. The miroplasma jet was suessfully generated by the developed miniaturized Marx generator. [1] K. Tahibana: Journal of Plasma and Fusion Researh, 76, 435(2000). [2] K. Tahibana: Journal of Plasma and Fusion Researh, 80, 825(2004). [3] T. Namihira, S. Tsukamoto, D. Wang, S. Katsuki, R. Hakam, K. Okamoto, H. Akiyama: IEEE Transations on Plasma Siene, 28,109(2000). [4] T. Heeren, T. Ueno, D. Wang, T. Namihira, S. Katsuki, H. Akiyama: IEEE Transations on Plasma Siene, 33, 1205 (2005). [5] A. Chatterjee, K. Mallik, S.M. Oak: Review of Sientifi Instruments, 69, 2166(1998). [6] C. Jaegu, M. Keita, Y. Hidekazu, S. R. Hosseini, N. Takao; K. Sunao, A. Hidenori: Japanese journal of applied physis, 48, 016001 (2009). 1343