Effect of Cathode Designs on Radiation Emission of Compact Diode (CD) Device

Transcription

1 J Fusion Energ (2013) 32:34 41 DOI /s ORIGINAL RESEARCH Effect of Cathode Designs on Radiation Emission of Compact Diode (CD) Device Muhammad Zubair Khan Seong Ling Yap Muhammad Afzal Khan Attiq-ur-Rehman Muhammad Zakaullah Published online: 23 February 2012 Ó Springer Science+Business Media, LLC 2012 Abstract A comparative study on the radiation emission such as X-ray yield and efficiency has been carried out in compact diode device. Two different designs of cathode having sharp-edged razor blade (of 0.5 mm thickness with width 2 mm) and a sewing machine needle (of 0.5 mm diameter at tip with length of 39 mm) have been tested for this study. The radiation emission (X-ray yield) was determined by employing two set of PIN diodes at fixed positions. The maximum X-ray yield depends on cathode designs and electrodes separation in few mm. The yield of X-ray is small in the case of sharp-edged razor blade cathode than the sewing machine needle cathode. The X-ray yield, measured by 4p-geometry, shows its dependence on the cathode designs. The maximum X-ray yield is found to be ± 65.7 mj with efficiency of ± %. This study indicates that the compact diode device could be optimized to a great extent for optimal X-ray yield by using an appropriate cathode design. M. Z. Khan S. L. Yap Plasma Research Laboratory, Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia M. Z. Khan Department of Physics, Federal Urdu University of Arts, Science and Technology, Islamabad 45320, Pakistan M. Z. Khan (&) M. A. Khan M. Zakaullah Department of Physics, Quaid-i-Azam University, Islamabad 45320, Pakistan mzk_qau@yahoo.com Attiq-ur-Rehman Centre of Advance Studies of Physics, Government College University, Lahore, Lahore, Pakistan Keywords Compact diode device X-ray generation Low energy plasma Electron beam emission Field emission Comparison Introduction First time, in 1895, Roentgen [1] discovered the X-ray in Wurzburg, Germany, it was suggested that the X-ray are electromagnetic waves of wavelength of the order of m (or 1Ao). Roentgen s gas tube was used until 1913 with some improvements. To increase the X-ray production, a heavy metal anticathode was introduce between the cathode and anode, and was made a curved shape to focus the electrons onto the anode. The potential difference between the anode and cathode was largely independent of the current through the tube. It was much complicated design because for any change in the voltage, it was necessary to change the gas pressure. That was solved by Coolidge in 1913, devised a way to overcome this problem. In compact diode (CD), X-ray emission process is same as in X-ray tubes. The difference is only that in a compact diode, the technique of the field emission is used instead of thermionic emission. The basic principle of compact diode is the emission of electrons from an unheated surface as a result of a strong electric field existing at that surface. During the last few decades attention has been given to the development of flash X-ray sources because of their large number of potential applications. A first step in the realization of intense pulsed X-ray source of laboratory scale was introduced by Bradley et al. [2] introduced a generator, which for 100 ns pulse duration, performed very well to the point, which was limited by fundamental consideration. High speed phenomena [3], bio-medical radiography [4],

2 J Fusion Energ (2013) 32: pre-ionization of high pressure gas discharge laser [5], photo-excitation of molecular [6], atomic systems for fluorescence studies [7], and time resolved X-ray diffraction [8] are useful X-ray sources. Khan et al. [9] studied the X-ray emission from a compact diode operated by a high inductance Capacitor discharge. The maximum X-ray emission yield was determined 34 ± 3 mj with wall plug efficiency ± 0.001% in the 4p-geometry. The efficiency of this device was very low because of high parasitic inductance. Sharif et al. [10] examined the performance of a low energy ( kj) Mather-type plasma focus device as Cu K apha emissions for argon and hydrogen filling. The system was found to generate X-ray with % efficiency with discharge energy of 1.8 kj at optimum conditions. They enhanced the X-ray flux three times in the side on direction with a cut at the anode tip. The modified geometry used as a radiation source for X-ray diffraction. Verma et al. [11] investigated the effect of Kr seeding in deuterium on X-ray yields from a 200 J miniature plasma focus device. On the basis of relative increase in the focusing duration coupled with enhanced focusing efficiency, they obtained the broadening of operating pressure regime of B0.4 mbar and significant enhancement in the X-ray yields for D 2 -Kr admixtures. The enhancement in X-ray yields at low pressure for admixture operation will help in getting better performance device efficiency for lithography and micro-machining applications. Neog et al. [12] optimized the anode length of a modified PF device operated with N 2 gas for the highest X-ray yields. With the 125 mm anode length, they found the highest X-ray yield of 4.5 J into 4p sr, which is 0.2% of the input capacitor energy. They established a plasma focus device be optimized for optimal X-ray yield with a suitable anode length. Habibi et al. [13] investigated behaviour of pinch versus working voltages at the optimum pressure, correlation of plasma focusing time with the pressure, behavior of HXR respect to working pressure, effect of insulator sleeve with different dimensions, and anisotropy of HXR around the anode tip. They introduced a new Mather-type PF device named as Amirkabir (APF) with 16 kv, 36 F, and 115 nh for the study of hard X-ray (HXR). Hussain et al. [14] studied X-ray emission from a low energy ( kj) PF device with different (Cu, Mo, W, Pb) inserts at the anode tip. They reported the variation of the X-ray yield and corresponding efficiency with filling pressure at different charging voltages. In 4p-geometry, the maximum X-ray yield was estimated about 67.6 J for Cu at 27 kv, 62.4 J for Mo at 27 kv, 64 J for W at 29 kv, and 46.5 J for Pb at 23 kv at optimum working gas pressure with corresponding efficiencies for X-ray generation were 1.5, 1.4, 1.2, 1.4% respectively. Mohammadi et al. [15] achieved highest average Neon SXR yield of 3.3J UNU-ICTP plasma focus with a longer than conventional anode. They concluded good Neon SXR yield at new optimum pressure with significant increase in the anode length from its optimum value and defined an efficient PF operation with new optimized pressure for SXR yield. Zhao et al. [16] investigated the X-ray photon energy range with a high-purity Ge detector on Lanzhou ECR Ion Source No. 2 Modified (LECR2M). They operated various conditions with Argon to enhance high charge state ion beam intensities, derived bremsstrahlung spectra with the spectral temperature of hot electrons Tspe. With the help of LECR2M, different parameters such as the RF frequency, power and magnetic confinement configuration were investigated. Mohanty et al. [17] studied the ion characteristics such as flux and energy within angular positions variation and operation gas pressure in a N 2 filling PF device. They used three different designs of cylindrical anode: hollow, solid and hemispherical tip to find the ion emission characteristics by employing three FC at various angular positions. With the time of flight (TOF) method measured the ion energy which shows dependence on the designs of anode. In case of the hemispherical anode design, they found maximum ion energy 830 kev at an angular position 5 o. Appropriate design of anode which has good effect of optimal ions yield in PF device. Wong et al. [18] constructed a low energy pulsed X-ray source (17 J electrical energy) based on the vacuum spark configuration. They demonstrated a potential application of the X-ray source with X-ray radiography of small biological sample and found an X-ray yield efficiency of 0.1%, 18 mgy/pulse as an average X-ray dosage. Barbaglia et al. [19] studied hard X-ray emission in small PF device. They used three different anode lengths for the study of X-ray emission at a constant pressure 1.8 mbar of deuterium as working gas, found varied current amplitude at the pinch time but there was no influence the X-ray yield due to change in the pinch current. They observed a threshold in voltage drop on the pinch for X-ray yield. Kashani [20] investigated two types of cathode electrodes: (1) bar and (2) tubular cathodes to see the influence of the cathode structure on discharges in a 7 kj PF device. Under the optimized conditions expect the cathode structure, it had a great influence on energy dissipation in the run-down phase and hence the neutron yield. In the rundown phase, the energy dissipation was less in the bar cathode than in the tubular cathode and as a result, in the bar cathode, the pinch current and the neutron yield were higher. Their results are quite different than other experimental results because of in different devices with the bar

3 36 J Fusion Energ (2013) 32:34 41 or tubular cathode. Under optimized experimental conditions of both types of cathode in the same device were investigated for a complete comparison. In this paper, the radiation emission such as X-ray yield and efficiency of a compact diode with different designs of cathode is reported. This study indicates that the compact diode (CD) device could be optimized to a great extent for optimal X-ray yield by using an appropriate cathode design. Our attempt might help to shape the Compact Diode (CD) device as an excellent X-ray source for the future technological applications. Experimental Complex The compact diode device is energized by a single 0.5 lf, 30 kv (225 J) capacitor. The compact diode device basically comprises of an anode, which is a flat plate of brass with the thickness of 10 mm and diameter of 30 mm. Different metal targets, for instance, titanium, Cu, Mo, tin can be mounted at the surface of anode. Two different design of Cathode having sharp edged razor piece and a sewing machine needle were used in this investigation, as shown in Fig. 1a, b. The photo of Compact Diode device Fig. 1 Schematic of compact diode device with sharp-edged razor blade cathode shape (a) and sewing machine needle cathode shape (b)

4 J Fusion Energ (2013) 32: Fig. 2 The photo of compact diode device at the place of testing Fig. 3 Block diagram of a compact diode device with mechanical and electrical parameters at the place of testing is shown in Fig. 2.The cathode holder is a brass plate of 30 and 20 mm diameter thickness in both experiments, which is fond of an outer brass plate of 10 mm thickness and 257 mm diameter. Outer brass plate is connected to ground terminal of the capacitor via six

5 38 J Fusion Energ (2013) 32:34 41 hexagonal brass rods and the anode header is connected to HV terminal of the capacitor via sparkgap as switch. A block diagram of the compact diode device is shown in Fig. 3; the electrodes are enclosed in a cylindrical vacuum chamber (57 mm thick wall) of etalon with four ports. Within the both experiments: two ports are used for two Quantrad PIN diodes with different filters; other two ports are used for a rotary van pump (10-2 mbar) and Edwards CG3capsule type gauge (to measure vacuum inside). To analyze the X-rays, First experiment: used the filters (17.5 lm Ni, 20 lm Co), the absorption edge of Co is kev and Ni is kev, and Second experiment: used the filters (50 lm Ag, 55 lm Pb), for study of the X-ray in the energy range *13 25 kev; along with response (absorption and transmission curves) of the PIN diode is given in Figs. 4 and 5, respectively. These curves are obtained by using the data given in the Handbook of (a) 1 ResCo(20mic) Spectroscopy [21]. Thus the difference of transmission in the filters may be considered corresponding to the line radiation. Further, PMT (XP2020 coupled with 50 mm 9 50 mm cylindrical plastic scintillator NE102A) is positioned at 13 ± 0.5 cm from the diode point, where the X-rays are emitted. The inputs are directly connected to the diode and outputs are connected to a four channel oscilloscope (200-MHz GOULD 4074A), which is coupled with computer through GPIB interfacing card. Results and Discussion System specification and experimental results at 25 kv for both design of Sharpe edged razor blade and sewing machine needle cathodes are summarized in Table 1. Table 1 summarizes different machine parameters and calculated parameters of the compact diode with both Sharpe edged razor cathode and Sewing machine needle Detector Responce(a.u) resni(17.5mic) Energy(keV) (b) Fig. 4 Absorption curve (a) and transmission curve (b) of Ni (17.5 lm) and Co (20 lm) filters in sharp-edged razor blade cathode Fig. 5 Absorption curve (a) and transmission curve (b) of Ag (50 lm) and Pb (55 lm) filters in sewing machine needle cathode

6 J Fusion Energ (2013) 32: cathode. A typical Rogowski coil signal of the Compact Diode device for discharge of capacitor at 30 kv is presented in Fig. 6. The peak current and system parasitic inductance can be computed using equations [22]. I 0 ¼ pc 0V 0 ð1 þ f Þ ð1þ T and T2 L 0 ¼ 4p 2 ð2þ C 0 For the detection of X-rays, two Quantrad PIN diodes mask with Ross-pair filters [Ni (17.5 lm), Co (20 lm)] and [Ag (50 lm), Pb (55 lm)] are mounted inside the compact diode chamber. According to the relation I = I 0 exp (-l(e)t), where I 0 and I are the intensities of incident radiation flux and transmitted flux respectively, l(e) is the absorption coefficient of the filter. The l is a function of energy. One can attenuate intensity of radiation through different Ross filters [23]. A photomultiplier tube XP2020 Table 1 System specification and experimental results at optimum condition Parameter Sharpe edged razor cathode Sewing machine needle cathode Charging voltage V 0 (kv) Capacitance C 0 (lf) Stored energy E (J) Parasitic inductance L 0 (nh) 353 ± ± 5 Impedance Z 0 (mx) Peak current discharge I o (ka) 35 ± 2 38 ± 2 Filters thickness (lm) Ni (17.5), Co (20) Ag (50), Pb (55) X-ray yield (mj) 34 ± ± 65.7 Efficiency of X-ray yield (%) 0.02 ± ± coupled with 50 mm 9 50 mm cylindrical plastic scintillator NE102A with 3 mm thick aluminum light shield is positioned at distance 12 cm from the diode point in both cathode shapes, from where the X-ray are emitted. The X-ray must pass 57 mm thick nylon body, besides the 3 mm thick aluminum shield to enter the plastic scintillator. It is estimated that the X-ray photons detected by photomultiplier tube are of energy exceeding 20 kev in both Sharpe-edged cathode and Sewing needle machine cathode shape. The typical signals of the photomultiplier tube are given in the Fig. 7, which shows that X-ray pulse width (FWHM) is 35 ± 02 ns. The pulse is recorded 200 ± 10 ns after the application of HV. The X-ray emission is about 170 ns after the application of HV because of 28 ns is internal transit time of PMT. The current signal of the Rogowski coil is represent a small dip in signal, which synchronizes with PMT signal. This observation reveals that discharge in CD undergoes Z- pinch type compression. The filters masking the PIN diodes may help to estimate the X-ray yield in 4p-geometry, and the system efficiency for X-ray generation. Energy radiated as X-ray is determined by the relation [24]. Y ¼ Q expð4pþ ð3þ dxsðeþtðeþ where, Z Vdt Q exp ¼ R ðcoulombsþ R Vdt = area under the waveforms with two respective filters, S(E) = average sensitivity of the detector (from the Quantrad brochure), T(E) = average transmission of the filter, R = 50X, in the recent experiments, dx = da/r 2 o (sr.) is the solid angle subtended by the detector at the center of the anode.where, da = pr 2, r = 0.4 cm, is the radius of the Fig. 6 A typical signal of Rogowski coil of compact diode device Fig. 7 A typical signal of photomultiplier tube (PMT)? NE102A

7 40 J Fusion Energ (2013) 32: Signal Intensity (nvsec) Separation (mm) Fig. 8 Variation of signal intensity recorded by photomultiplier tube (PMT) plus NE102A (sharp-edged razor blade cathode Cu anode) with separation of electrodes Fig. 10 X-ray yield and efficiency versus separation (sharp-edged razor blade cathode Cu anode) exposed area of each detector, r o = 26 ± 1.0 mm, is the distance from the detector to the center of the anode. The variation of X-ray emission as a function of separation of electrodes and the shapes of cathode can play effective role in generation of radiation in Compact Diode (CD) device. The variation of average signal intensity with the separation of electrodes is described in Fig. 8. It is found that the average signal intensity recorded by PMT attains its maximum value at 3 mm inter-electrode separation. In Fig. 9, the variation of average signal intensity with width of sharp-edged razor blade Cathode. It is found that the average signal intensity recorded by PMT attains its maximum value at 2 mm width of razor cathode Signal Intensity (nvsec) Width (mm) Fig. 9 Variation of signal intensity recorded by photomultiplier tube (PMT) plus NE102A (sharp-edged razor blade cathode Cu anode) versus width of cathode Fig. 11 X-ray yield and efficiency versus separation (sewing machine needle cathode Pb anode) The variation of X-ray yield and efficiency against the separation of electrode is shown in Fig. 10. In 4p Geometry, the maximum X-ray yield and efficiency for interelectrode separation (3 mm) and width of razor blade (2 mm) is 39 ± 02 mj and 0.02% in case of sharp-edged razor Cathode shape. This small X-ray yield is speculated due to the high parasitic inductance of the system. In Fig. 11, the maximum X-ray yield in 4p Geometry is estimated about 940 ± 46 mj and efficiency is about 0.41% at separation (4 mm) of electrode. The variation of X-ray yield with respect to shape of Cathode is described in Fig. 12. At 3 mm separation, maximum X-ray yield is 0.02% in sharp-edged razor

8 J Fusion Energ (2013) 32: Conclusions X-ray generation from a new and simple configuration of Compact Diode (CD) device consisting of two different shape of cathode (1) Sharp edge razor blade, (2) Sewing machine needle with flat plate anode is investigated. The maximum X-ray yield, measured by 4p-geometry, is found to be mj with efficiency of 0.41%. The X-ray efficiency is less in the case of sharp edged razor blade cathode (0.08%) than the sewing machine needle cathode (0.4%). The X-ray yield efficiency shows its dependence on the cathode design. Using an appropriate cathode design, CD device could be optimized to great extent for optimal X-ray yield and efficiency. Fig. 12 X-ray yield versus separation of sharp-edged razor blade cathode Cu anode and sewing machine needle cathode Pb anode cathode while maximum X-ray yield is 0.41% at 4 mm separation in case of sewing machine needle cathode. Therefore, cathode shape may play very effective role in the generation of X-rays in Compact Diode (CD) device. In Compact Diode (CD) device, X-ray emission is studied w.r.t Cathode shapes: sharp edged razor blade cathode and sewing machine needle cathode. The Cathode shapes are prepared to sharp edged to facilitate the electron beam emission due to strong E-field. With razor blade cathode and sewing machine needle cathode, the variation of the X-ray emission increases/decreases, and an optimum separation is obtained. When the separation is reduced or increased than optimum separation (3 mm), the X-ray yield decreases. The following reasons: 1. Reduce the separation of electrodes, the electron may penetrate deeper into target, which may cause enhanced self absorption of X-ray and hence reduced emission. 2. Increase the separation of electrodes that offers the reduced E-field for the field emission and hence lower X-ray emission. The X-ray yield efficiency with sharp-edged razor blade cathode-copper anode is 0.02% and sewing machine needle cathode-lead anode increases by an order of magnitude to 0.4%. The radiation emission is much advanced in sewing machine needle cathode than sharp-edged razor blade cathode with their specific anodes. The X-ray emission with sewing machine needle cathode shape with lead target is much higher and more reproducible than other targets [25]. The shape of Cathode may participate an important role in the enhancement of X-ray yield in Compact Diode (CD) device. Acknowledgments This work was partially supported by Higher Education Commission and Pakistan Science Foundation (PSF) Project No. PSF/R&D/C-QU/Phys (199). Author also acknowledges the Federal Urdu University of Arts, Science & Technology (FUUAST) Islamabad Pakistan regarding the financial support for higher studies in University of Malaya (UM) Kuala Lumpur Malaysia. References 1. D. Halliday, R. Resnick, J. Walker, Fundamentals of Physics, 5th edn. (Wiley, New York, 1997), p L.C. Bradley et al., Rev. Sci. Instrum. 55, 25 (1984) 3. E. Sato, H. Isobe, F. Hoahino, Rev. Sci. Instrum. 57, 1399 (1986) 4. E. Sato et al., Rev. Sci. Instrum. 61, 2343 (1990) 5. E. Sato et al., Rev. Sci. Instrum. 62, 2115 (1991) 6. J.I. Levatter, Z. Li, Rev. Sci. Instrum. 52, 1651 (1981) 7. C. Cachoncinlle et al., J. Phys. D23, 984 (1990) 8. I.V. Tomov, P. Chen, P.M. Rentzepis, Rev. Sci. Instrum. 66, 5214 (1995) 9. M.Z. Khan et al., J. Fusion Energ. 21, 211 (2003) 10. M. Sharif, et al., Plasma Sources Sci. Technol. 13, B7 B13 (2004) 11. R. Verma et al., Appl. Phys. Lett. 92, (2008) 12. N.K. Neog, S.R. Mohanty, E. Hotta, J. Appl. Phys. 99, (2006) 13. M. Habibi, R. Amrollahi, M. Attaran, J. Fusion Energ. 28, (2009) 14. S. Hussain et al., Phys. Lett. A 349, (2006) 15. M.A. Mohammadi et al., Plasma Sources Sci. Technol. 16, (2007) 16. H.Y. Zhao, et al., Rev. Sci. Instrum. 79, 02B504 (2008) 17. S.R. Mohanty et al., Jpn. J. Appl. Phys. 46, (2007) 18. C.S. Wong, H.J. Woo, S.L. Yap, Laser Part Beams 25, (2007) 19. M. Barbaglia et al., Plasma Phys. Control. Fusion 51, (2009) 20. M. Kashani, J. Phys. Soc. Jpn. 72, 3 (2003) 21. J.W. Robison, Handbook of Spectroscopy (CRC, Cleveland, OH, 1974) 22. S. Lee, J. Phys. D Appl. Phys. 16, 2463 (1983) 23. D.J. Johnson, Rev. Sci. Instrum. 45, 191 (1974) 24. M. Zakaullah, J. Fusion Energ. 19, 143 (2000) 25. S. Hussain et al., Phys. Lett. A 319, 181 (2003)