20 Brazilian Journal of Physi s, vol. 32, no. 1, Mar h, 2002 Industrial Appli ations of Plasma Fo us Radiation C. Moreno, M. Venere, R. Barbuzza, M. Del Fresno, R. Ramos, H. Bruzzone, Florido P. J. Gonz alez, and A. Clausse Interinstitutional Program of Dense Magnetized Plasmas, CNEA-CONICET-CIC-INFIP-UNMP-UNICEN, Argentina Re eived on 3 July, 2001 Appli ations of a small- hamber Plasma Fo us used as portable radiation generator is presented. The devi e was designed to maximize the uen e. The mean neutron yield was 3 x 108 neutrons of 2.45 MeV per shot, orresponding to a 106 neutrons/ m2 uen e on the external surfa e of the hamber. A te hnique to dete t the presen e of water in the neighborhood of a ompa t Plasma Fo us is presented. The measuring system is omposed by two neutron dete tors operated simultaneously on every shot. The rst dete tor is used to register the PF neutron yield in ea h shot; whereas the other one was designed for dete ting neutrons s attered by the blanket. The results indi ate that the system is able to dete t water ontents of few per ents in volume. The orrelation of the ounts re orded by the dete tors lo ated at di erent positions was mapped with the water distribution around the neutron sour e. The omplete dete ting system is very simple and inexpensive. Among many other potential appli ations, the te hnique is spe ially suited for soil humidity prospe tion. X rays radiation emitted by the ompa t Plasma Fo us operated in Deuterium has been used for introspe tive radiographi imaging of metalli obje ts. The samples were lo ated about 1 m away from the PF hamber wall. High-sensitivity, fast-response ommer ial radiographi lm was used as x-ray dete tor. A set of experimental images is presented demonstrating a very high penetration power of the x-ray beam. Among many other appli ations, the presented te hnique is spe ially suited for introspe tive visualization of pie es manufa tured on metal. Radiographi proje tions of a stainless steel BNC elbow taken at 8 di erent angles were pro essed to re onstru t transversal uts of the pie e. A omputer te hnique for 3D re onstru tions was ombined with radiographi images of obje ts X-rayed with a ompa t plasma fo us. The te hnique is able to automati ally determine the position of the rotation axis, re onstru t the 3D-attenuation map, and display inner uts. The system was demonstrated in introspe tive tomographi imaging of a stainless steel BNC elbow. I Introdu tion Plasma Fo us (PF) devi es ourished in the 70's and 80's as nu lear fusion devi es based in the pin h phenomenon o urring during the path of high ele tri urrents through the working gas. The operation of PF has been extensively studied by resear h laboratories around the world, where several PF on gurations has been developed over the years aiming to in rease the neutron emission [1-4℄. Currently, PF pulsors are among the heapest available neutron generators, with unique features of extremely short pulses (hundreds of ns) that suit them for a number of interesting appliations. There are also interesting possibilities to take advantage of x-rays (1-100 keV), ele tron and ion beams emitted during PF shots. The plasma-fo us phenomenon o urs at the open end of oaxial ele trodes when an intense ele tri al disharge between them is indu ed by external means. The oaxial ele trodes are lo ated inside a va uum hamber lled with deuterium gas at low pressure. A harged apa itor bank is onne ted to the losed end of the ele trodes through a swit h. After losing the swit h, a gas dis harge starts in the gap between the ele trodes forming an umbrella-like plasma layer. The azimuthal magneti eld lo ated in the toroidal volume en losed by the urrent, produ es a Lorentz for e that pushes the sheath toward the open end of the ele trodes. The run-down of the urrent sheath is a sweeping supersoni sho k that propagates olle ting the gas parti les ahead of the front. On its arrival at the open end (some s after triggering), the magneti eld starts to ontra t, 21 C. Moreno et al. a elerating the plasma toward the axis. Finally, the sheath lashes on the axis in the form of a small dense plasma ylinder (fo us). The lifetime of the fo us is about 300 ns. The emitted neutrons an be applied to perform radiographs [5℄ and substan e analysis, taking advantage of the penetration and a tivation properties of neutral radiation [6℄. Likely, the intense x-ray pulses produ ed by fo alized ele tron bremstralung are ex ellent andidates for radiography of moving and soft obje ts and for mi roele troni lithography [7℄. Had small portable PF devi es been available, the added value of the emissions would substantially inrease, for larger uen es an be provided in wider domains of appli ations. However, due to the strong intera tion between the hot plasma and the va uum hamber, the ele trode housing is usually big, leaving room for the plasma blast. The main trouble with having the ele trodes too lose to the ontainer wall is the gas ontamination with impurities, whi h onspires against performan e and regularity of the emissions [8℄. Along this arti le, appli ations of the ompa t PF devi e, pulsing at one shot per minute is presented. The asso iated x-rays emissions were applied to obtain three-dimensional introspe tive images of small metalli omponents, and the emitted neutrons were used to dete t water by neutron inelasti s attering. II The plasma fo us GN1 The Plasma-Fo us GN1 is a ompa t version of a Mather-type ma hine. Fig. 1 shows a diagram of the va uum hamber and the ele trodes. The anode is an ele trolyti - opper ylinder, 38 mm diameter, 1.5 mm thi k, 87 mm long. The athode is formed by 12 bronze bars, 3 mm diameter, 100 mm long, ylindri ally pla ed, and welded at the end to a bronze ring 72 mm diameter. The ontainer is a stainless-steel ylinder 157 mm long, 96 mm diameter with a lateral NW25 va uum port for pumping and gas loading. The insulator is a Pyrex glass ylinder 35 mm long and 4 mm thi k. Using a me hani al pump and an oil di user, base pressures down to 10 8 mbar an be rea hed. The external ir uit is a apa itor bank, divided in three dis harging modules; ea h of them omposed by ve Maxwell type 31161 ondensers. The total apa itan e is 10.5 F and the harging voltage is 30 kV. The three modules are red simultaneously and peak urrents of 350 kA are attained in a quarter of period (1.1 s). The system operates between 1 to 8 mbar of Deuterium. CHAMBER CATHODE ANODE INSULATOR Figure 1. S hemati s of GN1 hamber (sizes are detailed in the text). After ea h shot, the lling pressure in reases about 0.05 mbar due to the release of impurities from the hamber, ele trodes and insulator walls. Consequently, the hamber is pumped down (me hani ally) after ea h shot in order to assure onstant pressure onditions. The maximum shot frequen y was one shot per minute, limited by the harger. Under these onditions, the frontal wall temperature (top in Fig. 1) in reases about 20C over the ambient temperature after 30 shots, ooled passively by air natural onve tion and heat ondu tion through the metalli stru ture. The working gas is renewed after 10 shots. Upon ommissioning, the equipment was tested in a series of 1000 shots at di erent load pressures. The time derivative of the urrent owing to the anode, dI/dt, and the voltage a ross the ele trodes, V , were monitored for ea h shot by a Rogowski oil and a resistive voltage divider, and were registered using a 500 MHz, 1 Gs/s digitizing os illos ope. Fig. 2 shows typi al signals for lling pressures of 2; 4 and 6 mbar. Very intense voltage spikes (120 kV) are obtained at the time where the maximum ompression takes pla e, thus indiating good fo using. Su h peaks impose severe design onditions on the insulator, whi h should be onsiderably thi k in order to stand the stress (2 mm of Pyrex glass is destroyed with few dis harges). It was observed in Fig. 2 that the fo us o urs later for higher pressures. Fig. 3 shows the average dependen e of the fo us timing with the deuterium pressure. The dimensions of the ele trodes were determined using a omputer aided 22 Brazilian Journal of Physi s, vol. 32, no. 1, Mar h, 2002 design system whi h is based in a thermonu lear PF model [9℄. The ontinuous tra e is the fo ussing time derived from an analyti al snowplow des ription oupled with an isoentropi plasma ompression model. 1.4 tf (µ sec) 3 2 mbar 2 1.6 1.2 1.0 1 0 0.8 -1 2 4 6 8 p o (m b) 3 Figure 3. Variation of the fo using timing with lling pressure. The line orresponds to predi tions of a snowplow model [9℄. 4 mbar 2 1 0 3 -1 3 6 mbar 2 Yn / 10 8 2 1 0 1 -1 0.0 0.5 1.0 t (µs) 1.5 2.0 Figure 2. Signals proportional to the voltage a ross ele trodes (solid line) and urrent time derivative (dotted line). The signals units are Volts measured in a Rogowski oil and a voltage divider respe tively. The time integrated neutron yield was also measured for ea h shot by silver a tivation response as a fun tion of pressure. The neutron dete tor was pla ed at 60 m from the fo us in a line perpendi ular to the axis. The sensitive area was 900 m2 . The neutron measuring system was alibrated by omparison against TLD dete tors. Fig. 4 shows the dependen e of the average neutron yield per pulse with the lling pressure. The ontinuous tra e on Fig. 4 orresponds to same model of Fig. 3. The optimum average yield, 3 x 108 neutrons per shot, o urs at 4 mbar. This yield orresponds to 106 n/ m2 per shot in the external frontal wall. Di erent insulator lengths were tested in preliminary series of measurements aimed to investigate ea h design performan e. It was found that variations of 5 mm in that length lead to noti eable degradation of the neutron yield at all the explored pressures. 0 0 2 4 6 8 Po (mb) Figure 4. Neutrons yield at 90Æ o axis as fun tion of lling pressure. The line orresponds to predi tions of a snowplow model [9℄. III Defe tos opy PF an be advantageously used as high-intensity short-duration x-ray sour es. Due to the very fast plasma ompression attained in these devi es, parti ularly intense soft and hard x-rays pulses are emitted. High repetition rate small PF are urrently used for SXR lithography [10℄. High-Z working gases are used in these appli ations to enhan e the x-ray yield at redu ed wavelengths [5℄. We de ided to use the ompa t PF operated in Deuterium for making non- onventional radiographs. Our aim is to nd new appli ation elds for the PF as x-ray sour e, and to allow for the possibility of obtaining, in a future experiment, a simultaneous radiographi and neutrographi image of the same obje t. 23 C. Moreno et al. The samples to be imaged were pla ed outside the stainless steel hamber, on the ele trodes symmetry axis, and 83.5 m away from the fo ussing region (Fig. 5). Commer ial radiographi lm, Curix ST-G2 from AGFA was used together with AGFA suggested developer and xer for this lm. No spe ial pro edures were needed other than those re ommended by the supplier, to manipulate and develop the lms. Object to be tomographied X-Ray Film Device for angular variation Figure 5. Setup of the tomographi GN 1 Neutron Generator te hnique. A stainless steel BNC elbow was used as a sample. All the shots were made at lling pressures of 4-5 mbar pure deuterium. The sample was mounted on a small a ryli platform that rotates to allow for taking images at di erent viewing angles. The rotating axis was set verti ally and 8 viewing angles were used: 0, 30, 60, 75, 90, 105, 120, and 150 degrees. The axis was marked with a sharp metalli needle. Fig. 7 shows the set of radiographs of the pie e at di erent angles. It an be seen that, even ontrolling arefully the operation, there are always di eren es in brightness and fo alization. Therefore, in order to be useful in a tomographi system, we should be able to re onstru t inner uts pro essing imperfe t proje tions. Figure 6. Angular radiographs of a BNC. Figure 7. Tomographi re onstru tions using the radiographs shown in Fig. 6. The Monte arlo method is a exible te hnique than an to handle quality di eren es of the information input. Basi ally, Monte arlo tomographi re onstru tion is a sto hasti sear hing pro ess, where the omputer boun es randomly in the set of all possible digital 3D images ( alled instan es ), guided with a sele tion riterion that ensures the onvergen e toward the a tual inner attenuation eld. The re onstru tion algorithm is based on the omparison of the a tual radiographs with the proje tions that would produ e the instan e image. The general pro edure is as follows: Starting with an initial guess instan e, modify slightly the tone of a pixel hosen at random. Cal ulate the proje tions of the instan e in every dire tion. Cal ulate an error indi ator averaging the square deviations of the instan e proje tions respe t to the a tual proje tions. If the error of the new instan e is lower than the previous one, the modi ation is a epted. 24 Brazilian Journal of Physi s, vol. 32, no. 1, Mar h, 2002 The method was implemented using obje t oriented programming, in visual C++ . A visualization system ompletes the tool, allowing the fast inspe tion of inner uts of the attenuation eld. The hardware requirement is just a Pentium personal omputer, and the system runs in MS Windows environment. Fig. 7 shows the display of di erent uts of the BNC on the ontrol panel of the appli ation. The visualization of the inner uts shows details down to 0.3-mm resolution. in position inside the an. This dete tor was lo ated side-on, approximately at 30 m from the PF hamber. WATER 50 cm DETECTOR 1 75 cm 7 cm 50 cm 20 cm PLASMA FOCUS DETECTOR-2 Neutron e hography Neutrons an be used for dete ting Hydrogen in metals or for dete ting substan es by a tivation analysis [11℄. Either huge ssion rea tors or powerful deuteron a elerators are used in these appli ations as neutron sour es, whi h share the disadvantages of being nonportable and very expensive. On the ontrary, substan e interrogation system should be portable, reliable, e onomi ally ompetitive, and minimize the environmental impa t. PF devi es lled with deuterium gas, omplemented with a ouple of neutroni dete tors are a valid alternative, as they ful ll all the requirements mentioned before. We ondu ted an experiment to dete t water or other substan es ontaining low Z elements, by neutron inelasti s attering. The method is based on the same prin iple of the sonar or the e hographs, registering the neutrons s attered by the interrogated substan e, omparing the dete tor response when the interrogated substan e is absent. The measuring system is omposed by a neutron pulsed generator and two silver a tivation dete tors operated simultaneously on every shot. The rst dete tor is used to register the PF neutron yield; whereas the other one is used to dete t neutrons s attered by a water blanket. The ompa t PF devi e lled with deuterium was used to provide neutron pulses. The basi idea of the method is to use a suitable dete tor to register the neutrons s attered by the interrogated substan e (if present) and to ontrast the dete tor response when the interrogated substan e is absent. Shots were made in presen e of the interrogated substan e, then the proedure was repeated after removing the substan e, and the neutrons registered in both situations were ompared. Fig. 8 shows the experimental setup. A standard silver a tivation ounter pla ed side-on, 60 m away from the PF hamber was used as the referen e dete tor to take the shot to shot variation of the neutron yield into a ount. This dete tor was overed with paraÆn to moderate the fast neutrons. The se ond dete tor is omposed by three Geigers, ea h of them overed by a 0.3 mm thi k silver foil and pla ed inside a metalli an (10 m in diameter and 13 m in length). Some pa kaging plasti was used to keep the Geigers assembly Figure 8. Neutron e ho interrogation setup. 140 liters of water were arranged in a wall of 11 plasti ontainers. The water wall was put in di erent positions respe t of the dete tor systems. For every on guration the PF was shot about fteen times re ording the ounts measured in ea h dete tor. Fig. 9 shows the results obtained along the PF axis. Ea h point in the graphi orresponds to one shot, the ounts of ea h dete tor being the oordinates of the plot. The relative ounting of the moderated dete tor in reases as the water gets loser to the dete tion system. This e e t an be attributed to the hange in the solid angle o ered by the water. The ounts olle ted by the s attered neutrons dete tor when no water is present, orrespond to neutrons re e ted by the laboratory building. The slope of the least-squares lines orresponding to ea h position an be used to hara terize the sensitivity of the system. Fig. 10 shows the ontour map of the relative ounting slope. The oordinates orrespond to the lo ation of the water wall respe t to the PF head. The x- and y- oordinates are measured along lines perpendi ular and parallel to the PF symmetry axis respe tively. It an be seen that the relative ounting de reases faster when the water is moved away along the symmetry axis. 1000 Scattered neutrons (a.u.) IV 7 cm 800 20 cm 600 y = 50 cm 400 200 0 water absent 0 500 1000 1500 2000 2500 Reference neutrons (a.u.) Figure 9. Neutroni re e tion in along the PF axis. 25 C. Moreno et al. the position of the rotation axis, re onstru t the 3Dattenuation map, and display inner uts. 50 0.27 y (cm) 30 0.35 20 Referen es 0.23 40 0.31 0.39 0.43 10 0.47 0 20 40 60 80 x (cm) Figure 10. Contour map of the relative neutron ounting. Every (x,y) oordinate represent the position of the water. V Con lusions Two feasibility studies of industrial appli ations of the X and neutron radiation produ ed by a ompa t Plasma Fo us were presented. A te hnique to dete t the presen e of water was developed using two neutron dete tors operated simultaneously on every shot. The results indi ate that the system is able to dete t water ontents of few per ents in volume. The omplete dete ting system is very simple and inexpensive. Among many other potential appli ations, the te hnique is spe ially suited for soil humidity prospe tion. On the other hand, X-rays from the PF has been used for introspe tive radiographi imaging of metalli obje ts. The te hnique is spe ially suited for introspe tive visualization of pie es manufa tured on metal. Radiographi proje tions of a stainless steel BNC elbow taken at 8 di erent angles were pro essed to re onstru t transversal uts of the pie e. A omputer te hnique for 3D re onstru tions was ombined with radiographi images of obje ts X-rayed with a ompa t plasma fous. The te hnique is able to automati ally determine [1℄ A. Bernard, Nu lear Instruments Methods, 145, 191 (1977). [2℄ H. Conrads, Pro . 3rd Latinameri an Worshop Plasma Phys., Santiago, Chile, July 18-29 (1989). [3℄ A. Serban and S. Lee. Experiments on speed-enhan ed neutron yield from a plasma fo us. J. Plasma Physi s, 60, part 1, 3-15 (1998). [4℄ G. De ker, R. Wiene ke, Physi a 82C, 155-164 (1976). [5℄ M. Gibbons, W. Ri hards and K. Shields. 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