Plasma Focus as a High Intensity Flash X-Ray Source for Biological Radiography

A study on X-ray emission from a low energy (3.3 kJ) plasma focus (PF) device operated with hydrogen is reported. X-ray are detected by using an X-ray detector consisting of three Quantrad Si PIN-diodes with differential filtering and with a pinhole camera. The X-ray yield in different energy windows is measured as a function of hydrogen filling pressure. The maximum yield in 4-geometry is found to be 46.6 J and corresponding wall plug efficiency for X-ray generation is 1.40%. In particular, to demonstrate feasibility of the present PF as a high intensity flash X-ray source for good contrast biological radiography, an X-ray radiogram of a fish, is presented. The fine structure of the specimen can be seen in different parts. The PF, because of its high X-ray yield and good reproducibility is particularly suited for this application.     

Low-Energy Plasma Focus as a Tailored X-Ray Source

A low-energy (2.3 kJ) plasma focus energized by a single 32-F capacitor charged at 12 kV with filling gases hydrogen, neon, and argon is investigated as an X-ray source. Experiments are conducted with a copper and an aluminum anode. Specifically, attention is given to tailoring the radiation in different windows, e.g., 1.2–1.3 keV, 1.3–1.5 keV, 2.5–5 keV, and Cu-K line radiation. The highest X-ray emission is observed with neon filling and the copper anode in the 1.2–1.3 keV window, which we speculate to be generated due to recombination of hydrogenlike neon ions with a few eV to a few 10s of eV electrons. The wall-plug efficiency of the device is found to be 4%. The other significant emission occurs with hydrogen filling, which exhibits wall-plug efficiency of 1.7% for overall X-ray emission and 0.35% for Cu-K line radiation. The emission is dominated by the interaction of electrons in the current sheath with the anode tip. The emission with the aluminum anode and hydrogen filling is up to 10 J, which corresponds to wall-plug efficiency of 0.4%. The X-ray emission with argon filling is less significant.     

Forward and Backward Photoemission Yields from Metals at Various X-Ray Angles of Incidence

X-ray generated photoemission from thin metal foils backed by graphite was measured with radiation incident from the front and back sides at several angles. Irradiation was provided by a 100-kV x-ray tube with three different filters to harden the spectrum. The total 2? photoelectron emission current from a surface was measured; a biased grid retarded the low-energy secondary electrons, which added only 10-30% to the current at zero grid bias. Investigated metals were: Mg, Al, Ti, Fe, Cu, Ag, Ta, Au, and Pb; also the total emission from just the graphite support was measured. The front-to-back ratio of emission currents at normal incidence ranged from about 1.9 for Al and Mg down to about 1.1 for Ta. The photoelectron yield was found to be Ge ?a Se electrons/photon, where ?a and Se are the energy-dependent photon absorption cross section and computed electron mean path length in the emitter, and Ge is a constant assumed independent of photon energy in the range studied (but does depend on radiation angle of incidence). For the photon energy range of 20-70 keV, the measured emission current densities corresponded to the following average values for ?e: 0.37 ± 0.06 for C, 0.30 ± 0.03 for Al, 0.21 ± 0.02 for Cu and Ag, and 0.18 ± 0.02 for Ta.     

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

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 4π-geometry, shows its dependence on the cathode designs. The maximum X-ray yield is found to be 939.2 ± 65.7 mJ with efficiency of 0.4142 ± 0.0289%. 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.     

A Comprehensive Time-Resolved Study of X-Ray Signals in a 4?kJ Dense Plasma Focus Facility

In the present research, time-resolved of X-ray signals are investigated in a 4?kJ PF device using five filtered PIN-diodes and a Scintillation detector. At applied voltage 12?kV using nitrogen (N₂) gas, the optimum pressures for maximum yield of HXR and SXR emission were 3?torr and 4?torr respectively. At each operating pressure, multiple pulses in SXR signals with respect to amplitude and duration of emission were studied in a time span of 700–800?ns. According to energy response of filters used, the origin of X-ray pulses and also the approximate energy of X-ray photons were analyzed. It was found that at higher pressure 4?torr, the yield of SXR photons in different pulses increases 20–30% in comparison with the results obtained at pressure 3?torr while the yield of HXR decreases. The results confirm that the operating pressure is an effective parameter on the yield of SXR and HXR emitted from this device.     

Numerical Experiments on Oxygen Soft X-Ray Emissions from Low Energy Plasma Focus Using Lee Model

The X-ray emission properties of oxygen plasmas are numerically investigated using corona plasma equilibrium model. The Lee model is here modified to include oxygen in addition to other gases. It is then applied to characterize the Rico Plasma Focus (1 kJ), finding a oxygen soft X-ray yield (Ysxr) of 0.04 mJ in its typical operation. Keeping the bank parameters and operational voltage unchanged but systematically changing other parameters, numerical experiments were performed finding the optimum combination of pressure = 3 Torr, anode length = 1.5 cm and anode radius = 1.29 cm. The optimum Ysxr was 43 mJ. Thus we expect to increase the oxygen Ysxr of PF-1 kJ thousand-fold from its present typical operation; without changing the capacitor bank, merely by changing the electrode configuration and operating pressure. The modified version of the Lee model code is also used to run numerical experiments with oxygen gas, for optimizing the oxygen soft X-ray yield on the new plasma focus device PF-SY2 (2.8 kJ). The static inductance L₀ of the capacitor bank is progressively reduced to assess the effect on pinch current I_pinch. The experiments confirm the I_pinch, limitation effect in plasma focus, where there is an optimum L₀ below which although the peak total current, I_peak, continues to increase progressively with progressively reduced inductance L₀, the I_pinch and consequently the soft X-ray yield, Ysxr, of that plasma focus would not increase, but instead decreases. The obtained results indicate that reducing the present L₀ of the PF-SY2 device will increase the oxygen soft X-ray yield till the maximum value after that the Ysxr will decrease with I_pinch decreasing.     

Radiation Emission Correlated with the Evolution of Current Sheath from a Deuterium Plasma Focus

The time resolved emission of neutrons and X-rays (both soft and hard) is correlated with the current sheath evolution during the radial phase of a 3.2 kJ Mather-type plasma focus device operated in deuterium at an optimised pressure of 4 mbar. A three-frame computer-controlled laser shadowgraphy system was incorporated in the experiment to investigate the time evolution of the radial phase of the plasma focus. The dynamics of the sheath was then correlated with the time resolved X-rays and neutron emission. The time-resolved neuron and hard X-ray emission was detected by a Scintillator-photomultiplier system while the time resolved soft X-rays were detected employing filtered PIN photo diodes. The observations were recorded with a temporal accuracy of a few ns. For the reference, the total neutron yield was also monitored by an Indium Foil activation detector. The correlation with the High Voltage Probe signal of the discharge, together with the X-ray and neutron emission regimes enabled to identify the important periods of the sheath evolution i.e. the radial compression (pre focus), minimum pinch radius (focus) and the post focus phenomena. During the initial stage of the radial phase, velocities of 10–23 cm/μs, while at the later stage of the radial phase (up till the compression), velocities up to 32–42 cm/μs were measured in our experiment. For the discharges with the lower neutron yield (lower than the average value ~1 × 10⁸ n/discharge), the current sheath appears to be disturbed and neutron and hard X-ray signal profiles do not carry much information whereas the soft X-ray emission is significant. For the discharges with high neutron yield (higher than the average value), the current sheath has a smooth structure until the maximum compression occurs. Hard X-ray emission is maximum for the discharges with high neutron yield, especially whenever there is development of m = 0 instability compressing the column to very high densities. The neutron are emitted long after the maximum compression supporting the beam target fusion. For the discharges with High neutron yield, the soft X-ray production is less as compared to the discharges with low neutron yield.     

Deposition of Alumina Films on Si (1 0 0) Substrate Using a Low Energy Dense Plasma Focus Device

A 1.5 kJ pulsed low energy Mather type plasma focus (PF) is used to deposit thin films of alumina (α-Al₂O₃) on Si (1 0 0) substrates. The PF device with its anode made of aluminum was operated with argon-oxygen mixture as the filling gas. The Al₂O₃ thin film samples were prepared using 10, 20 and 30 successive shots with substrates placed at 60 mm from the top of the anode at approximately zero angular position with respect to the anode axis. The crystallography of the as-deposited and annealed samples was studied by X-ray diffractometry (XRD). Raman Spectroscopy studies verified the formation of α-Al₂O₃ phase in the annealed films. Scanning electron micrographs (SEM) of the annealed films present many different sized particulates (50–300 nm) distributed upon the film surface. The cross-sectional SEM micrographs show that the thickness of deposited alumina film is linear with a typical rate of 45 nm/shot at focus storage energy of 850 J.     

Study of X-ray Emission from a Compact Diode Operated by a High-Inductance Capacitor Discharge

A compact diode comprising a flat plate anode and a sharp-edged cathode (a piece of razor blade) energized by 0.5 F capacitor charged to 30 kV is investigated for optimization of X-rays emission vis-à-vis separation between electrodes and width of the cathode, which is responsible for electron emission by impact of electric field. It is a high-inductance system, the parasitic inductance is found to be 353 ± 5 nH, and the recorded peak discharge current is just 35 ± 02 kA. The maximum X-ray emission is observed for a 2-mm-wide cathode with an interelectrode separation of 3 mm. The X-ray yield in 4-geometry is found to be 34 ± 3 mJ with a wall-plug efficiency of 0.015 ± 0.001%. The X-ray emission occurs about 200 ns after the application of high voltage, synchronized with the dip in current wave form. The low efficiency of the system for X-ray generation is attributed to high parasitic inductance.     

Experimental Study of Hard X-ray Emission with Different Anode Tips in APF Plasma Focus Device

To investigate the effect of different anode tips on the hard X-ray (HXR) emission from APF plasma focus device (16 kV, 36 μf, and 115 nH) we considered two shapes of anode tips, i.e., flat or conic, with Cu, Al and W anode tip materials. The highest HXR intensity was observed with conic W anode insert and the lowest with flat Al anode insert. In comparison with the hollow anode tube the HXR signals obtained from flat and especially conic W inserts nearly tending to have the same intensity along the axis of the device. Therefore the shape and the material of the anode tip has significant effect on the production of HXR emissions. This study show that the employment of a convenient shape of anode tip results to more isotropic emission of HXR.     

Energy spectra measurements of X-ray emission from electron interaction in a dense plasma focus device

Electrons generated during a pinch implosion in a hollow anode Mather-like plasma focus device (PF) are considered as a possible X-ray source via the impinging of those particles on medium and high-Z targets. A usual PF device has been slightly modified to optimise the X-ray production and their measurements by means of a suitable and non-invasive spectrometer. This ensemble allows measurements of X-rays generated booth by electrons turning back to the anode and by target collision of the so-called relativistic electron beam. The spectrum of the emitted photons is evaluated by using a differential absorption based technique. The X-ray spectrometer consists of a stack of LiF dosimeters which act both as detectors and filters to give curves of attenuated intensities. Finally, the energy distribution is calculated from such attenuation curves using an iterative procedure based on spectral algebra formalism.     

Sahand Plasma Focus Emitted More Than 35 J in Yield Neon Soft X-ray

X-ray emission from neon plasma produced in Sahand Filippov type plasma focus device is investigated. Detecting system used in our experiments is a five channel pin diode detector. Soft X-ray emissions from neon gas at different charging voltages and working gas pressures are studied and optimum condition for production of soft X-ray for Sahand plasma focus facility is obtained. Results show that for every working pressure there is a charging voltage at which the average soft X-ray yield is maximum and this optimum charging voltage increases with increasing gas pressure. For 0.25, 0.50 and 0.75 Torr neon the optimum voltages are 12, 13 and 16 kV respectively. The highest average soft X-ray yields produced at 0.50 Torr which is about 35.87 ± 1.18 J. Also soft X-ray production decreased after a certain pressure which for our experiments is 0.50 Torr. The variation of relative hard X-ray yield with charging voltage and working gas pressure is also investigated. The results show that 0.50 Torr is also the best operating gas pressure for optimum hard X-ray production in Sahand Filippov type plasma focus device.     

Measurement of the Energy of Nitrogen Ions Produced in Filippov Type Plasma Focus Used for the Nitriding of Titanium

In this paper the nitrogen ion properties (maximum energy, current density and the most probable energy) are investigated by using Faraday cup in a time of flight method. These ions are produced in a Filippov type plasma focus (Sahand Facility) device and the Faraday cup was placed in a distance range of 18–24?cm from the top of the anode. Maximum and minimum most probable ion energies are 76 and 8.5?keV for the distance range of 18 and 24?cm, respectively. The displacement from 18 to 24?cm at top of the anode the ion current density varies from 4.5?×?10⁶ to 3.2?×?10⁵ (A?m^?2). For the investigation of the effect of ions bombardment of materials at different positions, at the optimum working conditions of 14?kV as a working voltage, and 0.25?Torr as a gas pressure, titanium samples are placed in a distance of 21, 22, 23 and 24?cm from the top of the anode (θ?=?0) and each sample is put under irradiation for 30 plasma shots. The structure of the nitrided surfaces and their morphologies are characterized by X-ray diffractometry and by scanning electron microscopy, respectively. The average crystallite size deduced for (200) and (222) planes of TiN deposited with 30 shots in different distances are estimate to be from ~13 to ~38?nm.     

Experimental Study of the Variation of Neutron Emission Anisotropy in a Filippov-type Plasma Focus Facility

This paper presents the results of experimental studies of the variation of spatial anisotropy in neutron emission with working conditions in a 90 kJ Filippov-type plasma focus device. The working gases are D₂ and D₂ + 1%Kr. The results of our experiments have shown that the anisotropy factor decreases with increasing the initial pressure and/or discharge energy. Furthermore, it has been observed that by using D₂ + 1%Kr as working gas, the variation in anisotropy factor with initial pressure and/or discharge energy is relatively high, but by using D₂ it changes slowly. The highest neutron yield has been achieved by using D₂ + 1%Kr and a conic insert anode. Thus, we have studied the correlation between neutron yield and anisotropy factor for this case at fixed working conditions from shot to shot. At 16 kV discharge voltage and pressures around optimum, the behavior of anisotropy factor is generally increasing with neutron yield, whereas at low and high pressures, the anisotropy factor does not change significantly with yield.     

An Optimized Design of Anode Shape Based on Artificial Neural Network for Achieving Highest X-ray Yield in Plasma Focus Device

In this paper, an optimized design of anode shape in order to achieve highest X-ray yield in a plasma focus device filled with nitrogen gas based on artificial neural networks (ANNs) is presented. Multi-layer perceptron neural network structure with the back-propagation algorithm is used for the training of the proposed model. The model has achieved good agreement with the training data and has yielded satisfactory generalization. This shows that the ANN model is an accurate and reliable approach to predict the highest X-ray yield in plasma focus devices.     

Temporal Variation of the Current Sheet Inductance from PACO Plasma Focus Device

The temporal variation of the current sheet (CS) inductance in a plasma focus device can be calculated using the current derivative and the voltage signal acquired on the anode electrode, which are very common measurements in this type of device. The value of that inductance contains important information about the discharge performed including the CS lift-off from the insulator, voltage between the pinch extremes, maximum energy of the X-ray, energy delivered to the pinch and information about the actuating fusion mechanisms if the filling pressure is deuterium. This work discusses the values of the CS inductance extracted from several discharges of the Plasma Auto Confinado (PACO) plasma focus, installed in the National University of the Center of Buenos Aires—Argentina (2 kJ total energy, capacitor bank of 4 μF charged to 31 kV and a maximum current of 250 kA).     

大束流场发射阴极X射线管的阳极设计

采用蒙特卡罗软件和有限元分析软件分析了固定阳极X射线管中电子轰击阳极靶的能量沉积和热量传输过程,固定阳极由铜棒和钨片组成。研究了在不同形状、尺寸和占空比的电子束脉冲轰击下,阳极靶面不同厚度的钨片和相邻铜棒的温度上升过程。结果表明,钨片厚度存在最优值,但最优值与具体使用条件相关;在脉冲成像方式下,固定阳极靶能承受更强束流强度的轰击。因此,采用固定阳极靶方案,研制140 kV高压、10 mA以上电流、直径1 mm以下有效焦点的场发射阴极X射线管是可行的。     

Using Mather-Type Plasma Focus Device for Fabrication of Tungsten Thin Films

Tungsten (W) thin films were deposited on stainless steel–304 substrates using a low energy (2?kJ) plasma focus device. The samples were synthesized at various distances with respect to anode tip (7, 10 and 13?cm) and using same number of focus deposition shots (25 shots). X-ray diffraction (XRD), energy dispersive X-ray atomic force microscopy (AFM) and micro hardness were used to investigate the prepared samples. XRD analysis revealed that the degree of crystallinity of deposited thin films decrease with increasing the distance from the anode tip. AFM results showed that size of the grains on the surface of the films and the surface roughness of deposited samples constantly increase with the increasing of the axial position. Moreover, the hardness measurements revealed that the highest mechanical hardness is obtained when the film is deposited at 7?cm axial position.     

Characteristics of X-ray Hot Spots in a 3.5 kJ Plasma Focus Device at Different Pressures of Argon and Air

The present study examined the formation of hot spots in the plasma column of a 3.5 kJ Mather-type plasma focus device. Experiments were performed with air and argon as operating gases at 0.2–1.5 mbar of pressures. X-ray source images were obtained using a pinhole camera with dental X-ray film as X-ray detector. The objective was to investigate the effect of the operating conditions and gas type on formation and characteristics of the hot spots. Results showed that when using air in comparison to argon, the total X-ray emission is increased and therefore, the hot spots are covered by this high intensity emission and would be observed less frequently in the image. Using metal filters to attenuate the low-energy X-rays revealed that the most energetic or the most intense radiation was emitted from the hot spots region. The images of the X-ray source obtained using argon at the middle pressures (0.4–0.6 mbar) showed both the plasma column and the photons emitted from the anode surface. A pressure of 0.8–1.5 mbar using argon was most likely to observe the hot spots. For argon gas, the 0.9 mbar was the pressure in which the hot spots were more frequently observed with high reproducibility of location and number. Measurements revealed that the typical size of a hot spot was 10–300 µm and the distance from the anode surface was 0.5–20 mm.     

Experimental Study of Electron Emission Characteristics of a Surface Flashover Trigger in a Low Pressure Environment

Characteristics of electron emission induced by a surface flashover trigger device in a low-pressure trigger switch were investigated. A test method to measure the emitted charges from the trigger device was developed, and the factors affecting the emitted charges were analyzed. The results indicated that the major emitted charges from the trigger device were induced by surface plasma generated by surface flashover occurring on the trigger dielectric material. The emitted charges and the peak emission current increased linearly with the change in the trigger voltage and bias voltage. The emitted charges collected from the anode were affected by the gap distance. However, the emitted charges were less affected by the anode diameter. Furthermore, the emitted charges and the peak emission current decreased rapidly with the increase in gas pressure in a range from 0 to 100 Pa, and then remained stable or changed slightly when the increase in gas pressure up to 2400 Pa.