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.     

Analysis of X-ray spectra in 14.5-GHz ECR ion source for optimizing operation conditions

An electron cyclotron resonance (ECR) ion source operating at 14.5 GHz was developed for the generation of charged ions at the Korea Atomic Energy Research Institute (KAERI). Experiments were carried out to study the plasma inside the ECR ion source by analyzing the X-ray spectra generated by it. The X-ray energy distribution and electron energy inside the plasma chamber are influenced by the status of the heated plasma. That status depends on various operation parameters such as microwave power, injected gas-pressure, and solenoid and trim coil currents. X-ray spectra were recorded to find the correlation between the plasma and the X-rays for variations in the operation parameters. A standard NaI(Tl) detector was used for that purpose. The X-ray energy distribution was studied in the range of 100–500 W for radiofrequency power. The influence of the injected gas pressure and the mirror ratio in the emission of X-rays were analyzed.     

Angular Distribution of Argon Ions and X-Ray Emissions in the Apf Plasma Focus Device

Angular distribution of ion beam emission from an argon gas-filled plasma focus devices has been investigated using an array of five Faraday cups. The argon ion beam emission is found to be highly pressure-dependent and reaches its maximum at the pressure of 1 torr. The ions flux decreased as the working pressure increased; the maximum ion density at 1 torr was estimated to be around 9.24 × 10²⁴ ions/steradian. Also, the study on the angular distribution of X-rays has been carried out using TLD-100 dosimeters. The intensity of ions reduced significantly at angles higher than ±11° but the X-ray distribution was bimodal, peaked approximately at ±15°.     

Soft X-Ray Emission in the Water Window Region with Nitrogen Filling in a Low Energy Plasma Focus

For operation of the plasma focus in nitrogen, a focus pinch compression temperature range of 74–173 eV (0.86 × 10⁶–2 × 10⁶ K) is found to be suitable for good yield of nitrogen soft X-rays in the water window region. Using this temperature window, numerical experiments using five phase Lee model have been investigated on UNU/ICTP PFF and APF plasma focus devices with nitrogen filling gas. The Lee model was applied to characterize and optimize these two plasma focus devices. The optimum nitrogen soft X-ray yield was found to be Y_sxr = 2.73 J, with the corresponding efficiency of 0.13 % for UNU/ICTP PFF device, while for APF device it was Y_sxr = 4.84 J, with the corresponding efficiency of 0.19 % without changing the capacitor bank, merely by changing the electrode configuration and operating pressure. The Lee model code was also used to run numerical experiments for optimizing soft X-ray yield with reducing L₀, varying z₀ and ‘a’. From these numerical experiments we expect to increase the nitrogen soft X-ray yield of low energy plasma focus devices to maximum value of near 8 J, with the corresponding efficiency of 0.4 %, at an achievable L₀ = 10 nH.     

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.     

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.     

The Effect of Applied Voltage and Operating Pressure on Emitted X-Ray from Nitrogen (N<Subscript>2</Subscript>) Gas in APF Plasma Focus Device

In the present work, the effect of applied voltage and operating pressure on behaviour of X-rays emitted from nitrogen gas (N₂) used in APF plasma focus facility is investigated. It was found that the optimum conditions for high emissions of SXR and HXR from the plasma focus (PF) are different. At four applied voltages of 10, 11, 12, and 13 kV, the optimum pressures for SXR and HXR emissions of nitrogen gas (N₂) were obtained. At lower voltages, 10, and 11 kV optimum pressure for SXR emission was 3.5 torr while for HXR emission was 2.5 torr. At higher voltages, 12, and 13 kV, the optimum pressures shift to higher values at 4 and 3 torr for SXR and HXR emissions, respectively. Among the applied voltages, the least intensity of both SXR and HXR was at voltage 10 kV and the most intensity was for 13 kV which confirm with increasing voltage, the intensity of X-ray emission increases. Also the results obtained by images of pin-hole camera were in compatible with the results of detected signals by different filtered Pin-diodes and Scintillation detector. Our results illustrate that the voltage and the pressure are effective parameters in X-ray emission from the PF.     

Detection of X-ray Radiations from Low Energy Focus Plasma Using Thermoluminescence Dosimeter TLD-500A

The present plasma focus device (0.1 kJ Mather type) is powered by a capacitor bank of 1 μF at 15 kV maximum charging voltage. The radiations emission was investigated using time-integrated thermoluminescence TLD-500. These detectors have been calibrated against standard X-ray machine (VIC model 4000M⁺) as well as standard gamma sources (Co⁶⁰ and Cs¹³⁷⁾ to identify the dose from X-ray delivered to the dosimeter. It is found that from the results of time-integrated detectors, the maximum radiation dose obtained at 0.1 Torr pressure and charging voltage 15 kV for different gases. At this pressure, the device favours a proper discharge dynamics so as to form a strong pinching i.e. transferring maximum energy into plasma.     

A unique high pressure apparatus for X-ray diffraction studies of phase transitions up to 5 kbar

A new high pressure apparatus equipped with a beryllium pressure cell for X-ray diffraction experiments up to 5 kbar is described. The apparatus is especially designed for studying phase transitions in crystalline solids. The pressure generator has the capabilities of controlling pressure with precision of ± 2 bars and of performing a quick change in pressure applied to a sample. The beryllium miniature cell is press-fitted with a supporting jacket which has a large aperture for X-ray beams; therefore, a greater portion in reciprocal space can be surveyed. By using synchrotron radiation X-rays, we have carried out a test experiment on the incommensurate-commensurate phase transition in [N(CH₃)₄]₂MnCl₄.     

Numerical Experiments on Oxygen Plasma Focus: Scaling Laws of Soft X-Ray Yields

Numerical experiments have been investigated on UNU/ICTP PFF low energy plasma focus device with oxygen filling gas. In these numerical experiments, the temperature window of 119–260 eV has been used as a suitable temperature range for generating oxygen soft X-rays. The Lee model was applied to characterize the UNU/ICTP PFF plasma focus. The optimum soft X-ray yield (Y_sxr) was found to be 0.75 J, with the corresponding efficiency of about 0.03 % at pressure of 2.36 Torr and the end axial speed was v_a = 5 cm/μs. The practical optimum combination of p₀, z₀ and ‘a’ for oxygen Y_sxr was found to be 0.69 Torr, 4.8 cm and 2.366 cm respectively, with the outer radius b = 3.2 cm. This combination gives Y_sxr ~ 5 J, with the corresponding efficiency of about 0.16 %. Thus we expect to increase the oxygen Y_sxr of UNU/ICTP PFF, without changing the capacitor bank, merely by changing the electrode configuration and operating pressure. Scaling laws on oxygen soft X-ray yield, in terms of storage energies E₀, peak discharge current I_peak and focus pinch current I_pinch were found over the range from 1 kJ to 1 MJ. It was found that the oxygen soft X-ray yields scale well with $ {\text{Y}}_{\text{sxr}} = 2 \times 10^{ - 7} {\text{I}}_{\text{pinch}}^{3.45} $ and $ {\text{Y}}_{\text{sxr}} = 6 \times 10^{ - 7} {\text{I}}_{\text{peak}}^{ 2. 9 2} $ for the low inductance (L₀ = 30 nH) (where yields are in J and currents in kA). While the soft X-ray yield scaling laws in terms of storage energies were found to be as $ {\text{Y}}_{\text{sxr,O}} = 5.354 \times {\text{E}}_{0}^{1.12} $ (E₀ in kJ and Y_sxr in J) with the scaling showing gradual deterioration as E₀ rises over the range. The oxygen soft X-ray yield emitted from plasma focus is found to be about 8.7 kJ for storage energy of 1 MJ. The optimum efficiency for soft X-ray yield (1.1 %) is with capacitor bank energy of 120 kJ. This indicates that oxygen plasma focus is a good soft X-ray source when properly designed.     

Experimental Study of Current Discharge Behavior and Hard X-ray Anisotropy by APF Plasma Focus Device

Amirkabir (APF) is a new Mather-type plasma focus device (16 kV, 36 μf, and 115 nH). In this work we present some experimental results as variation of discharge current signal respect to applied voltage at the optimum pressure, focusing time of plasma versus gas pressure, and variations of current discharge with different insulator sleeve dimensions. As we prospected optimum pressure tending to increase as we tried to higher voltage levels. The time taken by the current sheath to lift-off the insulator surface and therefore quality of pinched plasma depends on the length of the insulator sleeve. The results show that the insulator diameter can influence on pinch quality. Behavior of hard X-ray (HXR) signals with the pressure and also anisotropy of HXR investigated by the use of two scintillation detectors. The distribution of HXR intensity shows a large anisotropy with a maximum intensity between 22.5° and 45° and also between −22.5° and −67.5°.     

Practical Optimization of AECS PF-2 Plasma Focus Device for Argon Soft X-ray Operation

For operation of the plasma focus in argon, a focus pinch compression temperature range of 1.4–5 keV (16.3 × 10⁶–58.14 × 10⁶ K) is found to be suitable for good yield of argon soft X-rays (SXR) Ysxr. This is based on reported temperature measurements of argon plasmas working at regime for X-ray output. Using this temperature window, numerical experiments have been investigated on AECS PF-2 plasma focus device with argon filling gas. The model was applied to characterize the 2.8 kJ plasma focus AECS PF-2. The optimum Ysxr was found to be 0.0035 J. Thus, we expect to increase the argon Ysxr of AECS PF-2, without changing the capacitor bank, merely by changing the electrode configuration and operating pressure. The Lee model code was also used to run numerical experiments on AECS PF-2 with argon gas for optimizing soft X-ray yield with reducing L₀, varying z₀ and ‘a’. From these numerical experiments we expect to increase the argon Ysxr of AECS PF-2 with reducing L₀, from the present computed 0.0035 J at L₀ = 270 nH to maximum value of near 0.082 J, with the corresponding efficiency is about 0.03%, at an achievable L₀ = 10 nH.     

Precision X-ray spectroscopy on 8.5 MeVamu heavy ions

A new experimental capability has been developed at the Lawrence Berkeley Laboratory Super-HILAC to investigate questions relating to high resolution atomic spectroscopy. A key element of these measurements is a dual arm Johann spectrometer. The ion beam passes inside the Rowland circle of two curved crystals which are mounted such that diffracted X-rays have equal and opposite linear Doppler shifts. The X-ray lines are detected with high speed X-ray film mounted on the Rowland circle. The beam-crystal geometry is arranged so a spectral range θ_B ~ 30°–70° is detected. The spectrometer efficiency is high with useful exposures obtained with only 10 mC of beam. A wavelength calibration is obtained by simultaneously exposing the film with diffracted K and L X-rays from an X-ray tube. X-ray lines from the beam are slanted, with respect to the calibration lines, due to Doppler shifts arising from X-rays incident on the crystal at angles other than perpendicular to the diffraction plane. The slope of these lines provides an independent determination of the beam velocity, which is used to correct for the transverse Doppler shift. Typical results are presented.     

Plasma Stability Evaluation Based on MHD Activity and Hard X-Ray Emission in the IR-T1 Tokamak

Determinations of the poloidal beta, internal inductance, plasma energy, plasma pressure, plasma temperature, plasma resistance, plasma effective atomic number, magneto-hydrodynamics (MHD) activity, Runaway electrons energy and energy confinement time are essential for tokamak experiments and optimized operation. Also some of the plasma information can be deduced from these parameters, such as plasma toroidal current profile, and MHD instabilities. In this contribution we investigated about measurements of some plasma parameters as well as MHD activity and Runaway electrons energy. For this purpose we used the magnetic diagnostics and a hard X-ray spectroscopy in IR-T1 tokamak. A hard X-ray emission is produced by collision of the Runaway electrons with the plasma particles or limiters. The mean energy was calculated from the slope of the energy spectrum of hard X-ray photons. In this paper in order to measure energy of the Runaway electrons, we obtained hard X-ray energy in every 5 ms intervals, from the beginning to the end of plasma. Results indicated mean energy of Runaway electrons is maximum during the 0–5 ms interval.     

Dose Measurement of Hard X-ray Produced by Damavand Tokamak by Means of LiF:Mg, Cu, P TLDs

Damavand tokamak is the source of soft and hard X-ray by hydrogen working gas in plasma duration time. As such devices are widely used in fusion researches, it is required to comply with radiation protection standards and monitor radiation dose output. In this paper the dose measurement of hard X-rays produced by Damavand tokamak has been done in order to perform the necessary protection arrangement in torus area. All experiments were done by Thermoluminescence crystal dosimeter tools of the type LiF:Mg, Cu, P crystals. The results showed that radiation levels around the torus are very high (in the order of several mSv per shot) and various dose levels in different points (in terms of distance and height of device) imply the anisotropic spatial distribution of measured dose. According to the measurements during 100 shots of Damavand tokamak, the total dose in the shielding room which is 5 m away from torus, is above the permissible level. In order to control personnel safety, it was designed and constructed a lead shielding wall with 5 cm thick and 2 m × 7.5 m dimensions and the performed dosimetry operation after installation of wall shows a mean value of 96.33 % reduction in measured dose due to presence of lead shielding. Thus there will be possibility of doing 25,000 shots/year in safe condition.     

On the Production of Relativistic Runaway Electrons in Damavand Tokamak

Experimental observations in Damavand tokamak show that hard X-ray is produced by either disruption with I _p  < 20 kA or by shots with I _p  > 20 kA. Hard X-ray also persists from the initiation of plasma discharge to the end. Occurrence of multiple spikes in hard X-ray during the discharge is evident. The propagation of hard X-ray is attributed to runaway electrons. We observe runaway electrons in two regimes with different characteristics. Regime (RADI) is similar to the observations of other Tokamak during disruption on that the plasma current is reduced abruptly and interpreted by Dreicer theory. In the regime of RADII, hard X-ray and subsequently runaway electrons are observed from starting of plasma discharge which provides the condition that the most of runaway electrons contain the toroidal plasma current. Runaway electron beam excites whistler waves and scattered electrons in velocity space and prevent growing the runaway electrons beam.     

Comprehensive Study of Neon HXR and SXR Emitted from APF Plasma Focus Device

Experimental results related to HXR and SXR properties of Neon plasma on the APF plasma focus device is presented. The experiments were carried on over a wide range of neon pressure and at voltages 11, 12 and 13 kV using plastic scintillator (NE102A) coupled with high gain PMT and six filtered photo PIN diodes. For the charging voltages of 11–13 kV with 2.17–3.04 kJ stored energy, the optimum operating pressure in neon is found to be in the range of 3.5–5 torr and the highest HXR emission was observed in the pressure of 5 torr at the voltage 13 kV and the maximum average HXR production is (9.84 ± 0.59) ×10^–7 volt sec. The behavior of SXR intensities were registered by different filters and it was found out that Al-Mylar 6 μm and Cu 10 μm has the highest and lowest amount of X-ray transmission.     

External scanning micro-PIXE for the characterization of a polycapillary lens: Measurement of the collected X-ray intensity distribution

We developed a PIXE detection system for the analysis of medium-light elements which exploits a weakly focusing polycapillary lens for the transmission of the X-rays emitted from the target material to a Silicon Drift Detector. The polycapillary lens efficiently collects X-rays, while prevents back-scattered protons from impinging on the detector chip, thus avoiding electronics perturbation and consequent quality loss of PIXE spectra. The system is optimized for the detection of X-rays in the energy range 1–10 keV, when the emission from the target is induced by MeV proton beams with size of the order of a few hundreds of micrometers.This work reports the results of the lens characterization in terms of X-ray collection spot, i.e. the area of the sample actually “seen” by the lens, and its dependence on the X-ray energy. The lens properties have been measured using the external scanning microbeam facility of the Tandetron accelerator at LABEC-INFN in Florence. The detection system was used to detect X-rays from a set of pure elemental standards with an incident 3 MeV proton beam focused to a size of about 30 μm scanning an area of 1.9 × 1.6 mm². By measuring the spatial distribution of characteristic X-rays from each given material, the collection profile of the lens at the corresponding X-ray energy was obtained. Using several standards, the behaviour throughout the range 1–10 keV was examined. The sensitivity of the lens collection profile on the lens-sample out-of-focus distance was also investigated.     

Correlation of Neutron and X-ray Emission from Plasma Focus with Pre-ionization

The effect of pre-ionization caused by depleted uranium (₉₂U²³⁸) on the correlation of neutron and X-ray emission from 1.8 kJ plasma focus is investigated by employing photomultiplier tubes (XP2020) coupled with fast (50 × 50) mm² cylindrical plastic scintillators (NE102A) along with GM tube and Quantrad Si PIN diodes with a pair of appropriate filters. It is found that neutron and Cu–K_α emission along with total X-ray yield are significantly increases with pre-ionization as compared to those without pre-ionization. Moreover, pre-ionization improves the shot to shot reproducibility of the system and broadens the operating pressure regime both for neutron and X-ray emission.     

Soft X-ray Imaging using a Neon Filled Plasma Focus X-ray Source

A 3 kJ Mather-type UNU/ICTP plasma focus device with neon filling is used, for the first time, as a soft X-ray source for imaging of thin biological samples including insects. A charge-coupled-device (CCD) based pinhole projection system, placed in a differentially pumped chamber, is used for radiography using neon soft X-rays. The image brightness, contrast and resolution have been optimized by varying soft X-ray yield, pinhole size, camera chamber length and X-ray filters. The system can simply be modified for table-top soft X-ray microscopy of thin biological samples.