Effect of Resonant Helical Field (RHF) on Runaway Electrons in Tokamaks

The high energy current of runaway electrons during a major disruption in tokamak reactors can cause serious damage to the first wall of the reactor and reduce its life time. Therefore, finding a method to minimize runaway electron is much needed. Resonant helical field (RHF) is one of the methods for controlling the magnetohydrodynamic (MHD) activity. This paper attempts to examine the effect of RHF on the generation of runaway electrons. Main parameters such as plasma current, loop voltage, emitted hard X-ray intensity, MHD oscillation, H_α radiation and MHD activity modes, in the presence and absence of RHF (L = 2 and L = 3), were measured. The results show that applying this system can change runaway electrons generation.     

Effects of Hot Limiter Biasing on Tokamak Runaway Discharges

In this research hot limiter biasing effects on the Runaway discharges were investigated. First wall of the tokamak reactors can affects serious damage due to the high energy runaway electrons during a major disruption and therefore its life time can be reduced. Therefore, it is important to find methods to decrease runaway electron generation and their energy. Tokamak limiter biasing is one of the methods for controlling the radial electric field and can induce a transition to an improved confinement state. In this article generation of runaway electrons and the energy they can obtain will be investigated theoretically. Moreover, in order to apply radial biasing an emissive limiter biasing is utilized. The biased limiter can apply +380 V in the status of cold and hot to the plasma and result in the increase of negative bias current in hot status. In fact, in this experiment we try to decrease the generation of runaway electrons and their energy by using emissive limiter biasing inserted on the IR-T1 tokamak. The mean energy of these electrons was obtained by spectroscopy of hard X-ray. Also, the plasma current center shift was measured from the vertical field coil characteristics in presence of limiter biasing. The calculation is made focusing on the vertical field coil current and voltage changes due to a horizontal displacement of plasma column.     

Initial Plasma Formation in the GLAST-II Spherical Tokamak

This paper reports the initial plasma formation in glass spherical tokamak (GLAST-II) with electron cyclotron resonance pre-ionization assisted startup. Initially, a plasma current of 3 kA has been produced for duration of about 0.5 ms after establishing optimum conditions for microwave absorption at 2.45 GHz. Plasma current is then enhanced up to 5 kA by applying a small vertical magnetic field that provides additional plasma heating and shaping. Applied vertical field is optimized experimentally and optimal value is found to be 40 Gauss for this experiment. Plasma current and loop voltage are monitored by using Rogowski coil and toroidal loop of wire. A fast framing camera (5000 fps) is used for temporal investigation of plasma during the discharge scenario. A fast photodiode (BPX-65) and USB4000 spectrometer are used to record the signature of plasma current and the impurity content (O₂, H etc.). Cross-sectional average electron temperature is also estimated from plasma resistivity and found to be 6.1 eV for maximum plasma current of 5 kA.     

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.     

全超导托卡马克装置欧姆放电逃逸电子行为研究

电子发生逃逸在托卡马克等离子体中是较常见的现象,特别是在等离子体破裂阶段,会产生大量的逃逸电子。本工作利用硬X射线监测系统,并结合其它相关诊断系统研究世界上第1个运行的全超导托卡马克（EAST）装置在欧姆放电的不同阶段逃逸电子的行为。研究结果表明：在欧姆放电起始阶段,逃逸电子的初级产生过程占主导地位。随着放电的进行,逃逸电子的次级雪崩过程逐渐增长,在放电后期一直到等离子体破裂阶段,雪崩过程将占据主导地位。等离子体破裂后,因存在较高的环电压而产生了高能逃逸电子拖尾。     

Scaling Laws of Nitrogen Soft X-ray Yields from 1 to 200 kJ Plasma Focus

Numerical experiments are carried out systematically to determine the nitrogen soft X-ray yield for optimized nitrogen plasma focus with storage energy E₀ from 1 to 200 kJ. Scaling laws on nitrogen soft X-ray yield, in terms of storage energies E₀, peak discharge current I_peak and focus pinch current I_pinch were found. It was found that the nitrogen X-ray yields scales on average with $ {\text{Y}}_{\text{sxr,N}} = 1.93 \times {\text{E}}_{0}^{1.21} {\text{J}} $ (E₀ in kJ) with the scaling showing gradual deterioration as E₀ rises over the range. A more robust scaling is $ {\text{Y}}_{\text{sxr}} = 8 \times 10^{ - 8} {\text{I}}_{\text{pinch}}^{3.38} $ . The optimum nitrogen soft X-ray yield emitted from plasma focus is found to be about 1 kJ for storage energy of 200 kJ. This indicates that nitrogen plasma focus is a good water-window soft X-ray source when properly designed.     

Computational Investigation of Nonideal Higher Ionization Carbon Plasma Under Simulated High Heat Fluxes in Fusion Tokamak Reactors

Plasma-facing materials in future large tokamaks will suffer from ablation due to expected hard disruptions, which affects the reactor interior lining tiles and the divertor modules. Ablation and surface evaporation due to the intense heat flux from disruption is associated with ionization of the evolved particulates. Generated ions at such plasma conditions may allow for higher ionization states such that the plasma at the boundary can be composed of electrons, ions (first, second and third ionization) and excited atoms. The boundary layer is dense and tends to be weakly nonideal. The NC State University electrothermal plasma code ETFLOW used to simulate the high heat flux conditions in which the carbon liner tested for simulated heat fluxes for transient discharge period of 100 μs, with FWHM of ~50 μs, to provide a wide range for obtaining reasonable good fits for the scaling laws. Transient events with ~10 MJ/m² energy deposition over short transient of 50–100 μs would produce heat fluxes of 100–200 GW/m². The heat flux range in this simulation is up to 288 GW/m² to explore the generation of carbon plasma up to the third ionization C⁺⁺⁺. The generation of such heat fluxes in the electrothermal plasma source requires discharge currents of up to 250 kA over a 100 μs pulse length with ~50 μs FWHM. The number density of the third ionization is six orders of magnitude less than the first ionization at the lowest heat flux and two orders of magnitude less at the highest heat flux. Plasma temperature varies from 31,600 K (2.722 eV) to 47,500 K (4.092 eV) at the lowest and highest heat fluxes, respectively. The plasma temperature and number density indicate typical high-density weakly nonideal plasma. The evolution of such high-density plasma particles into the reactor vacuum chamber will spread into the vessel and nucleate on the other interior components. The lifetime of the PFCs will shorten if the number of hard disruptions at such extreme heat fluxes would be increasing, resulting in major deterioration of the armor tiles.     

Effects of External Resonant Fields on the Tokamak Edge Plasma Fluctuations

In this contribution, the effects of external resonant electric and magnetic fields on the tokamak edge plasma fluctuations have been investigated. For this purpose, the radial and poloidal electric fields and ion saturation current have been measured by two arrays of the Langmuir probes. An external resonant electric field was applied with the limiter biasing system. The biased electric voltage has been restricted to 0 < V _bias  < +320 and it has been applied with the limiter that is fixed in the r/a = 0.9. Moreover, the power spectra of particle flux Γ _r , Γ _p , Reynolds stress, coherency between E _p and I _s have been calculated. Fourier-based techniques have been employed to analyze the frequency of the Reynolds stress and particle flux. The results show that after positive biasing application (V _bias  = +200v), the Reynolds stress increases about 50 % while it doesn’t change remarkability after positive biasing with V _bias  = +320v. The Reynolds stress power spectrum confirms these results. The effect of positive biasing on Γ _r has been displayed a decrease about 60, 20 % by V _bias  = +200v, V _bias  = +320v respectively. Γ _p and I _s increase about 30 and 4 % in the present of biased (V _bias  = +200v) while they don’t change remarkability in another voltage. The power spectrum of Γ _r decreases about 15 and 10 % while the power spectrum of Γ _p increases about 80 and 20 % after positive biasing (V _bias  = +200v, V _bias  = +320v). Consequently, a better confinement obtained for biased with V _bias  = +200v. It means that, the magnitude of biased is important factor in modifying and control plasma turbulence. Also, results were compared with the effects of external resonant magnetic field.     

2-D temperature distribution and heat flux of PFC in 2011 KSTAR campaign

KSTAR has reached a plasma current up to 630 kA, plasma duration up to 12 s, and has achieved high confinement mode (H-mode) in 2011 campaign. The heat flux of PFC tile was estimated from the temperature increase of PFC since 2010. The heat flux of PFC tiles increases significantly with higher plasma current and longer pulse duration. The time-averaged heat flux of shots in 2010 campaign (with 3 s pulse durations and I_p of 611 kA) is 0.01 MW/m² while that in 2011 campaign (with 12 s pulse duration and I_p of 630 kA) is about 0.02 MW/m². The heat flux at divertor is 1.4–2 times higher than that at inboard limiter or passive stabilizer. With the cryopump operation, the heat flux at the central divertor is higher than that without cryopump. The heat flux at divertor is proportional to, of course, the duration of H-mode. Furthermore, a software tool, which visualizes the 2D temperature distribution of PFC tile and estimates the heat flux in real time, is developed.     

Experimental Verification of Modified Lee’s Model Using a Newly Designed and Constructed Filippov-Type Plasma Focus Device

A new 20 kJ Filippov-type plasma focus device has been designed and constructed in Isfahan University. The paper reports on the design and construction of the Iranian Filippov-type plasma focus device (UIPFF1) using modified Lee’s model. A Rogowski coil has been used to measure the experimental discharge current. Equivalent electric circuit of the device is RLC circuit; therefore the discharge current has a sinusoidal shape which its amplitude decreases exponentially during the time. The current signal contains a set of data from physical processes in the device as well as discharge current characteristics. In a typical discharge experiment these values were obtained: the discharge current was 181 kA, period of current signal 7.9 µs, the total inductance of the device 132 nH and electrical resistance of the circuit 77 mΩ. By averaging from data obtained with a set of five experiments the calibration factor was obtained 121 kA/V. Temporal changes in plasma focus discharge current, confirmed the occurrence of pinch at a specific pressure of argon, neon and nitrogen gases. UIPFF1 has been tested between 15 and 25 kV and wide range of pressure for various gases. Experiments at various pressures and voltages have also confirmed reproducibility and stability of the plasma focus device.     

Soft X-ray Emission Optimization Studies with Krypton and Xenon Gases in Plasma Focus Using Lee Model

The X-ray emission properties of krypton and xenon plasmas are numerically investigated using corona plasma equilibrium model. Numerical experiments have been investigated on various low energy plasma focus devices with Kr and Xe filling gases using Lee model. The Lee model was applied to characterize and to find the optimum combination of soft X-ray yields (Y_sxr) for krypton (~4 Å) and xenon (~3 Å) plasma focus. These combinations give Y_sxr = 0.018 J for krypton, and Y_sxr = 0.5 J for xenon. Scaling laws on Kr and Xe soft X-ray yields, in terms of storage energies E₀, peak discharge current I_peak and focus pinch current I_pinch were found over the range from 2.8 to 900 kJ. Soft X-ray yields scaling laws in terms of storage energies were found to be as $ {\text{Y}}_{{{\text{sxr}},{\text{Kr}}}} = 0.0003 \times {\text{E}}_{0}^{1.43} $ Y sxr , Kr = 0.0003 × E 0 1.43 and $ {\text{Y}}_{{{\text{sxr}},{\text{Xe}}}} = 0.0064 \times {\text{E}}_{0}^{1.41} $ Y sxr , Xe = 0.0064 × E 0 1.41 for Kr and Xe, respectively, (E₀ in kJ and Y_sxr in J) with the scaling showing gradual deterioration as E₀ rises over the range. The maximum soft X-ray yields are found to be about 0.5 and 27 J from krypton and xenon, respectively, for storage energy of 900 kJ. The optimum efficiencies for soft X-ray yields (0.0002 % for Kr) and (0.0047 % for Xe) are with capacitor bank energies of 67.5 and 225 kJ, respectively.     

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.     

Characterization of Short Intense Pulsed Electrothermal Plasma Capillaries for Use as Fusion and Launchers Heat Flux Sources

Electrothermal plasma sources operating in the confined capillary arc regime are characterized by the magnitude and shape of the discharge current. The desired plasma parameters at the source exit, especially the pressure and heat flux, are highly dependent on the arc due to the effect of the arc radiant energy that ablates the inner wall of the source. These sources have applications in fusion as drivers for pellet injectors and as high heat flux sources for fusion materials studies. The high-pressure high heat flux flow is also of application in mass accelerators and launch technology systems. The 1-D, time-dependent ETFLOW capillary code models the plasma generation and flow inside the capillary discharges and determines the plasma parameters. The input file to the code is the discharge current density providing the Joule heating in the energy equation. A circuit module has been developed and incorporated in the code to generate desired current shapes and magnitudes. The current pulse length was varied between 5 and 100 μs at constant amplitude of 50 kA, and then the pulse amplitude was varied between 10 and 200 kA at a constant pulse length of 20 μs. Increasing the pulse length while maintaining its amplitude increases the plasma density and the total ablated mass, which have accumulation behavior by increasing the pulse length, and subsequently increases the exit pressure from 60 to 410 MPa in the cases studied herein. The pressure increase allows the thermalization of the plasma particles through more collisions, which reduces the plasma temperature by about 0.2 eV. The bulk velocity follows the trend of the plasma temperature, but at shorter pulse lengths the total ablated mass is lower and enables the plasma to carry the particles with increasing velocity. Increasing the pulse amplitude up to 200 kA increases the density to about 18 kg/m³ and the bulk velocity, which varies between 6.1 and 10.7 km/s. A sharp increase in most plasma parameters occurs as a result of the increase in the pulse amplitude.     

Analytical Study of Quantum Magnetic and Ion Viscous Effects on p11B Fusion in Plasma Focus Devices

In this article we studied the feasibility of proton-boron (p¹¹B) fusion in plasmoids produced by plasma pinch devices like plasma focus facility as commercially sources of energy. In plasmoids fusion power for 76 keV < T_i < 1,500 keV exceeds bremsstrahlung loss (W/P_b = 5.39). In such situation gain factor and the ratio of Te to Ti for a typical 150 kJ plasma focus will be 7.8 and 4.8 respectively. Also with considering the ion viscous heating effect W/P_b and T_i/T_e will be 2.7 and 6 respectively. Strong magnetic field will reduces ion–electron collision rate due to quantization of electron orbits. While approximately there is no change in electron–ion collision rate, The effect of quantum magnetic field makes ions much hotter than electrons which enhances the fraction of fusion power to bremsstrahlung loss.     

Experimental Measurement of Asymmetric Fluctuations of Poloidal Magnetic Field in Damavand Tokomak at Different Plasma Currents

Toroidal and Poloidal magnetic fields have an important effect on the tokomak topology. Damavand Tokomak is a small size tokomak characterized with k?=?1.2, B _t?=?1T, R ₀?=?36?cm, maximum plasma current is about 35?KA with a discharge time of 21?ms. In this experimental work, the variation of poloidal magnetic field on the torodial cross section is measured and analyzed. In order to measure the polodial magnetic field, 18 probes were installed on the edge of tokomak plasma with ?θ?=?18°, while a limiter was installed inside the torus. Plasma current, I _p, induces a polodial magnetic field, B _p, smaller than the torodial magnetic field B _t. Magnetic lines B produced as a combination of B _t and B _p, are localized on the nested toroidal magnetic surfaces. The presence of polodial magnetic field is necessary for particles confinement. Mirnov oscillations are the fluctuations of polodial magnetic field, detected by magnetic probes. Disrupted instability in Tokomak typically starts with mirnov oscillations which appear as fluctuations of polodial magnetic field and is detected by magnetic probes. Minor disruptions inside the plasma can contain principal magnetic islands and their satellites can cause the annihilation of plasma confinement. Production of thin layer of turbulent magnetic field lines cause minor disruption. Magnetic limiter may cause the deformation of symmetric equilibrium configuration and chaotic magnetic islands reveal in plasma occurring in thin region of chaotic field lines close to their separatrix. The width of this chaotic layer in the right side of poloidal profile of Damavand Tokomak is smaller than the width in the left side profile because of Shafranov displacement. Ergodic region in the left side of profile develops a perturbation on the magnetic polodial field lines, B _p, that are greater in magnitude than that in the right side, although the values of B _p on the left side are smaller than that on the right side of the profile. The Left side of profile is close to the principal magnetic axis and the right side is away from Z axis of Tokamak.     

The Formation of a Tokamak-like Plasma in Initial Experiments Using an Outboard Plasma Gun Current Source

Solenoid-free tokamak startup via point-source DC helicity injection is demonstrated on the Pegasus Toroidal Experiment using a high current density, low impurity plasma gun mounted near the outboard midplane. A threshold in the vacuum vertical magnetic field strength that allows the injected current filament to relax into a tokamak-like topology is observed. A simple 2-D model of the vacuum magnetic field suggests this threshold is the maximum field strength that allows a toroidally connected field null to form. Discharges with I _p ≈ 17 kA are produced using less than 2 kA of injected current and no inductive drive. The tokamak-like discharges exhibit current decay times about five times longer than the injected current decay, expansion of the plasma into the vacuum region and a significant increase in the line-integrated density.     

Calculation of Internal Inductance and Poloidal Beta Using Solovev’s Assumption in a Circular Cross Section Tokamak

We calculated the internal inductance and the poloidal beta in a circular cross section Tokamak using Solov’ev assumption in the solution of Grad–Shafranov equation (GSE). GSE is solved by considering linear source functions and fixed boundary conditions. This solution has the three quantities (plasma current I _p , plasma minor radius r _p and $ \beta_{p} + l_{i} /2 $ ) that they are as input data. In two different discharges on IR-T1 Tokamak with this solution, we have shown that the internal inductance at a low value is required to extend the duration of Tokamak plasma discharge that it is consistence with other method.     

Initial Experiments at High Normalized Current in the Pegasus Toroidal Experiment

Initial experiments in the Pegasus ST exhibited an operational limit of I _p ∼ I _tf at A ∼ 1.15. With new power supplies, discharge control has greatly increased. Equilibria and stability modeling of the high I _p/I _tf regime has produced stable equilibria with I _p/I _tf up to 2 while values of I _p/I _tf approaching 3 show a potential upper bound on stability. A new operating regime has been accessed in recent experiments with the addition of electrostatic plasma sources. Using the sources as a pre-ionization technique, I _p/I _tf values of 1.5 have been accessed at very low toroidal fields. Using the sources as a means for non-inductive startup, values of I _p/I _tf up to 2.3 are attained. The MHD activity in these discharges is characteristically different than ohmic discharges, suggesting the new current sources are modifying the current profile to allow stable discharge evolution at minimal toroidal field.     

Pinch Current and Soft X-Ray Yield Limitations by Numerical Experiments on Nitrogen Plasma Focus

The modified version of the Lee model code RADPF5-15a is used to run numerical experiments with nitrogen gas, for optimizing the nitrogen soft X-ray yield on PF-SY1. The static inductance L ₀ 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 L ₀ below which although the peak total current, I _peak, continues to increase progressively with progressively reduced inductance L ₀, the I _pinch and consequently the soft X-ray yield, Ysxr, of that plasma focus would not increase, but instead decreases. For the PF-SY1 with capacitance of 25 μF, the optimum L ₀ = 5 nH, at which I _pinch = 254 kA, Ysxr = 5 J; reducing L ₀ further increases neither I _pinch nor nitrogen Ysxr. The obtained results indicate that reducing the present L ₀ of the PF-SY1 device will increase the nitrogen soft X-ray yield.     

Characterization of dose delivery in a hard X-ray irradiation facility

The Radiation Bioengineering Laboratory at Seoul National University (SNU) operates a user-constructed hard X-ray irradiation facility for radiation biology and radiation therapy physics studies. The system package of YXLON model 450-D08 operating at the anode voltage of up to 450 kV is a key part of the facility, which enables in vitro cell irradiation and animal irradiation for in vivo studies. In this article, dose delivery in the hard X-ray irradiation facility was characterized in terms of the dose vs. operational parametric combination of the facility. The operational parameters included beam tube anode voltage, beam tube current, irradiation time, and beam exit-to-sample distance. Bremsstrahlung X-rays at energy below approximately 20 keV were filtered out by a 3 mm-thick aluminum plate fitted over the 5 mm-thick beryllium window. Gafchromic EBT films were used as radiation sensor materials in dose measurement. The characterization was validated via experimental observation of the in vitro biological responses of cells to radiation exposure. The biological responses obtained using the new hard X-ray irradiator were highly comparable with those obtained using a commercial gamma-ray irradiator.