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.     

Measurement of Plasma Energy Confinement Time in Presence of Resonant Helical Field in IR-T1 Tokamak

Plasma energy confinement time is one of the main parameters of tokamak plasma and Lawson criterion. In this paper we present an experimental method especially based on diamagnetic loop (toroidal flux loop) for measurement of this parameter in presence of resonance helical field (RHF) in IR-T1 tokamak. For this purpose a diamagnetic loop with its compensation coil constructed and installed on outer surface of the IR-T1. Also in this work we measured the plasma current and plasma voltage from the Rogowski coil and poloidal flux loop measurements. Measurement results of plasma energy confinement time with and without RHF (L = 2, L = 3, L = 2 & 3) show that the addition of a relatively small amount of RHF could be effective for improving the quality of tokamak plasma discharge by flatting the plasma current and increasing the energy confinement time.     

Plasma Start-up in HIT-II and NSTX Using Transient Coaxial Helicity Injection

The method of transient coaxial helicity injection (CHI) has previously been used in the HIT-II experiment at the University of Washington to produce 100 kA of closed flux current. The generation of the plasma current by CHI involves the process of magnetic reconnection, which has been experimentally controlled in the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory to allow this potentially unstable phenomenon to reorganize the magnetic field lines to form closed, nested magnetic surfaces carrying a plasma current up to 160 kA. This is a world record for non-inductive closed-flux current generation, and demonstrates the high current capability of this method.     

Magnetic Reynolds Number and Neon Current Sheet Structure in the Axial Phase of a Plasma Focus

The Magnetic Reynolds Number (MRN) in neon is computed as a function of Neon shock speed. The magnetic field profiles at various positions in the axial run down phase of the INTI Plasma Focus device are measured over a range of pressures from 2 to 20 Torr. These profiles are assessed for good electromagnetic coupling including measuring the current per unit current sheet thickness as a comparative measure of current sheet diffusion. It was found that at an axial current sheet speed of over 3.5 cm/μs (corresponding to MRN > 15), the current sheet has a compact profile with current density of 55 kA/cm of sheet thickness whereas at speeds below 2.8 cm/μs (corresponding to MRN < 10) the profile is more diffuse with current density less than 30 kA/cm of sheet thickness. Based on these studies it is proposed to take a speed of 3 cm/μs corresponding to an MRN of 10 as the minimum speed of neon current sheet below which the electromagnetic coupling begins to weaken.     

Non-inductive Production of ST Plasmas with Washer Gun Sources on the Pegasus Toroidal Experiment

Formation of tokamak-like plasmas via electrostatic helicity injection in the ultra-low aspect ratio Pegasus Toroidal Experiment is reported. Two low-impurity, high-current (1 kA) washer gun current sources have been installed in the lower divertor region. These initially drive current along helical field lines produced by the applied toroidal and vertical fields. At sufficiently low values of externally applied vertical field, the poloidal field generated by the plasma is large enough to cause a poloidal flux reversal. In these cases the plasma relaxes into a tokamak-like configuration. Discharges with I _ϕ≈ 30 kA are produced with less than 2 kA of injected current. These discharges exhibit features indicative of tokamak plasmas, including reversal of poloidal flux at the center column, strong vacuum field deformation, increased current decay times, increased core heating, and characteristic MHD modes common to other helicity-injection-driven toroidal devices.     

Initial plasma start-up using partial solenoid coils in Versatile Experiment Spherical Torus (VEST)

Initial plasma start-up experiments based on ohmic discharge using partial solenoid coils located at both vertical ends of a center stack have been carried out in Versatile Experiment Spherical Torus (VEST) at Seoul National University. Ohmic discharges with the help of microwave pre-ionization have been performed according to the pre-programed start-up scenario which was experimentally verified by a series of vacuum field measurements using an internal magnetic probe array. A plasma current of around 0.4 kA has been achieved by ohmic discharge using partial solenoid coils, under the toroidal magnetic field of 0.1 T. The vacuum field calculation and fast camera image have revealed that the small plasma current even with significant amount of loop voltage up to 9.7 V is attributed to the imbalance of poloidal field for equilibrium. Modification of the start-up scenario and upgrade of power supplies are proposed to be carried out in order to achieve higher plasma current in the future experiments.     

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.     

Plasma Internal Inductance in the Presence of External Resonant Fields in IR-T1 Tokamak

In this paper we presented experimental investigation of effects of local limiter biasing (V_biasing = +200 v, V_biasing = +320 v) on the plasma parameters as plasma current, loop voltage, poloidal beta, plasma pressure, plasma energy, plasma resistance, plasma temperature, plasma displacement, Shafranov parameter and plasma internal inductance in IR-T1 tokamak. For these purposes, array of magnetic probes and also a diamagnetic loop have been used. The results show that applied biased voltage V_biasing = +200 v causes to decrease of about 40 % in plasma internal inductance. The plasma resistance and the plasma displacement have been decreased by V_biasing = +200 v. The main result of the application of V_biasing = +200 v is flatting the plasma parameters profiles. In other words, the addition of biasing voltage V_biasing = +200 v to plasma could be effective for improving the quality of tokamak plasma discharge by creating the steady state plasma. The plasma current, plasma pressure, plasma energy, plasma temperature and shift parameter have increased after the application of limiter biasing with V_biasing = +320 v but they decrease rapidly.     

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).     

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.     

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.     

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.     

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.     

Formation of Low Aspect Ratio Torus Equilibria by ECH in the LATE Device

By ECH under a steady By field, a closed field equilibrium of a low aspect ratio as low as R/a = 1.4 is spontaneously formed in the LATE device. After the spontaneous formation, the plasma current has increased further up to Ip = 7.2 kA by 2.45 GHz, 30 kW and Ip = 11 kA by 5 GHz, 120 kW, by increasing the microwave power with a slow ramp of By for the equilibrium of the plasma loop at larger currents. Both amount to 12% of the total toroidai coil current. ECH/ECCD at 2nd harmonic resonance of EBW supports the plasma. An outline of the theoretical considerations for the formation process is presented.     

Numerical Experiments on Radiative Cooling and Collapse in Plasma Focus Operated in Krypton

The Plasma Focus has wide-ranging applications due to its intense radiation of SXR, XR, electron and ion beams and fusion neutrons when operated in deuterium. The 5-phase Lee Model code has been developed for the focus operated in various gases including D, D–T, He, Ne, N, O, Ar, Kr and Xe. Radiation-coupled motion is included in the modelling. In this paper we look at the effect of radiation cooling and radiation collapse in krypton. The Pease–Braginskii current is that current flowing in a hydrogen pinch which is just large enough for the Bremsstrahlung to balance Joule heating. This radiation-cooled threshold current for a hydrogen pinch is 1.6 MA. It is known that in gases undergoing line radiation strongly the radiation-cooled threshold current is considerably lowered. We show that the equations of the Lee Model code may be used to compute this lowering. The code also shows the effect of radiation cooling leading to radiative collapse. Numerical experiments based on experimentally fitted model parameters are run to demonstrate a regime in which radiation collapse is observed in Kr at a pinch current of 50–100 kA.     

Flow Characteristics and Charge Exchange Effects in a Two-Dimensional Model of Electrothermal Plasma Discharges

Electrothermal (ET) plasma discharges are capillary discharges that ablate liner materials and form partially ionized plasma. ET plasma discharges are generated by driving current pulses through a capillary source with peak currents on the order of tens of kA and pulse lengths on the order of \(100\,\upmu \hbox {s}\). These plasma discharges can be used to propel pellets into magnetic confinement fusion devices for deep fueling of the fusion reaction, ELM mitigation, and thermal quench of the fusion plasma. ET plasma discharges have been studied using 0D, 1D, and semi-2D fluid models. In this work, a fully 2D model of ET plasma discharges is presented. The newly developed model and code resolve inter-species interaction forces due to elastic collisions. These forces affect the plasma flow field in the source and impede the development of plasma pressure at the exit of the source. In this work, these affects are observed for discharge current pulses peaking at 10 and 20 kA. The sensitivity of the model to the inclusion of charge exchange effects is observed. The inclusion of charge exchange has little effect on the integrated, global results of the simulation. The difference in total ablated mass for the simulations caused by the inclusion of charge exchange reactions is <1 %. Differences in local plasma parameters are observed during discharge initialization, but after initialization, these differences diminish. The physical reasoning for this is discussed and recommendations are made for future modeling efforts.     

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.     

Design,simulation and construction of the Taban tokamak

This paper describes the design and construction of the Taban tokamak, which is located in Amirkabir University of Technology, Tehran, Iran. The Taban tokamak was designed for plasma investigation. The design, simulation and construction of essential parts of the Taban tokamak such as the toroidal field(TF) system, ohmic heating(OH) system and equilibrium field system and their power supplies are presented. For the Taban tokamak, the toroidal magnetic coil was designed to produce a maximum field of 0.7 T at R?=?0.45 m. The power supply of the TF was a130 kJ, 0–10 kV capacitor bank. Ripples of toroidal magnetic field at the plasma edge and plasma center are 0.2% and 0.014%, respectively. For the OH system with 3 kA current, the stray field in the plasma region is less than 40 G over 80% of the plasma volume. The power supply of the OH system consists of two stages, as follows. The fast bank stage is a 120 μF, 0–5 k V capacitor that produces 2.5 kA in 400 μs and the slow bank stage is 93 mF, 600 V that can produce a maximum of 3 kA. The equilibrium system can produce uniform magnetic field at plasma volume. This system's power supply, like the OH system, consists of two stages, so that the fast bank stage is 500 μF, 800 V and the slow bank stage is 110 mF, 200 V.     

Ultra Fast Shutter Driven by Pulsed High Current

Radiation simulation utilizing plasma radiation sources (PRS) generates a large number of undesirable debris, which may damage the expensive diagnosing detectors. An ultra fast shutter (UFS) driven by pulsed high current can erect a physical barrier to the slowly moving debris after allowing the passage of X-ray photons. The UFS consists of a pair of thin metal foils twisting the parallel axes in a Nylon cassette, compressed with an outer magnetic field, generated from a fast capacitor bank, discharging into a single turn loop. A typical capacitor bank is of 7.5 μF charging voltages varying from 30 kV to 45 kV, with corresponding currents of approximately 90kA to140 kA and discharging current periods of approximately 13.1 μs. A shutter closing time as fast as 38 microseconds has been obtained with an aluminium foil thickness of 100 micrometers and a cross-sectional area of 15 mm by 20 mm. The design, construction and the expressions of the valve-closing time of the UFS are presented along with the measured results of valve-closing velocities.     

High Performance High Repetition Rate Miniature Plasma Focus Device: Record Time Averaged Neutron Yield at 200 J with Enhanced Reproducibility

A high performance high repetition rate plasma focus device with significant time averaged neutron yield and greater shot to shot reproducibility could be a highly valuable neutron source for Homeland Security applications. The single module fast miniature plasma focus ‘FMPF-2’ (2.4 μF, 56 ± 3 nH, 89 kA @ 14.0 kV, T/4~575 ns) was upgraded to four module ‘FMPF-3’ (2.4 μF, 34 ± 2 nH, 103 kA @ 14.0 kV, T/4~458 ns) device. The time averaged neutron output of (1.4 ± 0.6) × 10⁶ n/sec at 1 Hz operation was enhanced to the record value of (1.4 ± 0.2) × 10⁷ n/sec at 10 Hz operation for the burst length of 50 consecutive shots for deuterium filling gas pressure of 5.5 mbar in FMPF-3 device at storage energy of ~200 J. Other key findings of the investigation were: (i) the deuterium filling gas pressure for single shot and repetitive modes of device operation were needed to be optimized separately, and (ii) in the repetitive mode of operation the deuterium filling gas pressure was higher than that obtained for single shot mode and also yielded better reproducibility in neutron emission.