A new model of the L–H transition and H-mode power threshold

In order to understand the mechanism of the confinement bifurcation and H-mode power threshold in magnetically confined plasma,a new dynamical model of the L–H transition based on edge instability phase transition(EIPT) has been developed.With the typical plasma parameters of the EAST tokamak,the self-consistent turbulence growth rate is analyzed using the simplest case of pressure-driven ballooning-type instability,which indicates that the L–H transition can be caused by the stabilization of the edge instability through EIPT.The weak E?×?B flow shear in L-mode is able to increase the ion inertia of the electrostatic motion by increasing the radial wave number of the tilted turbulence structures,which play an important role for accelerating the trigger process of EIPT rather than directly to suppress the turbulent transport.With the acceleration mechanism of E?×?B flow shear,fast L–H and H–L transitions are demonstrated under the control of the input heating power.Due to the simplified scrape-offlayer boundary condition applied,the ratio between the heating powers at the H–L and L–H transition respectively differs from the ratio by Nusselt number.The results of the modeling reveal a scaling of the power threshold of the L–H transition,P_(L-H)?∝?n~(0.76) B~(0.8) for deuterium plasma.It is found finite Larmor radius induces an isotope effect of the H-mode power threshold.     

Algorithm Validation of the Current Profile Reconstruction of EAST Based on Polarimeter/Interferometer

The method of plasma current profile reconstruction using the polarimeter/interferometer(POINT) data from a simulated equilibrium is explored and validated.It is shown that the safety factor(q) profile can be generally reconstructed from the external magnetic and POINT data.The reconstructed q profile is found to reasonably agree with the initial equilibriums.Comparisons of reconstructed q and density profiles using the magnetic data and the POINT data with 3%,5%and 10%random errors are investigated.The result shows that the POINT data could be used to a reasonably accurate determination of the q profile.     

Study of Inverted Sawtooth Activities in EAST Plasma

On the EAST tokamak, during whole off-axis LHCD （low hybrid current drive） limiter discharge, inverted sawteeth oscillations on the SXR （soft X-ray camera） signals appear continuously, and no positive sawtooth is observed. It is thought that this phenomenon is caused by the curvature pinch, though it could partly be explained by the electron temperature profile observed on the PHA （soft X-ray pulse height analyzer） system. The off-axis LHCD and Ohmic heating generate a non-monotonic q profile. According to the curvature pinch effect, this q profile leads to a special electron density profile that has a valley ring. The non-monotonic q profile and the special electron density profile lead to this interesting phenomenon.     

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.     

Study on divertor heat flux under n = 3 and n = 4 resonant magnetic perturbations using infrared thermography diagnostic in EAST

Resonant magnetic perturbations (RMPs) with high toroidal mode number n are considered for controlling edge-localized modes (ELMs) and divertor heat flux in future ITER H-mode operations. In this paper, characteristics of divertor heat flux under high-n RMPs (n = 3 and 4) in H-mode plasma are investigated using newly upgraded infrared thermography diagnostic in EAST. Additional splitting strike point (SSP) accompanying with ELM suppression is observed under both RMPs with n = 3 and n = 4, the SSP in heat flux profile agrees qualitatively with the modeled magnetic footprint. Although RMPs suppress ELMs, they increase the stationary heat flux during ELM suppression. The dependence of heat flux on ${q}_{95}$ during ELM suppression is preliminarily investigated, and further splitting in the original strike point is observed at ${q}_{95}=4$ during ELM suppression. In terms of ELM pulses, the presence of RMPs shows little influence on transient heat flux distribution.     

On the Physics of Improved Confinement During Pulsed Poloidal Current Drive in MST Reversed-Field Pinch

Reduction of core-resonant magnetic fluctuations and improved confinement in the Madison Symmetric Torus reversed-field pinch (MST RFP) have been routinely achieved by applying the surface poloidal electric field. The created inductive poloidal electric field drives current in plasma which leads to the improved confinement. To study the effect we developed a relatively simple 1-D model in cylindrical geometry which assumes poloidal and axial symmetry during the drive. We use resistive magnetohydrodynamics model with realistic plasma parameters and assume that there is a vacuum gap between plasma boundary and conducting wall of the vessel. Evolution of plasma density is taken into account and plasma boundary moves self-consistently with momentum equation. We start from an initial unstable equilibrium and examine stability of plasma configuration at intermediate moments of time during the drive. For this we calculate the growth rates of unstable eigenmodes in the plasma. Our results show that the modifications to the plasma current profile during the drive are stabilizing. The initial stabilization is due to the direct modification of the current profile near the edge. It enhances later in time due to the flattening of λ profile in the core region as plasma and magnetic field compress inward during the drive.     

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.     

Experimental observation of reverse- sheared Alfvén eigenmodes (RSAEs) in ELMy H-mode plasma on the EAST tokamak

Reverse-sheared Alfvén eigenmodes (RSAEs) have been observed by using an interferometer and ECE diagnostics in NBI heated ELMy H-mode plasma on EAST tokamak. A typical feature of these modes is a fast frequency sweeping upward from ∼80 kHz to ∼110 kHz in hundred milliseconds during which the plasma temperature, density and rotation keeps no change. Only core channels of the interferometer can observe these modes, implying a core localized mode. The ECE measurement further showed that these modes located at about ρ=0.37–0.46, just around the position of q_min with ρ∼0.4. These core localized modes are very weak in the magnetic fluctuations measured by mirnov probes mounted at the machine vacuum vessel. A multiple frequency fluctuation component, seemingly the so-called ‘grand cascades’, was also clearly observed on the ECE signal at ρ=0.46. During the phase, a transient internal transport barrier (ITB) in ion temperature and toroidal rotation was observed and the ITB foot was just close to the position of q_min . A modulation of RSAE frequency by ELM event was observed and this modulation could be attributed to rotation decrease orq_min increase due to ELM. Further study of these modes in EAST can provide valuable constraints for the q profile measurement and will be important for the long pulse operation.     

Initial Plasma Startup Test on SUNIST Spherical Tokamak

The goal of the Sino-United Spherical Tokamak (SUNIST) at Tsinghua University is to extend the understanding of toroidal plasma physics at a low aspect ratio (R/a ≈ 1.3) and to demonstrate a maintainable target plasma by non-inductive startup. The SUNIST device isdesigned to operate with up to 13 kA of ohmic heating field current, and to 0.15 T of toroidal field at 10 kA of discharge current. All of the poloidal fields can provide 30 mVs of Volt-seconds transformer. Experimental results of plasma startup show that SUNIST has remarkable characteristics of high ramp rate (dIp/dt ≈ 50 MA/s ), high normalized current IN of about 2.8 (IN = Ip/αBT)，and high-efficiency (Ip/IROD ≈ 0.4) production of plasma current while operating at a low toroidal field. Major disruption phenomena have not been observed from magnetic diagnostics of all testing shots. Initial discharges with 52 kA of plasma current (exceeding the designed value of 50 kA),2 ms of pulse length and 50 MA/s of ramp rate have been achieved easily with pre-ionized filament.     

Preliminary Study of the Magnetic Perturbation Effects on the Edge Density Profiles and Fluctuations Using Reflectometers on EAST

The resonant magnetic perturbation(RMP) coils have been successfully designed and installed on the Experimental Advanced Superconducting Tokamak(EAST).Using the reflectometer systems,the density profile and the density fluctuations during magnetic perturbations(MPs) phase have been investigated.During the experiments,two different cases are studied separately:steady MPs and rotating MPs.In both cases,a strongly density pump-out has been observed.In the steady MPs cases,an enhancement of the low frequency(60 kHz)density fluctuations in H-mode phase has been observed.The plasma density boundary out-shifts~ 5%caused by the MPs.The pedestal density gradient is reduced by 50%,while the radial location nearly stays unchanged.In the rotating MPs,the line-averaged density,the D_α emission at the divertor region and the spectrum of the density fluctuations are modulated.The results suggest that the low frequency(60 kHz) density fluctuations may contribute to the strong density pump-out.     

Experimental study of double tearing mode on EAST tokamak

The double tearing mode(DTM)in a high βN(βN>1.5)discharge with internal transport barrier on EAST was investigated.A 15 kHz tearing mode(TM)(m≥3,n=3)appears at outer q=2 surface at first,which is stable in the highβN phase.Then a 2 kHz TM(m=2,n=1)occurs at inner q=2 surface.Soon after,high βN collapsed with the crash of ELM,and DTM formed during the collapse of βN.The positions of the two islands of the DTM are consistent with the q=2 surface.The temperature fluctuations are the strongest between the two magnetic islands.A statistical analysis of high βN discharges operating with the reversed magnetic shear configuration in the EAST 2015-2018 campaign revealed the existence of the DTM in many discharges.During the DTM phase,all βN are reduced by 10%-30% within 0.1 s.However,there are two different βN behaviors thereafter-with and without βN recovery.Studying the physical mechanism of βN recovery during the DTM phase will benefit steady-state operation with reversed shear configurations in the future.     

Compound Sawteeth and Analysis of Reversed Shear in HL-1M

1.IntroductionUsually,thesawtoothactivityissignificantlyaffectedbylocalizedchangesofplasmaparameters,particularlycurrentandtemperatureprofiles.Asinglesawtooth,forexample,canbesubstitutedbyacompoundsawtoothascurrentprofileevolvesasahollowedform,i.e.twoq=lsurfaces(whereqissafetyfactor)presentintheplasmacoreregion.Becausethecurrentprofilemaintainsthecharacteristicpo'akedshapeundertheconditionOfsinglesawtooth,thecompoundsawteethusuallystartasthetwoq=1surfaCesappear.Duringtherampofsawtooththecurre…     

The Essential Requirements of Transition to Non-equilibrium Burn Stage of DD Fuel in Simple Spherical Targets

In this research, the transition from equilibrium ignition to non-equilibrium burn is evaluated by calculating the energy balance equations analytically for targets which consist of inner DD fuel and surrounded by a high-Z pusher. It is expected that these targets can trap much of the produced charged particles, radiation or even fast neutrons because of their high-Z pusher. Accordingly, DD fuel can be ignited in volume ignition regime with low ignition temperatures of 35 keV compared to central ignition. Thus, to get a non-equilibrium burning stage, we have examined all the important gain and loss processes for these targets as the energy deposition of fusion products, thermal conduction, radiation flux, mechanical work, bremsstrahlung radiation and inverse Compton scattering as well as competition among them. These conditions have investigated for different areal densities of DD fuel in ρR?~?1–100 g/cm² and it is shown that as areal density rises, transition temperature decreases. But at high areal densities, the transition temperature does not vary significantly and the limiting temperature of ~?20 keV will be obtained. Also, transition into non-equilibrium burn is studied for such cases that thermonuclear burn occurs at stagnation moment, before and after that. It is observed that the positive and negative role of mechanical work on the transition conditions is very important and varies transition temperature remarkably. In all cases, transition temperature to non-equilibrium burn phase is always much lower than ideal ignition temperature in specific areal density.     

Measurement of Current Profile in a Tokamak Through AC Modulation

The plasma current is modulated with an alternating current （ac） component in a frequency range of 90 Hz - 900 Hz in the plateau discharge phase in the CT-6B tokamak. A plasma electric conductivity profile in a form of （1 - r^2/a^2）^α with a parameter α which is fitted with the experimental data, can be determined. The effects of magnetic shear in a tokamak field configuration on the current penetration are taken into account in the numerical simulation. The measurement method and obtained results are discussed.     

Characteristics of inductively coupled plasma(ICP) and helicon plasma in a singleloop antenna

Large area uniform plasma sources, such as high-density magnetized inductively coupled plasma(ICP) and helicon plasma, have broad applications in industry. A comprehensive comparison of ICP and helicon plasma, excited by a single-loop antenna, is presented in this paper from the perspectives of mode transition, hysteresis behavior, and density distribution. The E-H mode transition in ICP and the E-H-W mode transition in helicon plasma are clearly observed in the experiments. Besides, the considerable variation of hysteresis behavior from inverse hysteresis to normal hysteresis by the influence of the magnetic field is explored. The bi-Maxwellian and Maxwellian electron energy distribution functions in each discharge are used to explain this phenomenon, which is essentially related to the transition from a nonlocal kinetic property to a local kinetic property of electrons. In addition, we notice that the plasma density, in the radial direction, is peaked in the center of the tube in ICP, but a complicated distribution is formed in helicon plasma. In the axial direction, the maximum plasma density is still in the center of the antenna in ICP, whereas the highest plasma density is located downstream, far away from the antenna, in helicon plasma. It is believed that the reflected electrons in the sheath and pre-sheath by the upper metallic endplate and downstream propagated helicon wave will be responsible for this plasma density profile in helicon plasma. Due to the constrained electron motion in the magnetic field, an extremely uniform density distribution will be obtained with an appropriate axial magnetic field in the wave discharge mode.     

Simulation Study on the ITB Formation During LHCD in JT—60U

A transport simulation has been done by using a 1.5D time dependent transport code to reproduce a formation of the ITB on electron temperature profile during the long pulse LHCD in JT-60U tokamak.The transport coefficients were assumed to reduce with a reversed magnetic shear and the LH driven current profile was evaluated by fitting dynamic change in the measured current profile.The gradual increase in the central electron temperature could be explanined by the change in the current profile during LHCD in the present simulation model.THe estimated LH-driven current prefile by the transport code analysis shows a finite current density at the plasma center.Analysis of transport simulation suggests some mechanisms for broadening the LH-driven current profile at the central region of the plasma.     

Impact of core electron temperature on current profile broadening with radio-frequency wave heating and current drive in EAST

In recent EAST experiments, current profile broadening characterized by reduced internal inductance has been achieved by utilizing radio-frequency current drives (RFCD). In contrast to previous density scan experiments, which showed an outward shift of the current density profile of lower hybrid current drive (LHCD) in higher plasma density, the core electron temperature (Te(0)) is found to affect the LHCD current profile as well. According to equilibrium reconstruction, a significant increase in on-axis safety factor (q0) from 2.05 to 3.41 is observed by careful arrangement of RFCD. Simulations using ray-tracing code GENRAY and Fokker–Planck code CQL3D have been performed to thoroughly analyze the LHCD current profile, revealing the sensitivity of the LHCD current profile to Te(0). The LHCD current density tends to accumulate in the plasma core with higher current drive efficiency benefiting from higher Te(0). With a lower Te(0), the LHCD current profile broadens due to off-axis deposition of power density. The sensitivity of the power deposition and current profile of LHCD to Te(0) provides a promising way to effectively optimize current profile via control of the core electron temperature.     

Current Sheath Dynamics and its Evolution Studies in Sahand Filippov Type Plasma Focus

One of the most important factors for optimizing the plasma focus device operation is the dynamics of the plasma. In this paper, we investigated the profile and dynamics of the current sheath by measuring the velocity and distribution of current sheath in Sahand as a Filippov type plasma focus device. For this purpose, the discharge is produced in pure neon gas with capacitor bank stored energies in the range of 14–50 kJ. The current sheath is monitored using two sets of magnetic probes, one with four and other with three equi-distant probe coils. These probes, installed in both radial and axial directions near the edge of the interior electrode (anode), are used for monitoring the distributions and dynamics of the current sheath. The maximum current sheath velocities at radial and axial phase are 4 ± 0.13 and 3.51 ± 0.22 (cm/μs) respectively for 0.25 Torr. The decreasing of CS velocities in going move away from anode surface is one of the our results in this paper. In this paper we conclude that the current sheath velocity at radial phase in Sahand is greater than axial phase. The effect of the neon working gas pressure and working voltage on the current sheath dynamics and its spatial evolution is investigated and presented.     

Investigation on current loss of high-power vacuum transmission lines with coaxial-disk transitions by particle-in-cell simulations

Coaxial-disk transitions can generate non-uniform magnetic fields and abrupt impedance variations in magnetically insulated transmission lines(MITLs),resulting in disturbed electron flow and non-negligible current loss.In this paper,3D particle-in-cell simulations are conducted with UNPIC-3d to investigate the current loss mechanism and the influence of the input parameters of the coaxial-disk transition on current loss in an MITL system.The results reveal that the magnetic field non-uniformity causes major current loss in the MITL after the coaxial-disk transition,and the non-uniformity decreases with the distance away from the transition.The uniformity of the magnetic field is improved when increasing the number of feed lines of a linear transformer driver-based accelerator with coaxial-disk transitions.The number of input feed lines should be no less than four in the azimuthal distribution to obtain acceptable uniformity of the magnetic field.To make the ratio of the current loss to the total current of the accelerator less than 2％at peak anode current,the ratio of the current in each feed line to the total current should be no less than 8％.     

Magnetic Probe Measurements in INTI Plasma Focus to Determine Dependence of Axial Speed with Pressure in Neon

Current sheath dynamics generated in INTI plasma focus device operated with neon gas has been studied. A 3-turn Rogowski coil design has been used to measure derivative current. A new magnetic probe was designed and used to study of current sheath arrival time, current profile and velocity variation in the axial phase at different experimental conditions. The current sheath’s average velocity was found to vary with pressure^?0.51 with a R² value of 0.9 which agrees well with the theoretically expected variation of pressure^?0.5.