Unified Description of Tokamak Ideal MHD Instabilities（I）

By using a coordinate system associated with magnetic surfaces,a unified eigenmode equation for describing the tokamak ideal MHD instabilities is derived in the shear-Alfven approximation.Based on this equation having a general operator form,the eigen-mode equation governing the large-scale perturbation (such as the kink mode,the low-n ballooning mode and the Alfven mode) and small-scale perturbation(such as the high-n ballooning mode,the local mode) can be further deduced.In the first part of the present study,the small-scale perturbation is discussed in detail.     

Unified Description of Tokamak Ideal MHD Instabilities(Ⅰ)

By using a coordinate system associated with magnetic surfaces, a unified eigen-mode equation for describing the tokamak ideal MHD instabilities is derived in the shear-Alfvenapproximation. Based on this equation having a general operator form, the eigen-mode equationgoverning the large-scale perturbation (such as the kink mode, the low-n ballooning mode and theAlfven mode) and small-scale perturbation (such as the high-n ballooning mode, the local mode)can be further deduced. In the first part of the present study, the small-scale perturbation isdiscussed in detail.     

Dynamic stabilization of D—T burn in Tokamak reactors

A simple,engineeringly feasible dynamic method is supposed to control the deuterium-tritium burn process in Tokamak reactors operated in an advanced scenario.The thermal transport of the D-T plasma is described by an anomalous thermal conduction which is a radially increasing function and the central conduction value is proportional to the central temperature of the plasma.The dynamic external heating power is selected to be inversely proportional to certain power function of this temperature,As a result,the D-T burn can undergo in controllable way in different temperature regimes with different power output.Anomalous alpha particle transport effect is taken into account.It can affect the resultant plasma equilibrium ,the reactor efficency,the operation mode and so on.     

Influence of Superathermal Electrons on Central Plasma Relaxation oscillations during Localized Electron Cyclotron Heating on the HL—1M Tokamak

During initial studies of ECRH in the HL-1M tokamak,non-standard central MHD activities,such as saturated wawtooth,partially saturated sawtooth,double sawtooth,and the strong m=1 bursts have been observed while changing the heating location,the ECRH power.the plasma density,Complete suppression of sawtooth is achieved for the duration of the ECRH.when the heating power is applied on the high-fiedl side of low-density plasma,and exceeds a threshold value of power .The m=1 bursts riding on the ramp phase of sawtooth can only be excited when the ECRH locaxation activities are produced or suppressed are described.Experimental results imply that the energetic electrons generated during ECRH are responsible for the modification/or stabilization/or excitation of the instability.Near the q=1 surface,the passing electrons play the role of reducing the shear and tending to stabilize the sawtooth activity,while the barely-trapped electrons play the role of enhancing or driving an internal kink instability.     

Study of Plasma MHD Equilibrium in HL-2A Tokamak

EFIT（Equilibrium Fitting） code is successfully transplanted for HL-2A tokamak parameters. The evolution of the equilibrium configurations for shot 2898 is simulated. It is shown that the discharge starts with a limiter configuration and develops into a divertor configuration gradually after t = 200 ms. The latter lasts until the end of the discharge at t = 900 ms. The evolution of the major plasma shape parameters such as the boundary magnetic fluxes, the positions of the x-point and magnetic axis, and the minor radii obtained are calculated and compared with the experimental results. The agreement between the simulation and experiments are shown to be reasonable. The possibility for discharge quality improvement is discussed.     

A Research Program of Spherical Tokamak in China

The mission of this program is to explore the spherical torus plasma with a SUNIST spherical tokamak. Main experiments in the start phase will be involved with breakdown and plasma current set-up with a mode of saving volt-second and without ohmic heating system, equilibrium and instability, current driving, heating and profile modification. The SUNIST is a university-scale conceptual spherical tokamak, with R = 0.3 m, A 1.3, Ip -50 kA, BT < 0.15 T, and PRF- 100 kW. The only peculiarity of SUNIST is that there is a toroidal insulating break along the outer wall of vacuum vessel. The expected that advantages of this arrangement are helpful not only for saving flux swing, but also for having a deep understanding of what will influence the discharge startup and globe performances of plasma under different conditions of strong vessel eddy and ECR power assistance. Of course, the vessel structure of cross seal will be at a great risk of controlling vacuum quality, although we have achieved positive results on sim     

A Study of Energy Conversion Efficiency Versus Plasma Density by Lower Hybrid Current Drive in HT—7 Tokamak

Ramp-up experiments by means of lower hydrid wave on HT-7 superconducting tokamak have been performed and analyzed.A ramp-up rate of over 300kA/s is obtained and a conversion efficiency of over 10% has been achieved during the ramp-up phases.The study of the dependence of conversion efficiency on plasma density shows that the conversion efficiency is affected by the driven current,which is mainly dominated by the competition of impurity concentration with wave accessibility condition.In addition,the effect of current profile may play an important role in determining the conversion efficiency.     

Design of the Cryostat for HT—7U superconducting Tokamak

The cryostat of HT-7U tokamak is a large vacuum vessel surrounding the entire basic machine with a cylindrical shell,a dished top and a flat bottom.The main function of HT-7U cryostat is to provide a thermal barrier between an ambient temperature test hall and a liquid helium-cooled superconducting magnet.The loads applied to the cryostat are from sources of vacuum pressure,dead weight,seismic events and electromagnetic forces originated by eddy currents.It also provides feed-through penetrations for all the conecting elements inside and outside the cryostat.The main material selected for the cryostat is stainless steel 304L.The structural analyses including buckling for the cryostat vessel under the plasma operation condition have been carried out by using a finite element code.Stress analysis results show that the maximum stress intensity was below the allowable value.In this paper,the structural analyses and design of HT-7U cryostat are emphasized.     

Exraction of MHD Signal Based on Wavelet Transform

Mirnov signals mixed with interferences are a kind of non-stationary signal,It cannot obtain satisfactory effects to extract MHD signals from mirnov signals from mirnov signals by Fourier Transform.This paper suggests that the wavelet tranform can be used to treat mirnov signals.Theoretical analysis and experimental result have indicated that using the time-frequency analysis characteristics of the wvaelet transform to filter mirnov signals can remove effectively interferences and extract useful MHD signals.     

MHD Flow Layer Formation at Boundaries of Magnetic Islands in Tokamak Plasmas

Non-linear development of double tearing modes induced by electron viscosity is numerically simulated. MHD flow layers are demonstrated to merge in the development of the modes. The sheared flows are shown to lie just at the boundaries of the magnetic islands, and to have suffcient levels required for internal transport barrier （ITB） formation. Possible correlation between the layer formation and triggering of experimentally observed ITBs, preferentially formed in proximities of rational flux surfaces of low safety factors, is discussed.     

Excitation and Propagetion of Modified Fluctuation in a Toroidal Plasma in KT—5C Device

Understanding the propagation of the turbulent perturbation in the tokamak edge plasma is an important issue to actively modify or control the turbulence,reduce the anomalous transport and improve plasma confinement.To realize active modification of the edge perturbation,a high dynamic output,broad-band,low-cost power amplifier is set up,and used to drive the active probes in the experiments on KT-5C Tokmak.By using small-size magnetic probes together with Langmiur probes,It is observed that the modified perturbation by the active probes with sufficiently driving power may spread with electrostatic mode,and electromagnetic mode as well.     

A Calculation of Steady Pressure Drop and an Analysis of HT—7U CICC

Under the condition of steady,the pressure drop of coolant is mainly caused by friction along the cable.in the CICC(cable-in-conduit-conductor).helium flow within the conductor consists of two parallel interconnected tubes.The velocity distribution has some differece between the central channel and conductor space.The region of Reynolds number is from 10^3 to 10^6.This paper describes the calculation of pressure drop of HT-7U CICC at various mass flows.It is assumed that the coolant flows in two parallel,rough tudes during the caclulation.     

Toroidicity Dependence of Tokamak Edge Safety Factor and Shear

Using a simple analytical model equilibrium, the dependence of tokamak edge safetyf actor qedge which can be understood as q~ for a limiter device or q95 for a divertor device, and the shear value are calculated and compared with the ITR~recommended formula. This dependence relation is useful in designing the medium and small aspect ratio tokamaks and reactors.     

The Current Profile of Tokamaks in Minimum Dissipation States

The principle of minimum dissipation rate is applied to tokamak plasmas with energy and helicity balances imposed as two constraints.The analytical solution on toroidal current distrbution are derived from the resulting Euler-Lagrangian equation.Three typical forms of current distribution are derived from the resulting Euler-Lagrangian equation.Three typical forms of current profiles are found for low-aspect-ratio tokamake like NSTX.One of them decreases with r on equatorial plane,the second peaks in the inner half part on equatorial plane and the third may have a hole or reverse in the central part.     

Comparison Between Mitigation Effects of the Finite Larmor Radius and Sheared Axial Flow on Rayleigh—Taylor Instability in Z—Pinch Implosions

A magnetohydrodynamic (MHD) formulation is derived to investigate and compare the mitigation effects of both the sheared axial flow and finite Larmor radius (FLR) on the Rayleigh-Taylor (RT) instability in Z-pinch implosions. The sheared axial flow is introduced into MHD equations in a conventional way and the FLR effect into the equations via ?/?t → -i(w+ik⊥2pi2Ωi), as proposed in our previous paper [Chin. Phys. Lett. 2002, 19:217] , where k⊥2 pi2 is referred to FLR effect from the general kinetic theory of magnetized plasma. Therefore the linearized continuity and momentum equations for the perturbed mass-density and velocity include both the sheared axial flow and the FLR effect. It is found that the effect of sheared axial flow with a lower peak velocity can mitigate RT instability in the whole wavenumber region and the effect of sheared axial flow with a higher one can mitigate RT instability only in the large wavenumber region (for normalized wavenumber k>2.4); The effect of FLR can mitigate RT     

Construction of a 2kW／4K Helium Refrigerator for HT—7U

Superconducting magnets of toroidal field (TF) and poloidal field(PF) of HT-7U tokamak are all made of NbTi/Cu Cable-in-Conduit Conductor (ClCC),and cooled with a forced flow supercritical helium of 3.8K.A helium refrigerator with an equivalent capacity of 2kW/r K will be constructed.This paper presents the design of the helium refrigerator process.The thermodynamics of the refrigeration cycle and the refrigerator equipments.     

Experiment and Operation of a LHCD—35kV／2.8MW／1000s High—Voltage Power Supply on HT—7 Tokamak

A-35kV/2.8MW/1000s high-voltage power supply(HVPS) for HT-7 superconducting tokamak has been built successfully.The HVPS is scheduled to run on a 2.45 GHz/1MW lower hybrid current drive(LHCD)^[1] system of HT-7 superoonducting tokamak before the set-up of HT-7 superconducting tokamak in 2003.The HVPS has a series of advantages such as good steady and dynamic response,logical computer program controlling the HVPS without and fault,operationa panel and experimental board for data acquisition.which both are grounded distinctively in a normative way to protec the main body of HVPS along with its attached equipments from dangers.Electric power cables and other control cables are disposed reasonably,to prevent singals from magnetic interference and ensure the precision of signal transfer.This paper involves the experiment and operation of a 35kV/2.8 MW/1000s HVPS^[2] for 2.45 GHz/1 MW LHCD system.The reliability and feasibility of the HVPS has been demonstrated in comparison with experimental results of original design and simulation data.     

Preliminary Analysis of Liquid Metal MHD Pressure Drop in the Blanket for the FDS

Preliminary analysis and calculation of liquid metal Li17Pb83 magnetogydrodynamic (MHD) pressure drop in the blanket for the FDS have been presented to evaluate the significance of MHD effects on the thermal-hydraulic design of the blanket.To decrease the liquid metal MHD pressure drop,Al2O3 is applied as an electronically insulated coating onto the inner surface of the ducts.The requirement for the insulated coating to reduce the additional leakage pressure drop caused by coating imperfections has been analyzed.Finally,the total liquid metal MHD pressure drop and magnetic pump power in the FDS blanket have been given.     

JET中优化剪切放电的MHD稳定性

对JET优化剪切（OS）放电性能的主要限制是由MHD不稳定性引起的,主要以破裂极限形式出现。观察到作为破裂前兆的MHO模的结构,如从SXR和ECE诊断中测出的那样,表现为全域理想MHD模。测出的模结构与计算出的压力驱动扭曲模的模结合完全一致。破裂出现在较低的规范化β（1＜βN＜2）处,与计算出的n＝1压力驱动扭曲模的理想MHD稳定性极限一致,通常包括良性的、线性调频脉冲模。这些模出现于频率改变的爆裂中,具有与破裂前兆一样的模结构,但被快粒子驱动成不稳定的。