Radial electric fields and confinement in the TJ-II stellarator

Radial plasma potential profiles have been obtained in the TJ-II by the Heavy Ion Beam Probing (HIBP) diagnostics. Results show that the potential increases up to 1 kV near the magnetic axis in ECRH low-density plasmas. The secondary ion current profiles, which directly reflect the plasma density, are hollow. In low-density ECRH operation, radial electric fields are found to be positive in the plasma core, however, a reduction in these fields is observed with increasing density. Radial plasma potential profiles show evidence of structures in configurations with low-order rational surfaces. In particular, HIBP measurements have permitted characterization of the plasma potential profile during e-ITB formation. Experiments in TJ-II have shown that it is possible to modify the global confinement and edge plasma parameters with limiter biasing, illustrating the direct impact of radial electric fields on confinement properties. Plasma potential measurements by the HIBP diagnostic show a strong impact of heating method (ECRH versus NBI) on radial electric fields.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

The Role of Radial Electric Fields in the Tokamaks TEXTOR-94, CASTOR, and T-10

Radial electric fields (E _r) and their role in the establishment of edge transport barriers and improved confinement have been studied in the tokamaks TEXTOR-94 and CASTOR, where E _r is externally applied to the plasma in a controlled way using a biased electrode, as well as in the tokamak T-10 where an edge transport barrier (H-mode) is obtained during electron-cyclotron resonance heating (ECRH) of the plasma.The physics of radial currents was studied and the radial conductivity in the edge of TEXTOR-94 (R = 1.75 m, a = 0.46 m) was found to be dominated by recycling (ion-neutral collisions) at the last closed flux surface (LCFS) and by parallel viscosity inside the LCFS. From a performance point of view (edge engineering), such electrode biasing was shown to induce a particle transport barrier, a reduction of particle transport, and a concomitant increase in energy confinement. An H-mode-like behaviour can be induced both with positive and negative electric fields. Positive as well as negative electric fields were shown to strongly affect the exhaust of hydrogen, helium, and impurities, not only in the H-mode-like regime.The impact of sheared radial electric fields on turbulent structures and flows at the plasma edge is investigated on the CASTOR tokamak (R = 0.4 m, a = 0.085 m). A non-intrusive biasing scheme that we call "separatrix biasing" is applied whereby the electrode is located in the scrape-off layer (SOL) with its tip just touching the LCFS. There is evidence of strongly sheared radial electric field and E×B flow, resulting in the formation of a transport barrier at the separatrix. Advanced probe diagnosis of the edge region has shown that the E×B shear rate that arises during separatrix biasing is larger than for standard edge plasma biasing. The plasma flows, especially the poloidal E×B drift velocity, are strongly modified in the sheared region, reaching Mach numbers as high as half the sound speed. The corresponding shear rates ( 5×10⁶ s^-1) derived from both the flow and electric field profiles are in excellent agreement and are at least an order of magnitude higher than the growth rate of unstable turbulent modes as estimated from fluctuation measurements.During ECRH in the tokamak T-10 (R = 1.5 m, a = 0.3 m), a regime of improved confinement is obtained with features resembling those in the H-mode in other tokamaks. Using a heavy ion beam probe, a narrow potential well is observed near the limiter together with the typical features of the L-H transition. The time evolution of the plasma profiles during L-H and H-L transitions is clearly correlated with that of the density profile and the formation of a transport barrier near the limiter. The edge electric field is initially positive after the onset of ECRH. It changes its sign during the L-H transition and grows till a steady condition is reached. Similar to the biasing experiments in TEXTOR-94 and CASTOR, the experimentally observed transport barrier is a barrier for particles.     

Reduction of impurity confinement time by combined heating of LHW and ECRH in EAST

The core impurity confinement properties are experimentally investigated in the Experimental Advanced Superconducting Tokamak(EAST) plasma heated by lower hybrid wave(LHW) and electron cyclotron resonance heating(ECRH)(LHW+ECRH).It is shown that the impurity confinement time(τ_(imp)) in the L-mode plasma jointly heated by LHW and ECRH is weakly dependent on electron density but strongly dependent on the heating power,thus it is shorter than that in LHW-only heated L-mode plasma with the similar plasma parameters.The combined heating of LHW and ECRH can reduce the collisionality and indicates a more effective heating method for core τimp reduction and normalized poloidal beta(βP) ~(im)provement.It should be emphasized that in this high β_P operation window the small ELM regime can be accessed,and an L-mode level τ_(imp)(40 ms-80 ms) and high β_N(～1.7) can be obtained simultaneously.It means that this typical small ELMy H-mode regime has an advantage in avoiding the serious tungsten accumulation,and will be competitive in future long-pulse steady-state and high-performance operation with high-Z material plasma-facing components.     

Status and prospects of controlled thermonuclear fusion

Of all approaches to controlled thermonuclear fusion the tokamak experiments have been most successful. Over the last decade particularly three large devices have achieved plasma density,n, temperature,T, and energy confinement time, _E, in ranges necessary for a fusion reactor plasma. Such maximum values have, however, been obtained not yet simultaneously but only in separate pulses, although the crucial triple product,nT _E, has also been improved by several orders of magnitude. The high temperatures sufficient in a fusion reactor can be produced by injection of neutral atoms or by absorption of radio frequency waves in the ion cyclotron frequency range. The plasma confinement ( _E1s) is still not understood and is handled through empirical scaling laws. Particle densities have usually been on the low side (n5×10¹⁹ m^–3) because increased fuelling rates can easily lead to violent current disruptions. Progress in obtaining peaked density profiles with pellet injection has led to high density plasmas without disruptions. Serious unsolved problems concern the spoiling of the fusion rates by (nonhydrogenic) impurities, the plasma parameter control over longer periods of time and indeed the plasma heating by fusion alpha-particles (ignition, burning). The most urgent technological question refers to the lifetime of the first wall which is in direct contact with the plasma. An important step towards ignition has been made by the recent JET/DT experiments in which, for the first time, the actual reactor fuel component tritium has been used to produce neutrons. The next generation tokamak ITER is, at present, being planned and designed in a world-wide collaborative effort. It should be operating before the year 2010 and is intended to investigate an ignited plasma burning for several minutes.     

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在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、Hα辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

HL-2A装置的约束改善和边界净输入功率分析

在HL-2A装置孔栏位形放电的等离子体实验中,电子回旋辅助加热期间观察到了等离子体约束改善的现象,并对等离子体从低约束模式(L模)向约束改善模式转换时的等离子体线平均电子密度、等离子体储能、分界面内辐射功率、能量约束时间、H_α辐射等进行了研究。同时,分析了电子密度和等离子体辐射功率的空间分布随时间的演化。对改善约束的相关功率(辅助加热、欧姆加热功率和损失功率)进行了分析,并研究了等离子体约束改善转换时的边界净输入功率(阈值)与电子线平均密度和环向磁场的关系。     

Determination of electron density in a wall stabilized Ar-CO ${\mathsf{_2}}$ thermal plasma

The electron density is measured in an argon-CO₂ thermal plasma by optical emission spectroscopy. Electron density is deduced from the Stark broadening of the argon line and hydrogen (H ) line. Different theories are used and compared. The effect of CO₂ molecule upon this plasma is studied as a function of the Ar-CO₂ mixture composition and discharge current. The electron density is ranging from 3 x 10²¹ m^-3 to 5.6 x 10²² m^-3. The influence of the arc confinement is studied and the electron density gradients are evaluated. Departure from local thermal equilibrium is also discussed.Received: 5 September 2003, Published online: 21 October 2003PACS: 52.70.Kz Optical (ultraviolet, visible, infrared) measurements - 52.80.Mg Arcs; sparks; lightning; atmospheric electricity     

Multifaceted asymmetric radiation from the edge along improved confinement mode in LHCD plasmas with graphite limiters on HT-7 tokamak

Multifaceted asymmetric radiation from the edge (MARFE) phenomena during lower hybrid current drive (LHCD) Experiments on the HT-7 superconducting tokamak are summarized in this paper. The best correlation has been found between the total input (ohmic + LHCD) power and the product of the edge line average density and Z _eff. Studies show that the critical density of MARFE onset is observed in the region of Z _eff ^1/2f_GW = 0.6-0.9, where , (here is the maximum line average electron density and n_GW is the Greenwald density). These MARFEs generally appear to have the same characteristics as high f_GW MARFEs and are positionally stable throughout the LHCD pulse. Improved confinement mode induced by a MARFE is observed, and it is maintained for about 65 ms. MARFE cools the plasma edge, and the electron density profile is observed to become more narrow and peaked.     

Investigation of the plasma potential oscillations in the range of geodesic acoustic mode frequencies by heavy ion beam probing in tokamaks

Specific oscillations within a range of 20 kHz (20 kHz-mode) were investigated on the T-10 and TEXT tokamaks using Heavy Ion Beam Probe (HIBP) diagnostic. Regimes with ohmic heating on both machines, and with off-axis ECRH in T-10 were studied. It was shown that 20 kHz-modes are mainly the potential oscillations. The power spectrum of the oscillations has the form of a solitary quasi-monochromatic peak with a contrast range of (3–5). They are the manifestation of torsional plasma oscillations with poloidal wavenumber m = 0, called zonal flows. It was shown that in TEXT the radial electric field oscillations exist in a limited radial range of 0.65 > < 0.95. The frequency of 20 kHz-mode is varied in the region of observation; it diminishes to the plasma edge. In T-10, after ECRH switch-on, the frequency increases, correlating with the growth of the electron temperature T _e. In both machines the frequency of the 20 kHz-mode varies with local T _e: f T _e ^1/2 , which is consistent with theoretical scaling for geodesic acoustic modes (GAM): f _GAM c _s/R T _e ^1/2 , where c _s is the speed of sound. The absolute frequencies are close to GAM values within a factor of unity.Presented at the Workshop Electric Fields Structures and Relaxation in Edge Plasmas, Nice, France, October 26–27, 2004.     

Energy confinement and high-k turbulence on HT-7 tokamak

A CO₂ collective scattering system has been developed to measure medium and high-k density fluctuation on HT-7 tokamak. Energy confinement and high-k density fluctuation are studied in ohmic discharges. The saturation of confinement was observed at approximately . For kθ=23 cm⁻¹ turbulence, it is found that fluctuation levels have great increase for , and simultaneously a high frequency turbulence feature appear in the frequency spectra. For kθ=14 cm⁻¹ turbulence, it is observed that fluctuation levels have no changes in uncertainties and the spectra are similar at all discharges in this experiment.     

High Power Gyro-Devices for Plasma Heating and Other Applications

Gyrotron oscillators are mainly used as high power millimeter wave sources for electron cyclotron resonance heating (ECRH), electron cyclotron current drive (ECCD), stability control and diagnostics of magnetically confined plasmas for generation of energy by controlled thermonuclear fusion. The maximum pulse length of commercially available 1 MW gyrotrons employing synthetic diamond output windows is 5 s at 110 GHz (CPI and JAERI-TOSHIBA), 12 s at 140 GHz (FZK-CRPP-CEA-TED) and 10 s at 170 GHz (GYCOM and JAERI-TOSHIBA), with efficiencies slightly above 30%. Total efficiencies of 45–50 % have been obtained using single-stage depressed collectors (SDC). The energy world record of 160 MJ (0.89 MW at 180 s pulse length and 140 GHz) at power levels higher than 0.8 MW has been achieved by the European FZK-CRPP-CEA-TED collaboration at FZK. Operation at the 1^st and the 2^nd harmonic of the EC frequency enables gyrotrons to act as medium power step-tunable mm- and sub-mm wave sources in the frequency range from 38 GHz (fundamental) to 889 GHz (2nd harmonic) for plasma diagnostics, EC plasma discharges for generation of multi-charged ions, high-frequency broadband electron paramagnetic resonance (EPR) spectroscopy and medical applications. Gyrotrons have also been successfully used in materials processing. Such technological applications require gyrotrons with the following parameters: f 24 GHz, P_out = 4–50 kW, CW, 30%. Future applications which await the development of novel high-power gyro-amplifiers include high resolution radar ranging and imaging in atmospheric and planetary science as well as deep-space and specialized satellite communications and RF drivers for next-generation high-gradient linear accelerators (supercolliders). The present paper reviews the state-of-the-art and future prospects of gyro-devices and their applications.     

Radial scanning diagnostics of bremsstrahlung and line emission in T-10 plasma

The paper describes the scanning spectroscopic diagnostics designed for measurement of line integrated plasma radiation in two visible spectral ranges. This diagnostic system is aimed at measuring the bremsstrahlung absolute values and profile with high spatial resolution. The bremsstrahlung absolute values are used to determine the value and radial distribution of effective plasma ion charge Z _eff(r) in T-10 discharges. The importance of Z _eff measurement is due to its strong influence on plasma heating, confinement, and stability. The spatial distribution of emission for one of the chosen spectral lines is measured simultaneously with bremsstrahlung. The spatial resolution of measurements is ~1 cm, and the temporal resolution is up to 10 ms. The spectral equipment and methods for its calibration are described. Examples of line integrated brightness distribution in a “continuum window” of 5236 ± 6 Å and brightness of the lines C⁵⁺ (5291 Å), He¹⁺ (4686 Å), and D_β (4861 Å) are given. Flattening of the bremsstrahlung brightness profile in the central region of the plasma column in some discharges with sawtooth oscillations in the T-10 is observed. The measured effective ion charge profiles in ohmic discharges with high plasma density and low discharge currents demonstrate accumulation of light impurities at the column axis; as a consequence, quenching of sawtooth oscillations in some discharges is observed. The developed diagnostics provides necessary data for investigation of heat, particle, and current transport in the plasma of the T-10. Successful application of the obtained data on Z _eff(r) for investigation of geodesic acoustic modes of plasma oscillations in the T-10 should be specially noted.     

HCN laser scattering on the JIPP T-IIU tokamak

A 4-channel HCN laser scattering system has been developed and applied to the JIPP T-IIU tokamak. Main aim of this system is to study microturbulence excited in the plasma, especially in the high power ICRF heating experiment. The observed density fluctuations are in a frequency range of the electron diamagnetic drift wave and have broad frequency spectra. The main part of the wavenumber is found to satisfy the condition of . In the heating experiments, deterioration of the energy confinement time is observed, and the density fluctuation level increases with the increase of the heating power.     

Density Fluctuation Measurements Using FIR Interferometer on HL-2A Tokamak

Density fluctuations were first measured in the core region of HL-2A tokamak plasma using a newly developed multi-channel FIR interferometer system. In divertor ohmic discharges, we measured the radial density fluctuation levels of 5%, which increase to 10-20% during the appearance of MHD activity. Most of the power density in the density fluctuation spectrum is directly associated with m = 2 tearing modes. The fluctuation levels reduce to 1/3 and plasma confinement is improved during off-axis electron-cyclotron-resonance heating （ECRH）.     

ECRH in Tandem Mirror Machines

Electron cyclotron resonance heating (ECRH) is important in tandem mirror devices which incorporate thermal barriers to enhance plasma confinement. ECRH in the end cells generates hot mirror-confined electrons to form the thermal barrier and creates the plugging potential to reduce ion end loss from the central cell. Important considerations for heating include a) the proper choice of wave polarization and harmonic number for wave penetration and efficient absorption, b) the possibility for control of hot electron energy and anisotropy by spatially limited heating, and c) the particle feed for hot electrons due to RF trapping processes. Fokker-Planck and Monte Carlo computer codes are useful for understanding these effects. The Tandem Mirror Experiment Upgrade (TMX-U) was constructed to investigate plasma confinement for the thermal barrier mode of operation. We carried out experiments to generate the hot electrons that are required for the thermal barrier. Heating efficiencies as large as 30 percent were observed. By combining the hot electron population with neutral-beam-injected hot ions and ECRH at the location of the desired potential peak, we strongly reduced the end loss of ions and measured plugging potentials up to 700 V. These results are consistent with the thermal barrier model.     

Velocity Fluctuations and Transport in the JET Boundary Region

A new approach for turbulent fluxes and _E×B measurements in the bulk plasma is proposed. It is based in the measurement of fluctuations in the phase velocity of fluctuations. The structure of turbulence has been investigated in the JET plasma boundary region with a fast reciprocating Langmuir probe system. Fluctuations in the radial and poloidal phase velocity have been computed from floating potential and ion saturation current measurements. The correlation between density fluctuations and fluctuations in the radial velocity of fluctuations signals show a good agreement with the turbulent transport computed from the correlation between density and poloidal electric field fluctuations. These results suggest that turbulent transport might be computed in the plasma core from measurement of density fluctuations. E×B sheared flows, both constant and varying in time, are close to the critical value to trigger the transition to improve confinement regimes below the power threshold to trigger the formation of transport barriers.     

EAST 电子回旋共振加热效果的数值模拟和分析

为了给EAST 电子回旋共振加热物理实验提供理论依据和模拟预测,从电子热输运方程出发,运用 CRONOS 输运程序对不同等离子体和波参数下,电子回旋加热效果进行了数值模拟计算。给出不同电子回旋波功率、入射角、电子密度和纵场等参数对电子回旋加热效果的影响,预测在不同参数下,电子温度、等离子体总内能和能量约束时间的变化,分析了其原因,并与实验结果进行了初步的比较。     

Reconstruction of the 2D hole gas spectrum for selectively doped <Emphasis Type="Italic">p</Emphasis>-Ge/Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>ix</Subscript> heterostructures

The magnetic field (0≤B≤32 T) and temperature (0.1≤T≤15 K) dependences of longitudinal and Hall resistivities have been investigated for p-Ge_0.93Si_0.07/Ge multilayers with different Ge layer widths 12≤d _w ≤20 nm and hole densities p _s =(1–5)×10¹⁵ m^?2. An extremely high sensitivity of the experimental data (the structure of magnetoresistance traces, relative values of the inter-Landau-level gaps deduced from the activation magnetotransport, etc.) to the quantum well profile is revealed in the cases where the Fermi level reaches the second confinement subband. An unusually high density of localized states between the Landau levels is deduced from the data. Two models for the long-range random impurity potential (the model with randomly distributed charged centers located outside the conducting layer and the model of the system with a spacer) are used to evaluate the impurity potential fluctuation characteristics: the random potential amplitude, the nonlinear screening length in the vicinity of integer filling factors v=1 and v=2, and the background density of states (DOS). The described models are suitable for explanation of the observed DOS values, while the short-range impurity potential models fail. For half-integer filling factors, a linear temperature dependence of the effective quantum Hall effect plateau-plateau (PP) transition widths v₀(T) is observed, contrary to the expected scaling behavior of the systems with short-range disorder. The finite T→0 width of the PP transitions may be due to an effective low-temperature screening of a smooth random potential due to the Coulomb repulsion of electrons.     

Evolution of Annular Self-controlled Electron–Nucleus Collapse in Condensed Targets

We considered peculiarities of the evolution of a region with sharp boundaries that is filled with a partially ionized plasma and is a part of the volume of a condensed target. The creation of such a region in the near-surface layer of the target can be related to the action of an external impulse symmetric ionizator or to the action of an intense small-extension shock wave on the target surface. We defined the conditions such that their fulfilment during the establishment of the equilibrium between the Coulomb attraction of electrons and ions with atom ionization multiplicity Z^*₁ and the kinetic pressure of electrons causes both the compression of this region and its ionization to the state with Z^*₂ > Z^*₁. The last leads to a further additional compression and ionization. Under these conditions, the spontaneous avalanche-like ionization of atoms of the target to the state of bare nuclei occurs synchronously with the avalanche-like metallization and the self-compression of the target. We showed that the avalanche-like ionization and the self-compression of the target happen in the case where the gas of degenerate electrons has drift momentum. If the region with initial ionization has the form of thin spherical layer, the process of avalanche-like ionization and self-compression of the target in this region is accompanied by the accelerated movement of the plasma layer to the target center. One of the reasons for the accelerated movement is the surface tension in a bounded domain of the nonequilibrium plasma layer neutralized by ions of the target. With increase in the velocity of movement of this layer to the target center, the additional self-compression of the system of electrons and nuclei to the state of degenerate electron gas occurs. At the leading edge of the running layer with extremely high electron density which is neutralized by nuclei of the target, the formation of a collapse of the electron--nucleus system proceeds, and the binding energy maximum for the electron--nucleus system shifts from A60 to A 60. This result makes possible the fast synthesis of superheavy nuclei. The decay of the collapse state, a partial restoration of the target structure, its rapid cooling, and the condensation of a part of the products of nuclear reactions happen in the target volume at the trailing edge of the moving plasma layer. Upon such a scanning propagation of the wave with high electron density, all the target substance is involved, step-by-step, to the process of nuclear transformations. At the target center, the moving plasma layer is squeezed with the formation of the state of quasistationary collapse under inertial confinement. Then the collapse state decays irreversibly.     

Pulsed-laser deposition of MgB2 and B thin films

Thin films of the novel superconductor MgB₂ were deposited from an Mg-enriched MgB₂ target or by alternating ablation from Mg and B targets, depositing multilayers. The superconducting films were achieved in situ by a two-step process: deposition at low temperatures ranging from room temperature to 200 °C and subsequently heating to 600 °C. The color of the plasma originating from Mg or Mg-enriched MgB₂ targets during the deposition is an indicator of the constituents of the plasma and can be used to adjust the plasma parameters like pressure and energy density. The films showed a reduced critical temperature (T_c) compared to the bulk value (39 K), which is attributed to the small grain sizes and the relatively high base pressure of the system (10^-7 mbar) causing impurities (oxygen, carbon...). To investigate B oxidation and to determine the suitable deposition conditions for B, films made by pulsed-laser deposition (PLD) from B target were analyzed by XPS. The films are very sensitive to the ambient gas purity and the base pressure. We anticipate an improvement of T_c and the crystallinity of MgB₂ thin films by using PLD in high vacuum and with a high purity Ar and H₂ gas mixture. PACS 74.70.Ad