Plasma Fueling Research Articles

MHD stability analysis of diagnostic optimized configuration shots in JET

The plasma edge MHD stability is analysed for several JET discharges in the diagnostic optimized configuration. The stability analysis of Type I ELMy plasmas shows how after an edge localized mode (ELM) crash the plasma edge is deep in the stable region against low- to intermediate-n peeling–ballooning modes. As the pressure gradient steepens and the edge current builds up, the plasma reaches the low- to intermediate-n peeling–ballooning mode stability boundary just before the ELM crash. Increasing the plasma fuelling by gas puffing makes the second stability access against high-n ballooning modes narrower until it closes completely and the ELMs change from Type I to Type III. Reducing the plasma heating has a similar effect. Increasing the safety factor at the plasma edge improves the stability against low- to intermediate-n modes allowing steeper pressure gradients to develop before an ELM crash.

Assessment of the poloidal distribution of core plasma fueling and impurity sources in DIII-D

Measurements and modeling of the 2D poloidal Dα intensity distribution in DIII-D low density L-mode and medium density ELMy H-mode plasmas indicate that the core plasma is predominately fueled near the divertor X-point region. The neutral deuterium and ion carbon emission were measured in the divertor and inner main chamber scrape-off layer (SOL) using a plasma imaging technique, covering 85% of the poloidal cross-section. Typically, the peak emission in the inner main SOL at the tokamak midplane was three orders of magnitude lower than in the divertor. For discharges with the ion B×∇B drift direction toward the lower divertor the UEDGE/DEGAS codes predict strong core plasma fueling from the significantly higher density and lower temperature plasma calculated in the inner divertor leg. The concomitant carbon ion flow reversal in the inner divertor leg enhances the leakage of carbon from the divertor into the main SOL, and hence into the core.

Neutral Beam Injection Experiments and Related Behavior of Neutral Particles in the GAMMA 10 Tandem Mirror

Results of neutral beam injection (NBI) experiments in the GAMMA 10 tandem mirror plasmas are presented together with the neutral particle behavior observed in the experiments. A hydrogen neural beam was injected into the hot-ion-mode plasmas by using the injector installed in the central-cell for the plasma heating and fueling. High-energy ions produced by NBI were observed and its energy distribution was measured for the first time with a neutral particle analyzer installed in the central-cell. The temporal and spatial behavior of hydrogen was observed with axially aligned Hα detectors installed from the central midplane to anchor-cell. Enhancement of hydrogen recycling due to the beam injection and the cause of the observed decrease in plasma diamagnetism are discussed. The Monte-Carlo code DEGAS for neutral transport simulation was applied to the GAMMA 10 central-cell and a 3-dimensional simulation was performed in the NBI experiment. Localization of neutral particle during the beam injection is investigated based on the simulation and it was found that the increased recycling due to the beam injection was dominant near the injection port.

Development of Pellet Technologies for Plasma Fueling

This contribution presents recent results of pellet technologies development for plasma fuelling in magnetic confinement machines with open or closed magnetic configuration. The current status of ITV7 pellet injector for GOL3 multimirror linear machine, PGS2.2 pellet guide system of ITV4 in-situ pellet injector for TUMAN-3M tokamak and ITV5 centrifuge pellet injector for Globus-M spherical tokamak is reported. New results on modeling of tangential pellet injection into TUMAN-3M tokamak are discussed as well.

Pellet Injectors Developed at the Pelin Laboratory for Steady-State Plasma Fuelling

Pneumatic and centrifugal injectors for steady-state plasma refuelling by solid hydrogen, deuterium and tritium pellets have been designed at the PELIN Laboratory to meet requirements of LHD, TORE SUPRA, and ITER. Presented here is a review of these injectors' designs and results.

Penetration and attenuation of SMB fueling in tokamak plasma

In this paper, a discussion and an optimizing numerical analysis of the penetration and attenuation of the thermonuclear tokamak plasma fueling by the supersonic cluster molecular beam(SMB) has been presented.The cluster formation and dissolution,SMB adiabatic expansion,shielding and cooling effect are all taken into account.An optimized numerical model is applied in the analysis and shown to be in good agreement with the experimental observations.The possibility for fueling large tokamak plasmas with SMB injection is explored.

New ORNL Pellet Injection System and Installation/Initial Operations on MST

A compact pellet injection system that was recently developed at the Oak Ridge National Laboratory has been installed on the Madison Symmetric Torus (MST) at the University of Wisconsin and used in initial plasma fueling experiments. The system, referred to as a “pellet injector in a suitcase,” is a pipe gun device with a four-barrel capability (presently equipped with two 1.0-mm-bore barrels), and it uses a cryogenic refrigerator for in-situ hydrogen pellet formation (typically, D2 pellets). This new, portable, stand-alone pellet injection system was developed to provide a flexible means of plasma fueling on a wide variety of magnetic confinement devices, with relatively low costs for installation and operation. The injector has already been used to produce useful results with pellets on MST plasmas, including significant and rapid increases (almost 100%) in the line average density, and effectively depositing fuel in the plasma core (central densities of [approximately equal to ≈ 1.4 × 1019 m−3). In this paper, the injection system, its performance, and reliability will be described, and results from some initial MST pellet experiments will be highlighted.

Electrostatic method to accelerate nanoshells to extreme hypervelocity

Using an acceleration voltage of less than a few hundred kilovolts, it is unlikely that a charged solid object larger than a few micron (10−6 m) in all three dimensions can be accelerated to more than 10 km/s. Quasi-two-dimensional (Q2D) objects are unique forms of matter with two macroscopic dimensions, while the third approaches atomic dimensions. Well-known examples of Q2D objects are thin films. Another example of a Q2D object will be a sphere with a nm thick shell (nanoshell). In this letter, it is predicted that nanoshells can be accelerated to 100 km/s (extreme hypervelocity, or EHV) and above using the electrostatic method. The maximum velocity is limited by field emission and material strength. The two limits only allow a certain number of charges on a nanoshell before it starts to emit ions or electrons, or to break. “Table-top” EHV nanoshell beams can be used for high-temperature plasma diagnostics and fueling. EHV nanoshells can also be used to study hypervelocity-impact phenomena in a momentum space not accessible in the past.

Advanced tokamak operation using the DIII–D plasma control system

The principal focus of experimental operations in the DIII–D tokamak is the advanced tokamak (AT) [Proc. 18th Symp. on Fusion Technol., August, 1994, Karlsruhe, Germany, Elsevier, Amsterdam, 1 (1995) 731] regime, which requires highly integrated and flexible plasma control to achieve. In a high performance AT, accurate regulation of the plasma boundary, internal profiles, pumping, fueling, and heating must be well coordinated with MHD control action to stabilize such instabilities as tearing modes and resistive wall modes. Sophisticated monitors of the operational regime must provide detection of off-normal conditions and trigger appropriate safety responses with acceptable levels of reliability. Many of these capabilities are presently implemented in the DIII–D plasma control system (PCS), and are now in frequent or routine operational use. The present work describes recent development, implementation, and operational experience with AT regime control elements for equilibrium control, MHD suppression, and off-normal event detection and response.

Deuterium to helium plasma-wall change-over experiments in the JET MkII-gas box divertor

The deuterium and helium dynamics in the plasma and subdivertor regions of JET are compared during a sequence of similar ohmic and ICRH pulses where 100% He gas is injected into the JET vacuum vessel, whose graphite walls were previously saturated with deuterium. After the first six He fueled change-over discharges, only He plasma operation was performed. Following this investigation, the situation is reversed and the change-over from an initially saturated He wall is investigated when only D 2 plasma fuelling is used. The He concentration is measured in the subdivertor with a species selective Penning gauge. Comparison of the time dependence of the divertor concentrations with those at the edge and strike point shows significant differences during the first six discharges. This difference along with a global He particle balance is used to assess the status of the wall saturation over the initial 6–7 He change-over discharges.

Global analysis of ICRF wave coupling on Tore Supra

The Tore Supra tokamak is equipped with a multi-megawatt ion cyclotron range of frequency (ICRF) system for heating and current drive. The coupling of the fast wave to the plasma, characterized by the distributed coupling resistance along the radiating straps, is a crucial issue in order to launch large RF powers.Many factors can have an effect on ICRF wave coupling. Quantitative prediction from theoretical modelling requires the knowledge of the local inhomogeneous plasma density profile in front of the antenna for running sophisticated antenna codes. In this work, we have rather followed a `global' approach, based on Tore Supra experimental results, for the parametric study of the coupling resistance. From a large data base covering seven experimental campaigns (∼2250 shots), a scaling law of the coupling resistance including the main parameters of the plasma and of the antenna configuration is established. This approach is found to be reliable for the analysis of coupling in the different scenarios: He/D2 gas filling, gas/pellets for plasma fuelling, plasma leaning on inner wall/low field side limiter, limiter/ergodic divertor configuration, minority heating/direct electron heating.From one scenario to another, a significant variation of the coefficients of the scaling law is found. The study of these variations allows to get some insight on the main physical mechanisms which influence the ICRF wave coupling in a tokamak operation, such as the wall conditioning and recycling conditions, RF sheaths or frequency.

Pellet and Molecular Beam Injection Fueling on the HL-1M Tokamak

The Eight-shot Pellet Injector (EPI) and Molecular Beam Injector (MBI) as new plasma fueling methods have been developed and installed on the HL-1M tokamak for fueling experiments. The main structures and characteristics of the fueling device and the typical fueling experimental results with EPI and the MBI are reported. In these experiments, typical responses of plasma in discharges with PI and MBI are the peaked density profile Qn = ne(0)/〈ne〉 of >1.65 for MBI and of 2 for PI. The improvement of confinement time τE is usually better than 10 to 30% of Gas Puffing (GP) discharge in the same operation condition. In addition, the penetration depth and deposition region of fueling particles, the variance of soft X-ray sawteeth, the rotation and flow of plasma in edge region as well as the photographing of ablation clouds with PI and MBI are compared and presented in this paper.

Natural density formation as an H-mode operational limit

The natural H-mode density, i.e. the plasma density evolving in an H-mode discharge without active fuelling, reaches Greenwald fractions in JET typically higher than in ASDEX Upgrade. According to general thinking this reflects device-specific differences as regards recycling-induced fuelling and beam fuelling. This paper presents evidence for a different view, namely that at sufficiently low plasma fuelling rates any fuelling rate dependence of the plasma density vanishes and the plasma particle content is completely determined by the plasma itself. It is shown that this limit, which would impose a lower boundary on the H-mode density, is reached in JET and ASDEX Upgrade natural density discharges, and its scaling is determined. Possible overlapping with existing density limit scalings in next-generation tokamaks is discussed with a view to a narrowing or even vanishing H-mode operation window.

Overview of the HL-1M Tokamak Experiments

Experimental progress with the HL-1M tokamak has been made in many areas including confinement improvement, auxiliary heating, plasma fueling, and wall conditionings. An H-mode induced by a biased electrode was obtained with the formation of an internal transport barrier at the region of r/a ~ 0.4 to 0.5. Confinement improvement by lower-hybrid current drive (LHCD) was extensively investigated. Confinement improvement seems to be related to the production of the radial electron field during LHCD. In off-axis electron cyclotron resonance heating (ECRH), double sawteeth in soft X-ray radiation were observed, which implies that reversed magnetic shear could be formed during ECRH. At higher ECRH power, when the resonance position is near the q = 1 surface, fishbone instability was observed and investigated. An eight-shot pellet injector was used for the experiments. The pellet ablation process was investigated with a charge-coupled device (CCD) camera and an Hα emission detector array. Clearly, asymmetry in the pellet cloud was observed in both the toroidal and poloidal directions. It has been found that the pellet velocity slows down clearly after the pellet enters the plasma. The density limit has been investigated on HL-1M at different wall conditionings with three kinds of fueling methods. It was found that a higher density limit could be achieved under the following conditions: (a) a strong reduction of the impurity content after siliconization and (b) a peaked density profile with pellet injection and/or supersonic molecular beam injection. With a neutral beam injection (NBI) system of 1 MW, preliminary results of NBI experiments were obtained with an increase of ion temperature from 450 to 700 eV.

Analysis and modelling of plasma response to pellet injection in RFX

Pellet injection experiments have been performed on the RFX reversed field pinch (RFP). The main aim of the experimental campaign was the study of the target plasma response to get information both on the possibility of an efficient pellet fuelling of the discharge and on the associated transport modification. In this paper, we present an analysis of different experimental scenarios. The tool employed is a one-dimensional transport code coupled with an ablation calculation that yields the pellet particle source and trajectory. It is seen that, in standard plasmas, the pellets that reach the plasma centre trigger a phase of rapid profile flattening and poorer particle and energy confinement, and an increase of the transport coefficients is required to reproduce the post-pellet behaviour of the profiles. On the other hand, the slow pellets that suffer large deflection and miss the plasma centre do not have such a deleterious effect and, in this case, the unperturbed transport coefficients are able to reproduce the post-pellet evolution of the profiles. Finally, when the pellets are launched in combination with some method that induces an improved confinement, the transport degradation is less evident. In our case, pellet injection has been combined with pulsed poloidal current drive (PPCD), a technique that reduces magnetic field stochasticization and transport in the RFP core. When the effect of the PPCD is strong enough, a synergy between pellet injection and PPCD is observed and plasma fuelling without dramatic confinement reduction is possible.

A mock-up of a tritium pellet injector for ITER

The ITER tokamak is to be equipped with a pellet injector for plasma fuelling. A tritium compatible pneumatic pellet injector with a screw extruder has been designed, tested with protium, and is under installation for testing at the experimental tritium loop in the Russian Nuclear Center in Sarov. The rectangular rod 3×4 mm 2 from condensed protium was extruded within 1500 s at rate of ∼15 mm/s. Pellets ∼3 mm in size formed from the rod were accelerated up to 500 m/s by helium under a pressure of 1 MPa with an injection frequency of 1 and 2 Hz. The main injector subsystems and the current status of the work are described.

Simplified mass ablation and relocation treatment for pellet injection optimization

A simplified mass ablation and relocation treatment ofhigh-field-side/low-field-side pellet injection is proposed. It includesa model of pellet ablation, parallel flow model for the attachedablation channel and a simplified magnetohydrodynamic pelletdisplacement model. This treatment reveals satisfactory agreement withthe present-day experiments on ASDEX-U and DIII-D tokamaks. The modelprovides the optimization directions for deep plasma fuelling inexperiments.

Wall conditioning and density control in the TJ-II stellarator

Helium glow discharge cleaning (GDC) at room temperature, applied during the overnight periods between experimental days, has been used as standard method of wall conditioning in the TJ-II stellarator during the initial operation with ECRH plasmas in an all stainless steel scenario. However, the density control at medium and high power injection (more than 300 kW) becomes difficult due to the contribution of He implanted on the wall during GDC and desorbed by plasma discharges. He desorption rates of ∼4×10 10 cm −3 ms −1 have been estimated from residual gas analyser (RGA) postpulse measurements. In order to overcome this effect, an Ar GDC of a duration of 30 min prior to the TJ-II operation, and after the overnight He GDC have been applied in the last campaign. The Ar GDC removes of the order of 10 21 He atoms (about one monolayer) from the walls allowing a good control of the plasma density by means of external gas puffing. Values of Z eff∼2.0–2.5 in H 2 plasmas and mass spectra measurements of thermal desorption show that the contribution of Ar implanted to the plasma fuelling is not important.

Status of the Russian pellet injector R&D program for plasma fuelling

Basic trends of research within the Russian pellet injector R&D program for 1999–2001 are discussed. The attention is focused on injectors with a screw extruder and those with a porous generator capable of injecting pellets in a repetitive continuous steady state mode. Over 12 000 deuterium pellets of 2 mm diameter were produced and accelerated to 200 m/s at a rate of 10 Hz with a reliability over 99% by a pneumatic injector equipped with a screw extruder. Over 2000 hydrogen pellets of 2.4 mm diameter and 4 mm length were accelerated to 1 km/s at a rate of 0.3–0.8 Hz by a pneumatic injector equipped with a porous generator with no moving parts. The injector designs and experimental results are presented and a concept of a new centrifuge pellet injector is discussed.

A two-stage pellet injector with a cryogenic piston for plasma fueling of thermonuclear devices

A new two-stage gun was developed for solid-hydrogen-or deuterium-pellet formation and injection into plasma of thermonuclear devices. A cryogenic piston made from solid carbon dioxide was first used to compress and heat the propellant gas in the second gun stage. The piston was produced simultaneously with fuel pellets before each shot. Deuterium pellets with a 1.5-mm diameter and up to 5-mm length were accelerated to a speed of 1.7 km/s. The experimental results of cryogenic-piston formation with a 4-mm diameter and up to 30-mm length are presented.