ФИЗИКА ПЛАЗМЫ, 2011, том 37, № 1, с. 3-20
= ТОКАМАКИ
УДК 533.9
IMPACTS OF PELLETS INJECTED FROM THE LOW-FIELD SIDE
ON PLASMA IN ITER
© 2011 A. Wisitsorasak*, T. Onjun**
* Faculty of Science, Mahidol University, Bangkok, Thailand ** School of Manufacturing Systems and Mechanical Engineering, Sirindhorn International Institute of Technology,
Thammasat University, Pathumthani, Thailand Поступила в редакцию 05.04.2010 г.
Impacts of pellets injected from the low-field side (LFS) on plasma in ITER are investigated using the 1.5D BALDUR integrated predictive modeling code. In these simulations, the pellet ablation is described using the neutral gas shielding (NGS) model. The pellet ablation model is coupled with the plasma core transport model, which is a combination of the MMM95 anomalous transport model and NCLASS neoclassical transport model. The boundary conditions are assumed to be at the top of the pedestal, in which the pedestal parameters are predicted using a pedestal model based on the theoretical-based pedestal width scaling (either magnetic and flow shear stabilization width scaling, or flow shear stabilization width scaling, or normalized po-loidal pressure width scaling) and the infinite-n ballooning mode pressure gradient limit. These pedestal models depend sensitively on the density at the top of the pedestal, which can be strongly influenced by the injection of pellets. The combination of the MMM95 and NCLASS models, together with the pedestal and NGS models, is used to simulate the time evolution of the plasma current, ion and electron temperatures, and density profiles for ITER standard type I ELMy H-mode discharges during the injection of LFS pellets. It is found that the injection of pellets results in a complicated plasma scenario, especially in the outer region of the plasma and the plasma conditions at the boundary in which the pellet has an impact on increasing the plasma edge density, but reducing the plasma edge temperature. The LFS pellet has a stronger impact on the edge as compared to the center. For fusion performance, the pellet can result in either enhancement or degradation, depending sensitively on the pellet parameters; such as the pellet size, pellet velocity, and pellet frequency. For example, when a series of deuterium pellets with a size of 0.5 cm, velocity of 1 km/s, and frequency of 2 Hz are injected into the ITER plasma from the LFS, the plasma performance, evaluated in terms of Gfeaom can increase up to 72% of that before the use of pellets. It is also found that the injection of pellets results in an increase in the ion and electron densities, but does not enhance the central plasma density. On the other hand, it results in the formation of another peak of the plasma density in the outer region near the plasma edge. The formation of the density peak results in the reduction of plasma transports near the edge by decreasing the contributions of ion-temperature-gradient and trapped electron modes, as well as kinetic ballooning modes.
1. INTRODUCTION
It has been observed in self-consistent predictions for the International Thermonuclear Experimental Reactor (ITER) [1] using different integrated predictive modeling codes that the density profile in ITER is flat or relatively small peaking, which results in pessimistic nuclear fusion performance in some studies [2— 5]. The pessimistic performance causes a concern in the fusion community. If the central plasma density in ITER is increased, the fusion performance will increase to be in a more desirable range for burning plasma experiments. In general, peaking the plasma central density can be achieved by pellet injection [6], which has been successfully developed and widely used in many existing tokamak experiments. Density peaking using pellet injection has many advantages compared to other methods, such as the penetration depth into the plasma core [7] and the enhancement of impurities. However, density peaking with pellet injection yields a complicated plasma scenario, since the
pellet interactions with the plasma involve many complicated physical processes. Specifically, high-field side (HFS) pellet system is used for density peaking or plasma fueling, while low-field side (LFS) pellet system is used for an ELMs control. It is known that the installation of LFS pellet system is easier than that of HFS. As a result, it is interesting to investigate the behavior of LFS pellets in ITER plasma, which may allow an LFS pellet to be used for density peaking or plasma fueling in ITER.
Significant progress has been made in pellet injection technology since the first pellet experiment was achieved in 1954 by Spitzer and colleagues [8]. The first pellet injection system was used to inject small spherical hydrogen pellets 0.25 mm in diameter at a velocity of 10 m/s into a small experimental rotating plasma device [9]. A few years later, Foster and colleagues [10] used small spherical pellets of solid hydrogen at a velocity of about 100 m/s to perform the first pellet injection experiments on the Oak Ridge Toka-
mak (ORMAK). Nowadays, several advanced pellet injection systems have been developed and installed in many present-day tokamaks such as JET, DIII-D, JT-60U, and ASDEX-U. However, normal plasma parameters in these tokamaks differ from those in ITER. To successfully increase the central plasma density in ITER via pellet injection, an intensive investigation is required. Using self-consistent predictive simulations can lead to better understanding of the plasma behavior in ITER during the pellet injection phase.
During the last few years, both theoretical and experimental work on pellet penetration into the toka-mak has been carried out by many research teams in various countries [11—16]. When a pellet is injected into a hot plasma, it is exposed to the energy fluxes carried by the plasma (both electrons and ions). This results in the ablation of the pellet, the rate of which is defined by the balance between the energy flux available and the flux that is required to remove particles from the pellet surface and dissociate, ionize, and accelerate them [17]. As a result, the plasma density can increase. An excellent review for pellet studies can be found in [11].
This paper aims to demonstrate impacts of highspeed LFS pellets on the ITER plasma, especially the changes of the temperature and density profiles and, consequently, an impact on nuclear fusion production. It is widely known that the injection of HFS pellets yield better performance for density peaking and plasma fueling than that from LFS pellets because of the inward shift due to the E x B drift. However, the installation of HFS pellet system is more complicated due to the limitation of space available. In this study, it focuses on the impacts of high-speed LFS pellets on the ITER plasma. Self-consistence simulations of the ITER plasma are carried out using the 1.5D BALDUR integrated predictive modeling code [18]. In [19], the BALDUR code was used to study the density ramp-up and the ignition of thermonuclear plasmas by pellet injection, where the plasma simulation was not considered self-consistently. The predictive core and pedestal models are used together to simulate the complex plasma scenario during pellet injection. The injection of a pellet results in changes in the plasma, especially in the outer region of the plasma, which influences the pedestal structure and, consequently, the plasma core. It is worth noting that, when the pellet is applied, the pedestal density is increased, which results in a reduction of the pedestal temperature. The neutral gas shielding (NGS) module developed by Milora and Foster [20] is used to describe the behavior of the pellet in the plasma. This pellet ablation model considers only the shielding of the neutral cloud that releases from pellet surface. The predictions of this pellet model, such as the pellet penetration length, have been intensively tested in various plasma conditions, in which good agreements were obtained. To improve physics basis of the pellet model, it was extended to consider the shielding of both neutral and plasma, which is
called the "neutral gas and plasma shielding" (NGPS) model [21]. However, even though more realistic description of ablation processes was used, the NGPS model is less successful than the NGS model in comparing the measured pellet penetration depths.
According the ITER design, the pellet velocity of 0.3—0.5 km/s is planed for both the LFS and HFS with a frequency of 8 Hz. In this work, deuterium pellets with a size of 4 mm are considered to inject into ITER plasma from the LFS with a velocity of 1 km/s and frequency of 2 Hz. Note that because of using LFS pellets, injection of high-speed pellet is possible. Even though the effects of pellet injected from the LFS might be less efficient for density peaking and plasma fueling, it is believed to have interesting impact on the plasma with the high-speed pellets. The pellet type and its parameters will be varied to observe their impacts later. The NGS module is coupled with the core transport models to simulate the time evolution of the plasma current, temperature, and density profiles, where the boundary conditions are described using a predictive pedestal module. The plasma core transport is described by the combination of the theoretical-based multimode anomalous transport model (MMM95) [22] and the NCLASS neoclassical transport model [23]. In this work, the three best pedestal models in [24] are chosen. These pedestal models were developed by using the combination of the theoretical-based pedestal width models together with the pressure gradient limits imposed by ballooning mode instability. There are three choices of the pedestal width models considered: magnetic and flow shear stabilization (A rc p;s ) [25], flow shear stabilization (A ^sjpjRii) [24], and normalized poloidal pressure (A x Ryjp0 ped) [26], where A is the width
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