Journal of Applied Mathematics

Volume 2012, Article ID 957185, 8 pages

http://dx.doi.org/10.1155/2012/957185

## On a Quasi-Neutral Approximation to the Incompressible Euler Equations

College of Mathematics and Information Science, North China University of Water Resources and Electric Power, Zhengzhou 450011, China

Received 10 February 2012; Revised 25 April 2012; Accepted 26 April 2012

Academic Editor: Roberto Natalini

Copyright © 2012 Jianwei Yang and Zhitao Zhuang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We rigorously justify a singular Euler-Poisson approximation of the incompressible Euler equations in the quasi-neutral regime for plasma physics. Using the modulated energy estimates, the rate convergence of Euler-Poisson systems to the incompressible Euler equations is obtained.

#### 1. Introduction

In this paper, we shall consider the following hydrodynamic system: for and , subject to the initial conditions for . In the above equations, is 3-dimensional torus and is small parameter. Here , , denote the electron density, electron velocity, and the electrostatic potential, respectively.

System (1.1) is a model of a collisionless plasma where the ions are supposed to be at rest and create a neutralizing background field. Then the motion of the electrons can be described by using either the kinetic formalism or the hydrodynamic equations of conservation of mass and momentum as we do here. The self-induced electric field is the gradient of a potential that depends on the electron’s density through the linear Poisson equation .

To solve uniquely the Poisson equation, we add the condition . Passing to the limit when goes to zero, it is easy to see, at least at a very formal level, that tends to , where and In other words, is a solution of the incompressible Euler equations. The aim of this paper is to give a rigorous justification to this formal computation. We shall prove the following result.

Theorem 1.1. *Let be a solution of the incompressible Euler equations (1.3) such that and for . Assume that the initial value is such that
**
Then, there exist and such that for there is a solution of (1.1) satisfying
**
for any .*

Concerning the quasi-neutral limit, there are some results for various specific models. In particular, this limit has been performed for the Vlasov-Poisson system [1, 2], for the drift-diffusion equations and the quantum drift-diffusion equations [3, 4], for the one-dimensional and isothermal Euler-Poisson system [5], for the multidimensional Euler-Poisson equations [6, 7], for the bipolar Euler-Poisson system [8, 9], for the Vlasov-Maxwell system [10], and for Euler-Maxwell equations [11]. We refer to [12–15] and references therein for more recent contributions.

The main focus in the present note is on the use of the modulated energy techniques for studying incompressible fluids. We will mostly restrict ourselves to the case of well-prepared initial data. Our result gives a more general rate of convergence in strong norm of the solution of the singular system towards a smooth solution of the incompressible Euler equation. We noticed that the quasi-neutral limit with pressure is treated in [5, 6]. But the techniques used there do not apply here.

It should be pointed that the model that we considered is a collisionless plasma while the model in [6, 7] includes the pressure. Our proof is based on the modulated energy estimates and the curl-div decomposition of the gradient while the proof in [6, 7] is based on formal asymptotic expansions and iterative methods. Meanwhile, the model that we considered in this paper is a different scaling from that of [16]. Furthermore, our convergence result is different from the convergence result in [16].

#### 2. Proof of Theorem 1.1

First, let us set Then, we know the vector solves the system where . In fact, from (1.3), we get .

As in [16], we make the following change of unknowns: By using the last equation in (2.2), we get the following system: This last system can be written as a singular perturbation of a symmetrizable hyperbolic system: where denotes the th component of and where

Now, let us set and for with , It is easy to know that system (2.5) is a hyperbolic system. Consequently, for fixed, we have a result of local existence and uniqueness of strong solutions in , see [17]. This allows us to define as the largest time such that where which is such that when goes to zero will be chosen carefully later. To achieve the proof of Theorem 1.1, and in particular inequality (1.5), it is sufficient to establish that , which will be proved by showing that in (2.8) the equality cannot be reached for thanks to a good choice of .

Before performing the energy estimate, we apply the operator for with to (2.5), to obtain where

Along the proof, we shall denote by a number independent of , which actually may change from line to line, and by a nondecreasing function. Moreover and stand for the usual scalar product and norm, is the usual Sobolev norm, and is the usual norm.

Now, we proceed to perform the energy estimates for (2.9) in a classical way by taking the scalar product of system (2.9) with . Then, we have

Let us start the estimate of each term in the above equation. For , since is symmetric and , by Cauchy-Schwartz’s inequality and Sobolev’s lemma, we have that Next, since is skew-symmetric, we have that For , by a direct calculation, one gets Here, we have used the basic Moser-type calculus inequalities [18].

To give the estimate of the term , we split it in two terms. Specifically, we can deduce that Here, we have used the curl-div decomposition inequality

For , we have that

To estimate the last term, that is, , by using basic Moser-type calculus inequalities and Sobolev’s lemma, we have

Now, we collect all the previous estimates (2.12)–(2.18) and we sum over to find By using (2.8), we get with that Hence, by the Gronwall inequality, we get that

Consequently, if we choose , we see that we cannot reach equality in (2.8) for . This proves that and that (2.21) is valid on [.

#### Acknowledgment

The authors acknowledge partial support from the Research Initiation Project for High-Level Talents (no. 40118) of North China University of Water Resources and Electric Power.

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