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Plasma surface treatment debye shielding and Debye length introduction

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30 Oct,2020

Plasma surface treatment debye shielding and Debye length introduction:

 

If the plasma thermal motion disturbance of internal particles in plasma surface treatment, plasma body somewhere around power for q negative charge accumulation, due to the electrostatic field of the group charge effect, it will attract positive ions around the electronics, the results of positively charged "positive" clouds "negative". Seen from a distance, clouds of "positive charge" weaken the effect of "negative charge," the coulomb force of "negative charge" on distant charged particles, a phenomenon known in plasma surface treatment physics as "debay shielding." The length from the center of the "negative charge" to the edge of the "positive charge" cloud is called Debai length λD, and can be derived by solving a shielded Coulomb potential.

 

Plasma surface treatment debye shielding and Debye length assume that the center of negative charge is the origin of coordinates, for the balanced charged particle system with a space charge distribution of P (r), the potential distribution in the space from the center φ (r) satisfies the Poisson equation.

 

However, due to charge shielding, P(r) depends on the difference between positive and negative charge densities at R:

P (r) = e [Ni (r) - Ne (r)] (1-5)

Where, Ni(r) and Ne(r) represent the number density of positive and negative charged particles at the distance from the negative charge center r. In the absence of space charge accumulation, electrons and positive ions are uniformly distributed, and Nio=Neo=N. After charge accumulation, Ni(r) and Ne(r) are no longer uniformly distributed. In general, the electron with small mass first reaches thermal equilibrium, while the ion with large mass still stays in its original position. In this case, Ne(R) obeys Maxwell distribution.

 

In the formula, Ve(r)=-eφ (r) is the electric potential energy of the electron. For plasma surface treatment, the average thermal kinetic energy is much more than the average electric potential energy, namely kTe e. The Taylor series expansion of Equation (1-6) and the second-order approximation are made to obtain:

Ne(r)=Neo[1+eφ(r)/kTe] (1-7)

 

In general, the effective force path shielding the Coulomb potential is the debai length λD, or a sphere with radius λD, called the debai sphere. The coulomb potential outside the Debai sphere is negligible. The physical meaning of debye length is as follows:

(1) The plasma has a shielding effect on the potential acting on it, and the shielding radius is the debye length;

(2) debye length is a small space scale where the plasma is neutral. When r> λD, the plasma is neutral.

(3) debye length is the lower limit of the macroscopic spatial scale of plasma, namely the spatial scale of plasma L>> λD.

Plasma surface treatment debye shielding and Debye length introduction

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