INFLUENCE OF THE METASTABLE ATOMS LIFETIME ON THE RUNNING STRIATION EXCITATION

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INFLUENCE OF THE METASTABLE ATOMS LIFETIME ON THE RUNNING STRIATION

EXCITATION

P. Landa, Yu.V. Ponomarev

To cite this version:

P. Landa, Yu.V. Ponomarev. INFLUENCE OF THE METASTABLE ATOMS LIFETIME ON THE

RUNNING STRIATION EXCITATION. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-223-

C7-224. �10.1051/jphyscol:19797110�. �jpa-00219081�

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JOURNAL DE PHY5IQUE

INFLUENCE (3F THE METASTABLE ATOMS LIFETIME ON THE RUNNING STRIATION EXCITATION

P.S. Landa, YU.V. Ponornarev.

Moscow State University, Department of Physics.

It i s known t h a t i n the noble gas dis- charges inside a region of the running s t r i a t i o n excitation there can be a r e l a t i - vely small region of currents and pressu- r e s i n which there are no s t r i a t i o n

/I/.

The boundaries of t h i s region depend on the gas composition and discharge tube di- mensions /2,3/. The investigation of a non-striation region i s of great p r a c t i c a l i n t e r e s t because the operating ranges of pressures and currents i n helium-neon la-

s e r s usually overlap i t /3/.

In

t h i s work the presence of a non-striation region i s explained by the f i n i t e l i f e t i m e of the metastable atoms.

In most of t h e o r e t i c a l works on s t r i a t i - on excitation, f o r example /4,5/, the equa- t i o n f o r metastable atoms is not taken in- t o account. This holds true f o r great cur- r e n t s near the Pupp boundary, where the ef- f e c t i v e lifetime of metastable atoms i s

small and t h e i r concentration can follow t h a t of the electrons both i n time and space.

In

t h i s case the influence of meta- s t a b l e atoms displays i t s e l f only through the ionisation r a t e .

In

the region of small and medium currents the metastable atoms lifetime i s greater or i s of the same or- der than t h a t of the electrons, due t o which t h e i r influence becomes more consi- derable. The metastable atoms lifetime i s determined by both the diffusion on the tube walls and t h e i r disapperance i n the 4

volume due t o the processes of deexcitati- on and s t e p ionization. Therefore,

i t

i s useful t o introduce an e f f e c t i v e l i f e t i m e of metastable atoms

% e r

which depends on the average value of the electron concen- t r a t ion and consequently on the discharge

current. When the current changes 'E;ne a l s o varies i n broad l i m i t s .

It i s convenient t o devide the whole cur- r e n t region, i n which the s t r i a t i o n can e x i s t , i n t o three parts: ? ) g r e a t currents

Tm

>7

T,,,

> 7 2 m e

, 2)medium currents

C m

> > Z n - r m e

, 3)small currents

rrn ,-., Tme >>Tn ,

⁽

^Trr. , en are diffusion

l'fetirnes of electrons and metastable atoms).

d each of these regions the mechanism of the positive column i n s t a b i l i t y h a s i t s d i s t i n c t i v e p e c u l i a r i t i e s . For great and medium currents the metastable atoms diffu-

sion on the walls is negligible and t h e i r concentration s l i g h t l y depends on current.

In the region of small currents the met*

s t a b l e atoms concentration is proportional t o t h a t of electrons and therefore cosider- ably changes with the variatians of the cur- rent.

From the known experiments one can

drow

a conclusion t h a t i n the regions of great and small currents two d i f f e r e n t types of stri- a t i o n s are excited. In the region

of

medium currents, which i s t r a n s i t i o n a l , e i t h e r both types of the s t r i a t i o n s are excited or they are absent. The present work i s concerned with the e x c i t a t i o n conditions and dispersion c h a r a c t e r i s t i c s of s t r i a t i o n i n t h i s region of current.

Hydrodinamic equations of continuity were used a s i n i t i a l f o r concentration of elec- trons, metastable atoms and the electron gas energy

/6/.

The f i n a l value of r e s i s - t a m e of the discharge supply and boundary conditions f o r the positive column are taken i n t o account /5/. The account of the positive column s t a b i l i t y was c a r r i e d out a n a l y t i c a l l y a t d e f i n i t e parameter r e s t r i -

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797110

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ctions and by a computer.

To calculate stability the initial equa- tions were linearized relatively to small deviations from the stationary values.

Without taking into account the metastable atoms diffusion these equations expressed in terms of non-dimensional variables have the form

Here BIP is relative deviation from the sta- tionary value of the metastable atom con- centration; J is the value proportional to the constant component of the discharge current;

h)

2 ^{T, PT,}

^ktT

are non-dimensional temperature derivatives of the frequencies of formation of ions and metastable atoms and losses of electrons energy at colli- sions; 4, is a parameter characterizing step ionization; Pn is a parameter deter- mined by the dependence of frequency of me- tastable atom formation on the eiectrons concentration, (this dependence is due to the difference of the electron distribu- tion function from that of Maxwell

/4/);

other no!ations are the same as in /5/.

It follows from equations (1) that harmo- nic in time perturbation with frequency

C3

leads to the appearence of four waves and a syn-phase component related to the J

discharge current modulation, One of these waves is basio and near the boundary of striation selfexitation it is slightly en- hanced in the direction from the cathode to anode.The other waves greatly attenuate in the propagation direction. The conditi=

on of the stristion self-excitation is re- lated to the basic wave enhancement, name- ly, the space amplification factor of the basic wave )() should be greater than some boundary value J/, determined by the ratio of the discharge supply resistance to the positive column resistance / S / . Near the excitation boundary at the fixed value of the discharge current J the space am- plif ieation factor p depends resonantly on

the wave number ae , reaching its maximum value $&ax at some

Ee

. The regions of striation excitations correspond to those values of current J, for which $!J- 3 9,.

It follows from computations that these regions are mainly determined by the va- lues of parameters Pn and PT. Depending on these parameters the conditions of stria- tion self-excitation may be satisfied either for the whole region of medium cur- rents, or there can be a current region in which striations are absent (Fig .I 1, i

.e.

there is a non-striation region.

Gn both sides of the non-striation regi- on the striation of different types with different dispersion rules should be exc- ited: for the striation of m-type

C d

ae3=

=const, for those of n-type Uae=const.

If the non-striation region is absent the striations of m and n-types may either exist simultaneously, or a competitive de- pression of one of the striation types may occure, In the latter case at the current change the transition from one type of the striation to the other should be of a hysteresis character /7/.