Simple analytic expressions are derived to design uniform-field electrodes that are superior in compactness and field uniformity to those described in literature. When such electrodes are used in a TEA laser system, smaller electrodes can be used for the same gas-discharge width

Ernst, Gerard J.

English

Ernst, G.J.

University of Twente Research Information

Volume 49. number 4 OPTICS COMMUNICATIONS 15 March 1984 UNIFORM-FIELD ELECTRODES WITH MINIMUM WIDTH Gerard J. ERNST Department of Applied Physics, Twente University of Technology, Enschede, The Netherlands Received 1 September 1983 Simple analytic expressions are derived to design uniform-field electrodes that are superior in compactness and field uniformity to those described in literature. When such electrodes are used in a TEA laser system, smaller electrodes can be used for the same gas-discharge width. In order to obtain high output powers from trans- versely excited pulsed lasers it is important to have a very uniform energy loading of the active gas medium That is why there is a need for specially contoured electrodes that produce a very uniform field-strength distribution over a certain amount of surface. A num- ber of authors tried to solve this problem. Rogowski’s [ 1 ] and Chang’s [2] profiles are most commonly used up to now. For some applications, however, a smaller electrode width is required for a given dis- charge width than will be obtained by e.g. Chang’s formulas. This, for instance, is the case for large-aper- ture CO, and CO lasers, where the distance of the UV source from the electrode centre is an important mea- sure, or for large-aperture UV lasers, where the elec- trode inductance contributes significantly to the total circuit inductance. Recently [3] Chang profiles have been improved by modification of the conformal transformation. Although those profiles are excellent as for their specifications, the analytic expressions are rather complicated, which limits their use. We present here a family of analytic profiles for uniform-field electrodes that have minimum width and can produce almost any degree of field-strength uniformity at the electrode surface, and have analytic expressions as simple as Chang’s. We start with the following conformal transformation: [=w+ku sinhw+kl sinh2w, (I) where { =x + iy and w = u + iv, with x and y being the space coordinates and u and IJ being the flux and potential functions, respectively. For each value of IJ (I u I < n), the profile of the corresponding equipo- tential surface is given by x=u+kusinhucosu+kl sinh2ucos2u, (2) y = u + k, cash u sin u + k, cash 2u sin 2u, (3) where u is the running variable. This profile is sym- metric with respect to they axis and the +u and -u equipotentials are mirror images with respect to the x axis, which are prerequisites for a uniform-field electrode. From relations (2) and (3) it can be seen that the profiles are not uniquely determined. Three independent variables, k,, k, , and u, determine the form of the profile as well as the electric-field strength distribution. Once k, has been chosen as an indepen- dent variable, k, and u can be used to optimize the electric-field strength distribution over the surface. To find the optimum profile an expression is needed for the electric-field strength: E-2 = ld{/dw12 = 11 + k, cash w + 2k, cash 2w12 = f2(n) + g2(u), with (4) f(u) = 1 + k, cash u cos u + 2k, cash 2u cos 2u, (5) g(u) = kO sinh u sin u + 2k, sinh 2u sin 2~. (6) When the electric-field strength is expressed as a power-series expansion around u = 0, 275 Volume 49, number 4 OPTICS COMMUNICATIONS 15 March 1984 E = E,(k,, k,, 4 + E,(k,, k, > u)u* + E4(ko, k,, u)u4 + . . . . (7) the optimum profile can be found by requiring the lower coefficients E2 and E4 to vanish or E2 = -If(O)f<*‘(O) t (g”‘(O))*] /f3(0) = 0. (8) E4 = -If(0)fi4’(O) + 3(f12’(0))2 + 4g’l’(O)g’3’(0)] l&O) = 0, (9) where the exponent between brackets denotes the number of differentiations with respect to u. The results of our computer calculations are shown in table 1. As can be seen, the optimized values of IJ deviate only very slightly from rr/2 up to k, values of 0.1. For all practical profiles the u value can therefore be approximated by n/2. The value of k, can then be found from eq. (8) with u = n/2. This results in k; - 8k,(l - 2/c,) = 0 (IO) or k, = ; - ;(l - &“2, which often can be approximated by k, =ik;. (11) (12) Table 1 Optimized values of /co, kr, and u for the profile of eqs. (2) and (3) ko kz ” 0.001 0.1250002 E - 6 1.570796 0.0015 0.2812508 E - 6 1.570796 0.002 0.5000025 E - 6 1.570796 0.003 0.1125013 E - 5 1.570796 0.005 0.3125098 E - 5 1.570796 0.007 0.6125375 E - 5 1.570796 0.01 0.1250156 E - 4 1.570795 0.015 0.2813292 E - 4 1.570793 0.02 0.5002505 E - 4 1.570788 0.03 0.1126271 E - 3 1.570769 0.05 0.3134887 E - 3 1.570670 0.07 0.6163444 E - 3 1.570445 0.1 0.1266441 E - 2 1.569746 0.15 0.2901734 E - 2 1.567006 0.2 0.5 315494 E - 2 1.560798 0.3 0.1400316 E - 1 1.514851 276 So, the resulting profile is given by X=U- f[l -(l -ki)1’2]sinh2u, (13) y = n/2 + k, cash u, (14) where u is the running variable and k. an indepen- dent variable that determines the ratio between the electrode gap and the width of the electrodes (or the gas discharge). In fig. 1 the electrode profile is shown for a k. value of 0.02 according to eqs. (13) and (14) (curve B) and compared to Chang’s profile (curve A). The exact profile found from eqs. (2) and (3) with optimized values for ko, kl and u as well as the profile from [3] with an eighth-power function for k differ only slightly from curve B. All x and y values in the figures have been normalized toy0 = 7r/2 t k,. It can be seen that our profile is 10 to 15% smaller than Chang’s ones, and the expressions are simpel and have only one free parameter. Moreover, the uniform- ity of the field-strength distribution over the electrode surface is greatly improved. This is demonstrated in fig. 2, where a plot has been made of the deviation of the field-strength at the electrode surface from its central value, normalized to this central value. Curve A relates to Chang’s profile and the curves B, B’, and B” relate respectively to our new profile according to eqs. (13) and (14) the profile according to eqs. (2) and (3) and the profile from [3]. The curves B, B’, Fig. 1. The shape of the optimized profiles for three different cases. Curve A is Chang’s profile, curve B relates to eqs. (13) and (14); curve C to eqs. (16) and (17). For all cases ko = 0.02. Volume 49, number 4 OPTICS COMMUNICATIONS 15 March 1984 1 LIE E (0) I lo- lo- lo- lo- lo- 4 t Fig. 2. The normalized deviation of the electric-field strength at the surface of the electrodes from the central value for Chang’s profile (curve A), for the profile from eqs. (13) and (14) (curve B), for the profile from eqs. (2) and (3) (curve B’), for the profile from [ 31 (curve B”), and for the profile from (16) and (17) (curve C), plotted against the normalized position at the electrode. For all cases ko = 0.02. and B” all lie close together and show a much better field-strength uniformity than curve A. So, it can be expected that our smaller profiles produce a wider gas discharge. If necessary, further improvement of the profile is possible by adding extra terms in the conformal transformation, for instance: {=w+kosinhw+kl sinh2w+k2sinh3w. (15) Now the parameters k,, k,, k,, and u can be used to further optimize the field-strength distribution by re- quiring all three coefficients E,, E4, and E6 from eq. (8) to vanish. The calculations show that the optimiz- ed value of LJ lies closer to n/2 than in the former case. For instance, for k. = 0.02 the optimized values for k,, k,, and u are: k, = 0.5000125 E - 4, k2 = 0.8334357 E - 7, and v = 1.570796. So, the profile is given by: x=u-kkl sinh2u (16) y = 1r/2 + k, cash u - k, cash 3u, (17) and the optimized coefficients k, and k2 can be found by requiring the coefficients E2 and E4 to vanish, which gives the cubic equations with the following roots: k, = ; (1 - [l - (k. - 9k2)2] “2}, (18) k,=&k,(l-[l-$(l-8k,/k;+64kf/k;)]’IZ}. (19) The values of k, and kz can be easily found after some iteration steps. A good approximation often is k, = ek; (20) k, = & k;. (21) A plot of this profile, with k, = 0.02, is shown in fig. 1, curve C, and its field-strength distribution is shown in fig. 2, curve C. Again, improvement of the uniformity of the field-strength distribution is found. Of course, further improvement can be achieved by adding extra higher-order terms in the conformal trans- formation, so that in principle any degree of uniformi- ty in field strength at the electrode surface can be ob- tained. However, this will be of little practical impor- tance. Moreover, it is obvious that our procedure, re- sulting in simple analytic formulas, give the minimum possible electrode width. It should be pointed out that truncation of the electrodes according to eqs. (16) and (17), which can be done at the electrode backside, has only little in- fluence on the uniformity of the field-strength distri- bution at the central part of the electrodes. References [l] W. Rogowski, Arch. Electrotech. 12 (1923) 1. [2] T.Y. Chang, Rev. Scient. Instr. 44 (1973) 405. [3] G.J. Ernst, Optics Comm. 47 (1983) 47. 277 

Uniform-field electrodes with minimum width

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