fig3 and 4 show a high - frequency - discharge excited gas laser according to a first embodiment of the present invention . reference numeral 1 designates metallic electrodes , each of which is a pipe made of iron , for example , and reference numeral 2 designates coverings ( hereinafter referred to as dielectric layers ) formed of dielectric ceramic pipes having ballast effect which is effective to obtain uniform electric discharge . each dielectric layer 2 covers the respective metallic electrode 1 with a clearance 3 . two electrodes each formed by the metallic electrode 1 and the dielectric layer 2 are arranged in a laser tube , not shown . as a high frequency voltage , generated by a high frequency power source 4 , is applied between these metallic electrodes 1 , 1 , the laser gas flowing in the laser tube is excited to thereby create laser beams . the high - frequency - discharge excited gas laser of the present embodiment differs from the conventional one in that the metallic electrode 1 is separated from the dielectric layer 2 by the clearance 3 , and there is no further structural difference between the two . incidentally , it should be noted that the provision of the clearance 3 is not necessarily required , but the dielectric layer 2 can be laid on the metallic electrode 1 without a clearance therebetween , so long as the layer 2 is not fastened to the metallic electrode . also it is possible to fill the clearance 3 with a highly electric conductive paste . the voltage drop v that occurs in the dielectric layer 2 formed as above is given by the following equation , which is obtained by a simple calculation . where i 0 is the current density , ε the dielectric constant of the dielectric layer 2 , ω the applied voltage frequency , and d the thickness of the dielectric layer 2 . in order to stabilize the electric discharge , the voltage drop v ought to be a value in the vicinity of the electric discharge sustaining voltage , so that to find an appropriate thickness d of the dielectric layer 2 , v 0 is substituted for v in the equation ( 1 ), and the resulting equation is solved for d to obtain an equation : in the case of the embodiment shown in fig3 since there are two dielectric layers 2 , 2 , the thickness of each dielectric layer 2 ought to be one half of the value d obtained by the equation ( 2 ). the electric field intensity e created inside the dielectric layer 2 is given by an equation : if e 0 is the dielectric breakdown electric field intensity of the dielectric layer 2 , then the dielectric breakdown is avoided so long as the electric field intensity e is equal to or lower than e 0 , i . e ., which tells that the maximum allowable current density is given as e 0 εω , so that in order to increase the maximum allowable current density , either the frequency should be increased or a material whose dielectric constant ε and dielectric breakdown electric field intensity e 0 give a greater product e 0 ε should be selected . therefore , by increasing the maximum allowable current density and hence increasing the tolerable level of input electric power per unit volume for electric discharge , it is possible to embody a high - frequency - discharge excited gas laser capable of generating an output whose intensity exceeds 1 kw . since the maximum allowable current density does not depend on the thickness of the dielectric layer 2 , the thickness can be selected so as to avoid formation of pinholes , etc ., and therefore adaptable manufacture of the dielectric layer 2 is possible . in view of the electrical properties mentioned above , appropriate materials for the dielectric layer 2 include barium titanate , quartz , alumina , mylar , and polyimide . in the embodiment described above , both metallic electrodes 1 , 1 are covered with dielectric layer 2 , but it is also possible to construct an equally effective laser by having only one of the metallic electrodes covered with dielectric layer 2 and the other naked . also , the cross section of the electrode 1 need not be circular , but may be of any shape such as quadrilateral . fig5 shows a coaxial type high - frequency - discharge excited gas laser according to a second embodiment of the present invention , wherein reference numeral 21 refers to a laser tube in the shape of a rectangular parallelepiped pipe made of dielectric ceramic such as alumina . a hollow 21a having a cross section of square l × l in size is formed in the laser . as metallic electrodes , metallic films 22 , 23 are respectively formed on a pair of opposite outer surfaces of the laser tube 21 , i . e ., a top face 21b and a bottom face 21c by means of fused metal injection , for instance . these metallic films 22 , 23 are connected to a high frequency power source 4 , so that the laser gases ( a mixture of co 2 , n 2 , he and so on ) flowing through the hollow 21a of the laser tube 21 are excited . since the laser tube 21 , itself as a structural component , has a sufficient mechanical strength , the metallic films 22 , 23 need to have good electrical conductivity alone , and thus they can be made as thin as one can desire . consequently , unlike the conventional case , in the case of the present embodiment there is no strict requirement concerning the conformity in characteristics between the dielectric body and the metallic film such as thermal properties , so that the choice of the dielectric material for the laser tube 21 is allowed to be made purely from the viewpoint of the electric properties . in particular , when the metallic films 22 , 23 are formed by fused metal injection -- which is done , for instance , by highly ionizing metallic powder such as copper and aluminium in he gas , and injecting the powder onto the surfaces 21b , 21c of the laser tube 21 to form metallic films 22 , 23 of thickness 30μ --, the resulting metallic films 22 , 23 become porous . therefore , even if the thermal expansion coefficient of the films 22 , 23 is not equal to that of the dielectric body 21 , there is little fear of cracking and peeling of the films . in contrast , in the conventional laser pipe in which the dielectric body is formed of a dielectric layer adhered to the metallic electrode , if the porous dielectric layer is formed on the metal by fused injection , the laser pipe is quite useless , because the applied current easily penetrates the porous dielectric layer , and hence no ballast effect is obtained . it is therefore only through forming a metallic film on the dielectric body , as in the present invention , that it becomes possible to form a laser electrode combining a dielectric body and a metal which are dissimilar in thermal expansion coefficient and other properties . also , since only 1 ampere or so of electric current need flow through the metallic films 22 , 23 , the thickness of the films can be as thin as 30μ , which is very thin compared with that of the dielectric body ( which is generally 2 - 3 mm ). this fact renders it unnecessary to form the films 22 , 23 by fused metal injection ; plating or evaporation will suffice to form the films , and further the films may be adhered to the dielectric body with electrically conductive adhesive . in order to make it doubly sure that the metallic films 22 , 23 won &# 39 ; t peel off , they may be formed on the outer surface of the laser tube 21 in a square wave pattern , as shown by reference numeral 5 in fig6 and this can be done through fused metal injection , evaporation , or plating . further , the metallic film 5 may be glued with an adhesive . alternatively , many metallic foil pieces 6 may be adhered to the laser tube 21 , as shown in fig7 with the foil pieces 6 being connected together via a leading wire 7 . those walls of the laser tube 21 on which the metallic film 22 or 23 is not formed are made relatively thick ( e . g ., 5 mm in thickness ) to increase the mechanical strength of the laser tube 21 . on the other hand , the upper and lower walls on which the films 22 and 23 are respectively formed have the smaller thickness d , which is given by equation ( 2 ) as vεω / 2i 0 , whereby the voltage drop through these walls becomes equal to the electric discharge sustaining voltage of the laser plasma to thereby stabilize the electric discharge . the electric field intensity e created inside the dielectric body is given as i 0 / εω by equation ( 3 ); in order to avoid dielectric breakdown of the dielectric body , the value i 0 / εω should be less than the dielectric breakdown electric field intensity e 0 of the dielectric body , wherefore the maximum allowable current density i 0 max and the maximum allowable input electric power wmax are given as follows , respectively ; where l is the overall length of the laser tube 21 , 1 the width of the metal film , ε 0 the absolute dielectric constant , and k the relative dielectric constant . to increase the thickness d of the upper and lower walls for the purpose of increasing the mechanical strength of the laser tube 21 it is necessary to lower the current density , to increase the voltage , to increase the frequency , or to use a material of higher dielectric constant . similarily , to increase the maximum allowable input electric power wmax , it is necessary to increase the voltage , to increase the frequency , or to use a material of greater dielectric breakdown strength eo . the table below lists the required thickness d and the maximum allowable input electric power wmax , respectively , of various materials , in the case where l = 3 cm , l = 1 . 6m , electric discharge sustaining voltage v = 1 kv , and ω / 2π = 100 khz . ______________________________________dielectrics k e ( kv / cm ) d ( mm ) wmax ( kw ) ______________________________________batio . sub . 3 1200 30 53 . 4 96 . 2sio . sub . 2 3 . 8 160 0 . 17 16 . 2beo 10 . 1 236 0 . 45 63 . 6al . sub . 2 o . sub . 3 9 . 4 394 0 . 42 98 . 9______________________________________ incidentally , as shown in the equations ( 3 ), ( 4 ), power w is proportional to both the product v of the voltage v and the applied frequency ω , so that by increasing either or both of these values v and w , it is possible to obtain values greater than those listed in the above table . according to the table , alumina ( al 2 o 3 ) is a satisfactory material . although the dielectric body , which is required to provide the mechanical strength of the electrodes , is made of ceramic in the above embodiments , the dielectric body need not be ceramic but can be made of any other dielectric material that has sufficient mechanical strength and desirous electrical properties . in the second embodiment , the laser to which the present invention is applied is of a coaxial type , but it is applicable to a laser of an orthogonal type .