fig5 shows the first embodiment of the present invention . fig5 corresponds to fig3 described earlier , and the base 10 is omitted from fig5 for the sake of convenience . in fig5 parts that correspond to parts in fig3 are given the same reference numerals and the descriptions thereof are omitted . referring to fig5 the present embodiment employs an anode electrode 16 that is provided to oppose the base 10 ( not shown ) as well as to the insulating layer 11 provided on the upper major surface of the base , and a negative voltage is applied to the anode electrode 16 instead of a positive voltage . there , a negative voltage is applied to the anode electrode 16 by closing a switch sw when effecting a cleaning process . at the same time , a negative voltage is applied to the cathode electrode 12a and a positive voltage applied to the cathode electrode 12b , so that electrons are emitted from the emitter 15a by the field emission effect and reach the emitter 15b along a path connecting the emitter tip 15a to the emitter tip 15b . according to this embodiment , since a negative voltage is applied to the anode electrode during a cleaning process , electrons emitted from the emitter tip 15a reach the emitter tip 15b with high efficiency , so that a cleaning process is effected efficiently . in normal operation , the switch sw is opened , and a positive voltage is applied to the anode 16 . fig6 represents a potential distribution formed in a field emitter array when a voltage of - 1 v is applied to the emitter tip 15a , a voltage of + 1 v to the emitter tip 15b , and a voltage of - 1 v to the anode electrode 16 . as in the case of fig3 fig6 is reversed left to right in relation to fig5 . in the calculation of fig6 it is assumed that the gate electrodes 13a and 13b are grounded . as can be seen from fig6 electrons emitted from the emitter tip 15a are repelled by the electric field created by the anode so that regardless of the angle of incidence of the electrons with respect to the anode , the electrons are urged to return to the emitter tip 15b . some of the electrons are captured by the gate electrodes 13a and 13b , while others are captured by an intense electric field formed around the emitter tip 15b and collected by the emitter 15b . comparing fig6 with fig4 it will be noted that potential distributions near the emitter tip 15a differ significantly over the potential distribution around the emitter tip 15b . it is evident that a potential distribution around the emitter tip 15b shown in fig6 facilitates the collection of the electron beam to the end portion of the emitter tip 15b . while the magnitude of a negative voltage applied to the anode electrode depends on the configuration of the field emitter array , it is generally effective in this embodiment to set the magnitude of a negative voltage applied to the anode electrode to be larger than the voltage applied to the emitter tip 15a . next , a description of the second embodiment of the present invention will be made with reference to fig7 illustrating a field emitter array 30 . in the second embodiment , a pair of electron - beam sources are selected consecutively , starting from one end of an electron - beam source array and proceeding to the other end , and the above - mentioned excitation voltage is applied to the selected pair to form the electron beam connecting therebetween , as shown in fig1 . referring to fig7 ( a ) through 7 ( c ), the field emitter array 30 comprises : an insulating layer 32 formed on an insulating base 31 ; cathode electrodes 33a , 33b , . . . provided at the boundary between the above - mentioned insulating layer base 31 and the insulating 32 ; through - holes 32a formed in the above - mentioned insulating layer 32 to expose the above - mentioned cathode electrodes 33a , 33b . . . ; emitter tips 34a , 34b , . . . provided in correspondence to the above - mentioned through - holes ; gate electrodes 35 provided on the upper major surface of the above - mentioned insulating layer 32 ; and an anode electrode 36 provided to oppose to the above - mentioned emitter tips 34a , 34b , . . . . the emitter tips 34a , 34b , are arranged into a plurality of groups and form electron - beam sources a , b , c , d , . . . . in this illustration , the electron - beam source a is formed on one end of the electron - beam source array . in a state shown in fig7 ( a ) , the electron - beam source a and the neighboring electron - beam source b are selected and an electron beam is formed to extend from the beam source a to the source b . thereby , the emitter tip 34b in the beam source b is cleaned by the electron beam . after the electron - beam source b is cleaned in the process of fig7 ( a ), the process proceeds to a state shown in fig7 ( b ) wherein the electron - beam source b and the neighboring electron - beam source c are selected and an electron beam is formed to extend from the beam source b to the source c . thereby , the emitter tip 34c in the beam source c is cleaned . next , in a state shown in fig7 ( c ), the electron - beam source c and the electron - beam source d are selected , and the emitter tip 34d in the electron - beam source d is cleaned by an electron beam radiated from the electron - beam source c to the electron - beam source d . in such a cleaning process , it should be noted one has to apply a large negative voltage to the electron - beam source a which is selected first for causing the emission of the electrons . it should be noted that the electron - beam source a is not subjected to any earlier cleaning process and hence a large excitation voltage is required to cause the desired electron emission . on the other hand , the electron - beam source b , which effects an electron emission in the process shown in fig7 ( b ), or the electron - beam source c , which effects an electron emission in the process shown in fig7 ( c ), has been cleaned already in the earlier process , so that a voltage required for field emission of electrons therefrom becomes lower than the excitation voltage used for the electron - beam source a . fig8 ( a ) through 8 ( e ) are time charts illustrating how the above - mentioned cleaning process proceeds . fig8 ( a ) shows voltages applied to the above - mentioned electron - beam source a and timings of that application ; fig8 ( b ) shows voltages applied to the above - mentioned electron - beam source b and timings of that application ; fig8 ( c ) shows voltages applied to the above - mentioned electron - beam source c and timings of that application . similarly , fig8 ( d ) shows voltages applied to the n - 1th electron - beam source and timings of that application ; fig8 ( e ) shows voltages applied to the nth electron - beam source and timings of that application . as shown in fig8 ( a ) and ( b ), a negative voltage v e1 is applied to the electron - beam source a in an interval t 1 , and a positive voltage v x1 is applied to the electron - beam source b at the same timing . after an electron beam is radiated from the electron - beam source a to the electron - beam source b in this state , a negative voltage v e2 smaller in magnitude than the voltage v e1 is applied to the electron - beam source b in an interval t 3 , as shown in fig8 ( c ) and ( d ). at the same time , a positive voltage v x2 , smaller in magnitude than the voltage v x1 , is applied to the electron - beam source c . as a result , an electron beam path from the electron - beam source b to the electron - beam source c is formed , so that the emitter tip of the electron - beam source c is cleaned . after that , the electron - beam sources are cleaned consecutively by sequentially selecting a next pair of the electron - beam sources and applying the voltages v e2 and v x2 between the selected electron - beam sources . as shown in fig8 ( d ) and 8 ( e ), at the end of the process , the positive voltage v x2 is applied to the above - mentioned n - 1th electron - beam source , and the negative voltage v e2 is applied to the nth electron - beam source which sources are located at the other end of the electron - beam source array . the above - mentioned process can repeat itself a plurality of times as indicated in fig8 as &# 34 ; 1st cycle &# 34 ; and &# 34 ; 2nd cycle &# 34 ;. when repeating the process , in consideration of the fact that each of the electron - beam sources a , b , c , . . . . has already been subjected to at least one cleaning process , the applied negative voltage v e3 is set to be smaller in magnitude than the above - mentioned voltage v e2 , and the applied positive voltage v x3 is set to be smaller in magnitude than the above - mentioned voltage v x2 . thereby , it is possible to minimize the wear of the emitter tips by gradually decreasing the level of excitation voltage as the cleaning proceeds . in the present embodiment , it is particularly advantageous to provide the electron - beam source a as a special , cleaning - purpose - only electron - beam source for initiating the cleaning process at the end or marginal region of the electron - beam source array . the voltage applied to the electron - beam source for effecting a cleaning process may be fixed at v x for easy control hereof . next , a third embodiment of the present invention will be described with reference to fig9 ( a ) and 9 ( b ). in fig9 ( a ) and ( b ), those parts that were already described are given with the same reference numerals as in the previous drawings , and the description thereof will be omitted . in fig9 ( a ) and ( b ), electron - beam sources are identified by the numerals given to the cathode electrodes . referring to fig9 ( a ), a plurality of electron - beam sources , independently driven during normal operation , are grouped into two , mutually adjacent electron - beam source groups 33a and 33b during the cleaning process . in a state shown in fig9 ( a ), a positive voltage is applied to the electron - beam source group 33a , and a negative voltage is applied to the electron - beam source group 33b . a negative voltage is applied tc the anode electrode 36 by closing the switch sw . in this state , an electron beam is radiated from each electron - beam source group 33b to respective sources of the source group 33a , so that the emitter tips in the electron - beam source group 33a are cleaned . for example , the electron - beam source group 33a may represent the electron - beam source group corresponding to drive lines having an odd number , and the electron - bean source group 33b may represent the electron - beam source group corresponding to drive lines having an even number . see the perspective view of fig1 and the arrangement of the cathode and gate electrodes 12 and 13 shown therein . in a process shown in fig9 ( b ) following the process shown in fig9 ( a ), the voltage applied to the electron - beam sources is reversed , i . e ., a negative voltage is applied to the electron - beam source group 33a , and a positive voltage is applied to the electron - beam source group 33b , while the positive voltage applied to the anode electrode 36 remains the same . in this state , the emitter tips in the electron - beam source group 33b are cleaned by the electron - beams emitted from the electron - beam source group 33a . the cleanness of the emitter tips in each electron - beam source group is gradually improved , by repeating the processes shown in fig9 ( a ) and 9 ( b ) in an alternating manner . fig1 ( a ) and 10 ( b ) show voltages applied to the electron - beam source groups 33a and 33b when repeating the processes shown in fig9 ( a ) and 9 ( b ) in an alternating manner , wherein fig1 ( a ) shows voltages applied to the electron - beam source group 33a , while fig1 ( b ) shows voltages applied to the electron - beam source group 33b . as can be seen from fig1 ( a ) and 10 ( b ), at the interval t 1 , the negative voltage v e1 is applied to the electron - beam source group 33a , and the positive voltage v x is applied to the electron - beam source group 33b . at the next interval t 3 , separated from t 1 by the interval t 2 , the positive voltage v x is applied to the electron - beam source group 33a , and the negative voltage v e2 , smaller in magnitude than the previous negative voltage v e1 is applied to the electron - beam source group 33b . as the above - mentioned process is repeated , the magnitude of the negative voltages is controlled to decrease as per v e3 , v e4 , v e5 , . . . . upon reaching the voltage v e5 , the negative voltage is maintained at a constant level . by setting the excitation voltage in this way , a maximum cleaning effect is achieved while minimizing wear of the emitter tips . the number of electron - beam sources contained in the electron - beam source groups 33a and 33b and cleaned simultaneously may be set as appropriate depending on a adsorption capability of the getter not shown in the drawing . next , a description will be given of the fourth embodiment of the present invention with reference to fig1 , wherein fig1 illustrates a field emitter array 40 according to the fourth embodiment of the present invention . referring to fig1 , the field emitter array 40 is formed on an insulating base 41 , on which base formed an insulating film 42 . cathode electrodes 43a and 43b , corresponding to electron - beam sources 43a and 43b , are provided at the boundary between the insulating film 42 and the base 41 . a plurality of through - holes , corresponding to the cathode electrodes 43a and 43b , are formed in the insulating film 42 . on the surfaces of the cathode electrodes 43a and 43b , there are provided one or more emitter tips 44s each having a cone shape in correspondence to the part exposed by the through - holes . further , gate electrodes 45 are formed on the upper major surface of the insulating film 42 . further . there is provided an insulating base 47 above the above - mentioned base 41 as illustrated in fig1 , and the base 47 carries thereon a plurality of electrically separated anode electrode elements 48a , 48b , . . . at the side facing the above - mentioned electron - beam sources . the electrode elements 48a , 48b , . . . and the insulating base 47 as a whole form an anode 46 . fig1 further shows a configuration by which the emitter tips 44 are cleaned in a field emitter array of this configuration . in the illustration , the negative voltage v e1 is applied to the emitter tips 44 formed on the cathode electrode 43b , and the positive voltage v x is applied to the emitter tips 44 formed on the cathode electrode 43a , to that an electron beam is radiated from the plurality of emitter tips in the electron - beam sources 43b to the plurality of emitter tips in the electron - beam sources 43a , so that the emitter tips 44 in the electron - beam sources 43a are cleaned . in this embodiment , as in the previous embodiments , a negative voltage is applied to the anode electrode elements 48a , 48b , . . . . this embodiment is unique in that three kinds of power supplies for generating negative voltages vh1 , vh2 , vh3 are provided as anode power supplies ( vh1 & lt ; vh2 & lt ; vh3 ), and these negative voltages vh1 , vh2 , and vh3 are sequentially applied to three anode electrode elements 48f , 48e , and 48d arranged in a row , and also to the anode electrode elements 48c , 48b , 48a arranged in a row . as a result of this arrangement , an asymmetric potential distribution is formed increasing in magnitude from the anode electrode element 48f to the element 48d , and also from the anode electrode element 48c to the element 48a , with the result that a trajectory , along which the density of the electron beams becomes maximum , is bent toward the electron - beam sources 43a , and electrons are captured by the emitter tips 44 with high efficiency . the values of the voltages vh1 , vh2 , and vh3 are set , for example , to increase generally linearly with the positions of the electrode elements . for example . vh1 and vh3 are controlled to be 20 % different from each other in magnitude . the above - mentioned cleaning process may be achieved at the vacuum sealing process of the field emitter array , which process is included in the processes for manufacturing a field emitter array . the volatile substance is absorbed onto the surface of the emitter tip more or less immediately after a sealing process thereof , so there is a need for a cleaning process to be effected before shipping the device . in such a process carried out before shipping , it is effective to apply the intense negative voltage v e1 to the electron - beam source a specifically provided for the cleaning purpose as described with reference to fig8 ( a ). it is convenient , in a case where a field emitter array is built into an electronic apparatus and then shipped , to carry out a cleaning process right after turning on the power of an electronic device . generally , a variety of checking and diagnosing programs are executed right after turning on the power of an electronic device , therefore , by effecting a cleaning process during this initial period , an amount of extra time , required for a cleaning process , could be saved . also , since such a cleaning process does not require a high temperature , there is no fear of adversely affecting other parts of an electronic apparatus . it is possible , in order to deal with emitter tip gas absorption related to age thereof , to form a configuration such that an operating time of a field emitter array is monitored by means of a timer , so that a cleaning process be initiated after a predetermined period of time elapses . another configuration is possible such that a decrease of an anode current is monitored , and an alarm lamp is lighted when the anode current drops below a predetermined level , thus indicating a need for a cleaning process . the present invention is not limited to the above embodiments , and various other changes and modifications may be made without departing from the scope of the claims .