ELECTRICAL DESIGN OF ELECTROSTATIC DEFLECTORS FOR SECTORFOCUSED CYCLOTRONS [report]

B. H. Smith, H. A. Grunder
1963 unpublished
Because of the advance of the art of magnet design and improvements i n the r e g u l a t i o n of the e l e c t r i c a l parameters of t h e machine, t h e beam of t h e sector-focused cyclotron is more s t a b l e and b e t t e r focused than it w a s i n the conventional cyclotrons. The higher energy of these new machines r e q u i r e s a higher e l e c t r i c a l gradient than the older type, but i t is not p r o h i b i t i v e l y high and simply r e q u i r e s t h e same
more » ... s a s t h e other p a r t s of the machine. W e have been a b l e t o e x t r a c t t h e p a r t i c l e s t h a t need almost t h e highest gradient, 50 MeV protons, from the 88 Inch Cyclotron. The key t o d e f l e c t o r design is the a r t of t a i l o r i n g the e l e c t r o d e s t o the 1 ) geometry of the beam . For t h e 88 Inch Cyclotron the highest gradient required is 150 kV/cm. A t e x t r a c t i o n r a d i u s t h e height of t h e beam of u s e f u l p a r t i c l e s is about 0.25 in. The required d e f l e c t o r gap at t h e entrance is governed by the r a d i a l o s c i l l a t i o n s of t h e beam. Further down, the d e f l e c t o r gap is determined by t h e requirement of acconmtodating t h e d i f f e r e n t t r a j e c t o r i e s of t h e various beams. To provide a gradient of 150 kV/crn a t t h e entrance, we need a gap of about 0.25 in, and a voltage of 95 kV. Because of sparking, t h i s condition should be provided with as much margin as possible. W e decided t o build a test-model d e f l e c t o r t o obtain design d a t a f o r a f i n a l d e f l e c t o r f o r t h e 88 Inch Cyclotron (Fig. 1). To proceed with t h i s experiment we adopted t h e following hypothesis. The d e f l e c t o r high-voltage electrode oantains innumerable electron-emitting spots each of which s e r v e s as t h e cathode f o r a vacuum spark. These cathodes c o n s i s t of such t h i n g s as foreign contaminating material, spicules, and t h e gradient magnifying edges of the c r y s t a l s of the e l e c t r o d e material. If we assume gradient magnification, t h e ground electrode of t h e d e f l e c t o r provides s u f f i c i e n t gradient at t h e cathode s p o t s t o produce some f i e l d emission. The dimensions of t h e cathode a r e so small t h a t t h i s f i e l d emission involves a s u f f i c i e n t l y high current d e n s i t y t o heat t h e cathode t o thermionic emission temperature. Thus, t h e emission c u r r s n t increases with the e l e c t r i c gradient a t t h e cathode. The e l e c t r o n s follow t h e magnetic f i e l d l i n e s u n t i l they reach the spark anode. The power d e n s i t y here is thus proportional t o the t o t a l voltage and t h e e l e c t r o n c u r r e n t density. When the power d e n s i t y is s u f f i c i e n t t o vaporize the anode, a gaseous discharge occurs which we c a l l a spark. In the design of a n e l e c t r o s t a t i c d e f l e c t o r f o r a cyclotron, we believe t h a t the (*) Work done under auspices of the U. S. Atomic Energy Commision.
doi:10.2172/4707573 fatcat:4cxwkezgbjabpflcvkvgbax5cq