L. H. GRAY 



Ltd., employs the axial component of the electric field associated with 10 cm 

 radio waves travelling along a loaded cylindrical wave guide to accelerate 

 bunches of electrons to an energy of around 1 -5 MeV. Radio-frequency 

 power from a 2 MW magnetron is supplied in pulses of approximately 

 2 [xsec duration, as in the 4 MeV travelling wave accelerators in use as a 

 source of X radiation in a number of Radiotherapy Departments-^- -''. Our 

 accelerator differs from the clinical instruments in the construction of the 

 waveguide, which is designed for maximum current at 1 to 1-5 MeV, and 

 does not employ feedback of unused power from the output end of the wave- 

 guide. This has important advantages for single pulse operation and does 

 not necessarily waste any appreciable amount of power. Figure 2 shows the 



10 



fc 0-8 



o 



^0-6 



CO 



c2! 



04 

 0-2 



^Remanent power - 

 S<^ in absorbing load 



> 



20 



L. 



0) 



c 



LU 



10 



0-1 0-2 0-3 OA 0-5 06 07 



Pulse beam current amp 



Figure 2. Performance of high current accelerator — input power 

 approximately 1 • 8 MW 



(Reproduced by kind permission of C. W. Miller and J. IT. Boag) 



current voltage and power characteristics of our instrument when the power 

 input is 1 • 8 MW. It is evident that up to • 7 A of electron current may be 

 obtained during the 2 jasec pulse. Under these conditions the ratio of electron 

 beam power to input power is 0-43, which is slightly less than the ratio 

 usually obtained in the 4 MeV instruments which employ feedback. Im- 

 proved injection appears to be the key to any further increase in beam 

 current. A study of the problem of optimum power conversion by Saxon'^^ 

 indicates that it should be possible to achieve a ratio of 0-8 to 0-9 in a 

 practical waveguide which does not employ feedback. 



As supplied by the makers, the machine operates with repetitive pulses 

 at frequencies of up to 500 pulses per second. For studies of the kinetics of 

 irradiated chemical and biological systems reproducible single pulses are 

 required. These cannot readily be obtained from the standard modulator 

 which employs resonant charging of the pulse forming network. To obtain 

 single pulses, the pulse forming network is therefore trickle charged to 20 kV 

 from a low power D.C. rectifier set, the discharge being effected through the 

 usual hydrogen thyratron, to which a single triggering pulse can be applied. 



For certain experiments it was desired to obtain a pair of similar pulses 

 separated by an interval which could be varied from a few microseconds 

 upwards. Since the shorter intervals allow no time for recharging the pulse 

 20 289 



