It is demonstrated in this paper that it is also possible to obtain higher proton ratio in penning discharge of low pressure and low current. A penning ion source has been designed. It's outer diameter can be as small as 25mm; and it's proton ratio

as high as 30~50% under the pressure of 5×10

-5

mmHg and in 200~400 μA discharge current. It meets pretty well the needs of small sealed-off neutron tube.

It has been studied through experiments how the proton ratio is affected by some parameters

such as pressure

anode voltage

anode

diameter. magnetic field and recombination coefficient. An effective way has been found to raise the proton ratio. It is favourable for raising proton ratio to operate ion source at the upper limit of low pressure mode (~1×10

-4

mmHg)

to increase the strength of magnetic field and at the same time

to reduce properly the diameter of anode cylinder.