Answer:
The value is 
Explanation:
From the question we are told that
The radius of the cylinder is 
The charge density of the plasma is 
Generally the difference in potential energy of the Tritium ion before entering the cylinder and at the axis of the cylinder is mathematically represented as

Here
is the permitivity of free space with value

So

Generally from energy conservation we have that

Given that the velocity at the axis of the cylinder should be zero then
it means that the kinetic energy at this axis will also be zero
So

Here KE_i is the initial kinetic energy of the Tritium ion which is mathematically represented as

So
=> 
=> 
=> 
=> 
Here m is the mass of Tritium ion which is given in the question
=> 
=> 