Some important modifications were made to the Crooke's tube to make it a more efficient x ray source. Why was the coolant added to the anode plate?
Most of the kinetic energy of the electrons get converted into heat upon falling on the anode plate, turning it very hot.
As has been studied, bulk of the energy (around 99.9%) of the incoming electrons get converted to heat upon collision with the anode plate. Only the rest is actually used up in x ray production. This makes the anode plate very hot and unfit for long time use. The coolant is added so that the heating of the plate is kept under control and the Coolidge tube can be kept ON for longer periods of time.