Cooling techniques

In regard to cooling the ions, two methods have been used. One is to admit helium at moderately low pressure. The helium is roughly at the temperature of the apparatus and the collisions of the heavier ions with the helium atoms transfer kinetic energy to the helium on the average and cool the ions. The temperature will always be somewhat higher than that of the apparatus, but is usually low enough so that trapping times of as long as hundreds of seconds are achieved. The collision of the ions with helium atoms does produce a small frequency shift because the wave functions of the ions are perturbed during the collisions. This frequency shift can be estimated by varying the helium pressure and extrapolating to zero pressure.