Electron Bubbles
Helium atoms near an electron will exprience a short range repulsive potential and will form an entity called “electron bubble”. Such a bubble can be considered as a wall of helium atoms surrounding an electron, which responds to external forces as a whole. An electron bubble typically has a radius of ~18Å and a hydrodynamic mass m* ≅ 243 mHe. With a density lower than that of the liquid helium, even without any external electric field, the bubble will drift upward at 2g due to buoyancy alone.
With an electric field set up properly, the drift velocity of a bubble can be controlled. Since vortices will form once the velocity of bubble exceeds 40 m/s in the liquid, special care must be taken so that bubble will not travel faster than this critical velocity. Besides choosing a suitable field to apply, one can also add 3He into the liquid, which works as scattering sites to slow down bubbles. However, additional elementry excitations generated this way will effectively render quantum evaporation signals useless.
Once the electron is very close to the surface, a very high field is applied to help the electron tunnel through, followed by another high electric field to accelerate it to the detector. More information about pulling election out of liquid helium can be found in the article