A fixed-atom force field was a type of force field. It created a shield of positrons which would painfully repel any intruders. The weakness was that the shield would block entry from only one source, meaning that if two people tried to get through at the same time from the same side, it would only block one.
A fixed-atom force field was used to imprison the Miaxa. The Fifth Doctor was able to use the weakness of the force field to get inside his chamber to rescue him. This set off the emergency execution protocol, causing the field to contract until they were crushed to death. The Warden eventually decided to rescue the Doctor using the same trick, but refused to let the Miaxa out. Both were killed by the force field. (AUDIO: 1001 Nights)