Very high gas throughput is commonly adopted, and neutral-beam systems have custom vacuum pumps among the largest ever built, with pumping speeds in the range of million liters per second. If there are no space constraints, a large gas cell length formula_18 is adopted, but this solution is unlikely in future devices due to the limited volume inside the bioshield protecting from energetic neutron flux (for instance, in the case of JT-60U the N-NBI neutraliser cell is about 15 m long, while in the ITER HNB its length is limited to 3 m).