The
gas stopping station will function as a high efficiency, high pressure,
gas-filled ion source for exotic nuclei created at the NSCL.The
gas stopping cell is designed to slow down and thermalize the extremely
fast ions produced in nuclear reactions in the A1900.The
fast ions penetrate through an entrance window and then enter the
cell that is designed to operate with helium gas at ~1 bar (1 atmosphere)
pressure. This high pressure is needed to stop the high-energy ions in
a short distance. The ions and the helium gas leave the collection cell
through a hole that has the shape of a supersonic nozzle. After exiting
the gas cell a low-energy ion beam will be formed by stripping off the
neutral helium gas. The ions then go into the ion-optical beam
transport section of LEBIT.
The
exotic ions will pass through a degrader system and an entrance window,
losing most of their kinetic energy. The
remainder of the ion’s energy will be dissipated in the helium gas cell,
contained in a cell inside of Cross-A, shown in the adjoining picture. The
ion will be carried downstream to a supersonic nozzle by electric potentials and then out through the nozzle by the very high
velocity gas flow. The helium gas will expand after the nozzle and is pumped
away by a VERY large vacuum pump. The ions, on the other hand, are
held on the axis by a RFQ ion guide in Cross-A. A second RFQ ion
guide will constrain the ions as they move into Cross-B and then into Cross-C.
The helium will expand and be pumped away so that the pressure becomes
lower as the ion moves away from the nozzle.
Each
of the differentially pumped chambers downstream of the skimmer will contain
a RF structure. The first RFQ can be seen in the adjoining photograph.
The ions move in the central region of the four circular rods.
The rods are electrically isolated and segmented to provide both a drift
field and a focusing field for the ions. The final task for the gas
stopping station is to provide the ions with a kinetic energy of 5 keV
so that they can be sent to the ion accumulator and buncher on the other
side of the concrete wall.