Transcript Ion Motion Through an RFQ

Ion Motion Through an RFQ
Varying the q-value
Varying the Buffer Gas Pressure
The q-value
We define the q-value for an ion inside an RFQ device to be:
q
2eV pp
mr 
2
0
2
Where:
e = the charge on the ion
Vpp = the peak-to-peak voltage applied to the RFQ
m = the mass of the ion
r0 = the radius of the RFQ
Ω = the angular frequency of the applied RF-field
The q-value governs the stability of the ions paths in the RFQ. In
an RFQ the ion motion is stable for q ≤ 0.908. For q-values
higher than 0.908 the ion motion is unstable and hence the ions
are not trapped.
The Effect of Varying the q-value
q=0.1
q=0.3
q=0.5
q=0.7
q=1
Buffer Gas Cooling
An ion beam can be cooled via collisions with an inert buffer gas
however such collisions cause the beam to diverge. The RFQ
provides a force that pushes the ions onto its Z-axis hence an ion
beam can be cooled inside an RFQ via collisions with a buffer gas
without the beam diverging. By varying the buffer gas pressure
one can control the time it takes for cooling to take place.
The Effect of Varying the Buffer Gas Pressure
q=0.4_p=1^-2
q=0.4_p=1^-3
q=0.4_p=2.5^-2
q=0.4_p=5^-2
q=0.4_p=5^-3