Ion Motion Through an RFQ

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• 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 =  2eVpp

mr02 W2

• 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
W = 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

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 (mBar)