Ion Motion Through an RFQ

1

• 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

• 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
• O 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.

2
The Effect of Varying the q-value
q0.1
q0.3
q0.5
q0.7
q1
3
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.
4
The Effect of Varying the Buffer Gas Pressure
q0.4_p1-2
q0.4_p1-3
q0.4_p2.5-2
q0.4_p5-2
q0.4_p5-3