Resonant Cavities (BCMs)

• General Description

  Two resonnant cavities are positionned in the beam line behind and in front of the Unser monitor; they are respectively called "Upstream" and "Downstream" cavities. These cavities are two cylindrical wave guides made out of 304 stainless steel. Their dimensions are 15.48cm in diameter and 15.24cm in length. The resonnant frequency is adjusted to the 1497MHz frequency of the Cebaf beam by a stub tuner mounted on a micrometer that can be moved in and out of the cavity. Inside the body a magnetic field probe, a loop antenna, is located on the circumference, where the electrical field is minimum and the magnetic field is maximum. The probe is coupled to one of the resonnant modes of the cavity and used to couple the beam signal out of the cavity.

• Theory of operation

  The electron beam passing through the cavity will excite the resonnant transverse electromagnetic mode TM010 at 1497MHz and the probe loop will provide an output signal proportionnal to the current. This signal represents in fact the power induced by the beam inside the cavity, and is proportionnal to its Q factor. Q is the ratio between the total energy W stored in the cavity and the total dissipated power Pd, weighted by the resonnant frequency f:
  

  Electrical & Magnetic fields inside the cavity body

  Q=W.f/Pd or if we assume that the dissipation of the energy occurs only on the walls of the cavity: Q=Volume/Area Q=3000 for these cavities.

  
  

• Temperature dependance

  From the equations one see that Q is sensitive to the resonnant frequency of the cavity and consequently to its dimensions. Since the change of the dimensions is related to the temperature variation, an accurate measurement requires a constant temperature.
  
  

• Position dependance

  The power coupled from the beam to the cavity at radius r also depends for the TM010 mode (this mode is relatively insensitive to the position of the beam) on the variations of the electrical field along the z axis Ezr:
  

  P=const.Ibeam.Ezr

  
  thus the variation of Ez from the change in the beam position represents the position dependance of the current measurement.
  The effect of the beam displacement is about 0.06%/cm.
  
  

• Performances and limitations

  Unlike the Unser the cavities do not provide an absolute measurement of the current; this is a relative measurement because you collect a signal which is proportionnal to the beam intensity. So they need to be calibrated using an absolute device like the Unser monitor.
  However, as they are very stable and linear, once they have been well calibrated you can use them for the regular measurement on all the dynamic range.