When the electron beam passes through the cavity it excites the resonant transverse electromagnetic modes TM010 at 1497 MHz. The large area probe loop provides an output signal that is proportional to the current. The output signal represents power induced by the beam inside the cavity, and it is proportional to the Q factor of the cavity, which is defined as the ratio of the total energy W stored in the cavity to the total dissipated power Pd, weighted by the resonant frequency .
If we assume that dissipation of energy occurs only on the walls of cavity, then For our cavities Q is about 3000. One see from (1) and (2) that Q is sensitive to change in the resonant frequency, which is caused to change in the dimension of cavity. Since changes of the dimensions of the cavity are related to changes in temperature, when performing measurements one must be sure that the temperature inside current box system is kept constant. The temperature controller mounted in the system keeps T as low as 0.2 Co. This temperature fluctuations leads to an error of 0.07 % in the measurement of the beam current.The power from the electron beam current is coupled into the large area antenna through TM010mode, which is relatively insensitive to the radial displacement of the beam position from the axis of the cavity. However, there is some radial dependence. This can be expressed in term of the radial dependence of the axial component of the of electric field (Ez()):
Thus variation of Ez by change in beam position represents the position dependence of the current measurement. The effect of beam displacement is about 0.06 % for 1 cm of radial displacement.The output from each cavity is a high frequency signal, 1497 MHz, that has a high attenuation, even in heliac 0.5 inch cables. For that reason down converters are installed in the hall close to the cavities, to transform 1.497 GHz signal to 1 MHz signal. The 100 meters long cables that carry the 1 MHz signal should be separated from each other as much as possible, otherwise cross talk between the cables can introduce additional errors. These errors can be as large as 0.1 %. In the counting house the 1MHz signal is filtered and amplified by a factor of 4 dB by RF units. These units should be checked periodically, since they can produce noise of 10 percent. If it is noisy the signal should be fed directly to the digital voltmeters, avoiding the units. However, the linearity of DVM is better at higher signal value and because of their gain it is preferable to use the RF filters. For signals between 1 to 5 V the linearity of the DVM is better than 0.01 percent and for signals from 100 mV to 1 V the error introduced by the DVM nonlinearity is about 1 %.