The same leakage X-rays can also be detected more directly by the use of a high-Z blade in combination with a small in-air ionisation volume. This ‘electric version’ of an IAX detector consists of two parallel plates (or blades of 23 x 23 mm) separated by a small distance (typically 1 mm) in air. One of the blades is connected to a DC bias voltage (type 50V), while the other blade is connected via a short coaxial cable to a high impedance electric measurement device.

Interaction of the high energy X-rays with the high-Z material causes ionisation of the air molecules in the small volume between the two blades. This conductive state together with the applied DC bias voltage allows measurement of a strong voltage signal.

With the detector positioned on the slope of a ~ 100 um fwhm vertical profile the recorded voltage signal is extremely sensitive to vertical beam position variations. The linearity is acceptable for beam displacements of ± 20 um.

The performance in the AC range up to 400 Hz is shown in recordings with the global feedback system either active or off (Figure 155). Extensive tests have been carried out to assess the resolution and to compare with other diagnostic methods. The in-air blade monitor, that benefits from its location on a high vertical beta in the machine lattice, demonstrates a superior resolution to both the fast electron beam position monitors (BPMs) and the classical X-ray beam position monitors using blades sensitive to UHV. Eight of these devices are scheduled for installation in 2007. The interest of such monitors is in the further stabilisation of the orbit in the vertical plane beyond the present state limited by the noise in electron BPMs.

Fig. 155: Vertical beam positioning [um] vs time and associated FFT observed through an electric IAX monitor with or without the position feedback in operation.