Measurement of the Instantaneous 200-MHz Bunch Currents at 750 keV for the CERN LINAC 2

The efficiency of the longitudinal matching of the low energy proton beam with a double drift harmonic buncher (DDHB) depends on the correct setting of RF levels of the first and second harmonic buncher cavities and their respective phase. The adjustment is done by means of a retractable destructive broadband probe situated in front of the Linac input. It is essentially a piece of a well matched 50 Ω line intercepting the beam perpendicular to its axis (Fig. 1). At the beam side it is an asymmetric double strip line, forming a 1 mm gap between a highly transparent grid (90%) and a tantalum insert stopping the incoming beam on the inner conductor. One side of the line is open ended, the other smoothly tapered over a length of 30 mm into a coaxial line, continued outside the vacuum with a flexible low loss cable to observe the bunch signal on a 1.2 GHz real time oscilloscope. This signal should give the longitudinal proton density distribution and permits to optimize the bunching efficiency and to adjust the phase between the two buncher cavities to about 10 at 200 MHz. For high intensity beams the interpretation of the bunch shape requires some precaution as secondary and thermal electrons modify considerably the form of the expected signals.