Development of a rapid beam emittance measurement system using a real-time beam profile monitor

We have developed a rapid beam emittance measurement system for the K140 AVF cyclotron at Research Center for Nuclear Physics (RCNP). So far, an emittance monitor of conventional type section of a medium energy beam transport (MEBT) system to analyze the quality of the injected beam to the K400 ring cyclotron. Two kinds of emittance monitors were supplemented in the low energy beam line for evaluation of ion beams from ion sources. One of them is a conventional type consisting of position-variable slits and a three-wire profile monitor (TPM), similar to the one installed in the MEBT system of the AVF cyclotron. It takes about 30 minutes to get emittances in both the horizontal and vertical planes. For quick emittance measurements, we have developed a new system equipped with a fast moving slit with a fixed gap and a real time beam profile monitor (BPM83) with a rotating helical wire made of tungsten. The wire rotating speed is 18 rps. The BPM83 is controlled by a driver module BPM-FP3A. With this system the measurement time was considerably reduced to 70 seconds for both the horizontal and vertical emittances. The fast moving slits consist of two collimation blocks made of brass, and are inserted into the beam path at an angle of 45 degrees. The slits are scanned in the vertical plane at first, and then in the horizontal plane. The slits gap is variable, but at present it is fixed to 2 mm. A stepping motor for the slit is driven by a PLC system, and it takes only 70 seconds for a full stroke within 290 mm. The slit position is detected by a linear potentiometer. While moving the slits, the beam profile output from BPM-FP3A and the voltage of potentiometer are recorded at every position step simultaneously. The data acquisition system uses CAMAC and trigger signals generated by BPM-FP3A and NIM modules are provided for production of each beam profile at a frequency of 18 Hz, with 4.5 kHz sampling rate. A DAQ program runs on CC/NET (crate controller of CAMAC). When data acquisition finished, the data file is uploaded to a LINUX computer and analyzed automatically. First we calculate the slit position for each profile, then average profiles at every second millimeters. Time scale of the measured profile is converted into wire position. From the slit and the wire positions, we calculate the angle from a beam line axis. The measured slit position, the angle of the beam direction and a current density of the moment provides two dimensional beam current density distribution in phase space for estimation of emittance. Data analysis program is written by C, and the output is represented by gnuplot. Data analysis takes about 1 second. In total the horizontal and vertical phase space plots can be completed within 75 seconds from starting the first program. This rapid emittance measurement system will definitely make it easier to optimize parameters of ion sources and the beam transport system.