BCM Beam Current Monitor
Description from the Design Manual:
5.9.2 Beam Current Monitors
The current circulating in the Ring must be monitored to provide a measure of the cycle
intensity. Beam Current Monitors (BCM) will provide information of the average current, charge density distribution within the bunch.
18.104.22.168 Design Requirements
The beam current in the SNS Ring will be measured non-destructively using several
beam current transformers. The design average current (or charge per turn) will vary from 9
mA (5 x 1010 protons) for the first turn to 34 A (2 x 1014 protons) on the last turn injected.
Within the bunch the peak current may be twice this value. It is not intended to measure charge in the gap with this system. The limitations of the system were recognized early, and instead another system will address beam charge in the gap (BIG).
Average beam current in the SNS Ring will be measured by the BCM. The resolution
must be better than 0.1% of full scale and available on a turn by turn basis. The detector system, which will be a current transformer and electronics, will have a droop of less than 0.1% over the full cycle.
The charge density distribution within the bunch will be measured with the fast current
transformer (FBCM). The FBCM must have a rise time of 1ns or better to observe the 550 ns
bunch length, and a droop time constant of at least 100 us. Acquisition will be through the 100 MHz bandwidth of the Bunch Data Acquisition System.
22.214.171.124 Design Description
A standard BergozÒ FCT (Fast Current Transformer) has been selected for this application. It provides a 50 turn output winding developing 0.5 Volts per Amp into a 50 Ohm load. This FCT will have < 1ns rise time and a droop of <0.1% per microsecond making it suitable for the FBCM application. To achieve the low droop for the average beam current measurements, digital droop compensation will be employed in the electronics. To achieve a 1000 to 1 improvement in droop, the time constant of the transformer must be measured to an accuracy of about 0.1%. It is proposed to make measurements during the transformer recovery time between macro-pulses. This recovery signal will be analyzed by an exponential fitting routine to determine the transformer time constant. Numerous measurements can be averaged to provide an estimate within 0.1%. In addition, this transformer will include a second winding with 10 turns to act as a calibration winding. The calibration winding will be electrically isolated from the output winding permitting isolation of grounds. This transformer is available in a 220mm ID, sufficient for use on the 210mm beam pipe. For commonality it is proposed to use the same size transformer throughout the HEBT, Ring, and RTBT.
By taking the integral of the current in the bunch and sampling at the end of the
bunch, the turn-by-turn charge can be displayed.
The FBCM will use a commercially available sensor, a Fast Current Transformer (FCT),
manufactured by BergozÒ , with a risetime of <1 ns but at the sacrifice of a droop time constant of about 100 m S (0.1%/m s).
Units will be housed on a 0.5 meter section of beam pipe with a ceramic break to prevent the image currents from passing through the transformer aperture. A properly designed
outer cover will allow the wall current to gracefully pass around the transformer section.
To properly compensate the transformer droop, baseline restoration is necessary and will be accomplished digitally. The signal will be digitized with an 80MSPS ADC (AD6645-80). The analog output to the digitizer will also be available for direct viewing if desired. Adjustable gain will be provided to accommodate the wide dynamic range of the signal (1000 to 1). This will be achieved by providing multiple paths for the signal with different gain in each. All of the paths will be summed, and selection will be made by switching an amplifier equipped for disabled operation (OPA680) to an "OFF" state. These amplifiers switch in about 100ns permitting gain changes during the "gap" time. In this way it will be possible to change gain with no loss of turn information. The digital processing will average the signal prior to the beam pulse to determine the DC offset. This will be subtracted from all digitized data to remove the offset. The droop compensation will employ an IIR filter that provides a zero and a pole. The zero will be set to cancel the lower corner frequency of the transformer and a new corner frequency of 0.159Hz (1 second time constant) will be established to set the droop to 0.1%/ms. The current will be integrated digitally. An ADC running at 64 times the revolution frequency will read the output signal. The analog signal will also be available to display the current stacking.
The FBCM will require a wideband data acquisition system. This could be a high speed oscilloscope with network programming capability. Such an instrument will also provide the ability to set gain to allow viewing current of a particular set of turns. A wide bandwidth multiplexer will be used to switch signals to the oscilloscope, thereby sharing the oscilloscope with other acquisition systems.
|TechNote 62 Study of Ring BCM Requirements, Parameters and Feasibility|
|TechNote 104 Considerations for the Design of the Beam Current Monitor System for SNS|
EPAC2000 SNS Project-Wide Beam Current Monitors
BIW 2000 SNS Project-Wide Beam Current Monitors