HAPPEX Charge Asymmetry Feedback
The HAPPEX DAQ is run to provide beam charge asymmetry feedback for the Polarized 3He experiments. This page will briefly describe the the system, and how to recover it if it does not work.
Introduction - The Feedback Loop
Summary of the components (pink boxes):
- Polarized Source - IA Cell
The electron beam is created by illuminating a GaAs Photo Cathode with circularly polarized light. Before this light is circularly polarized (with a randomly flipped 30Hz Pockels Cell), the linearly polarized beam traverses through the "IA Cell". This system contains it's own Pockels Cell whose voltage is controlled at the same 30Hz random flip as the main Pockels Cell. It's purpose is to vary the intensity of the linearly polarized light, in a beam helicity correlated way. This provides the ability to control the amount of current is generated off of the photo cathode per helicity state.
- Hall A - Beam Current Monitor
The beam current is measured by the Hall A Beam Current Monitor. The signals from this monitor is coupled to the RF of the beam, and is then down-converted to generate a voltage signal proportional to the beam current. There are two separate down converter electronics in the Hall A Counting house. One is used by the HAPPEX DAQ, and the other by the HRS (and BigBite) DAQ.
- HAPPEX DAQ - Counting House Crate
The Counting House Crate of the HAPPEX DAQ contains special ADCs made to integrate over long periods (several milliseconds). The Timing/ADC Gate is supplied by a JLab custom built "timing board" which is triggered by the MPS helicity signal. The HAPPEX ADCs have TwinAx inputs, so the BCM signal must be converted from BNC to TwinAx.
- Parity Analyzer (PAN) - compiled specifically for feedback
PAN is a ROOT based application, specifically built to analyze the data structure from the HAPPEX DAQ. A special version of PAN has been compiled to take in the input directly from CODA, sort the events in terms of helicity windows, and form pairs to calculate helicity differences and asymmetries. The information about the IA Cell's linear response to it's control voltage is then used to determine what voltage change must be made to minimize the beam charge asymmetry. Once a value is calculated (after a set 2 minutes of beam), it is communicated to the IA cell via EPICS.
Summary of the communications (arrows)
- CW Electron Beam
- BCM Downconverter - TWINAX Cable
- CODA - Event Transfer (ET)