Difference between revisions of "Compton Cavity State Identification (d2n)"
(New page: =Background= In the Hall A Compton Polarimeter, Compton scattering occurs inside a Fabry-Perot cavity fed by an infrared (1064 nm) laser. Over the course of a run, the cavity is periodica...) |
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* Is the laser on and locked to the cavity, or unlocked? This is the difference between a few milliWatts of power and 400+ Watts. | * Is the laser on and locked to the cavity, or unlocked? This is the difference between a few milliWatts of power and 400+ Watts. | ||
* Are the photons in the cavity left-circularly or right-circularly polarized? | * Are the photons in the cavity left-circularly or right-circularly polarized? | ||
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| + | =Cavity State Variables= | ||
| + | |||
| + | During d2n running, two types of signals were used to keep track of cavity state data: "real-time" bits read in through the TIR module, and EPICS variables (which can be much larger), which are read into the datastream roughly every 50 MPSes. One obvious drawback of EPICS data is that, due to its less frequent readout, it lags actual changes. | ||
| + | |||
| + | The variables at our disposal are: | ||
| + | |||
| + | * Cavity power | ||
| + | ** A "real-time" bit that is supposed to read LOW when the cavity is empty, and HIGH when the cavity is filled. This bit is buggy. | ||
| + | ** A floating-point EPICS variable that is supposed to report the actual cavity power in Watts. | ||
| + | * Cavity photon polarization direction | ||
| + | ** A "real-time" bit that is LOW when the photons are left-circularly polarized, and HIGH when the photons are right-circularly polarized. | ||
| + | ** An EPICS variable that reads -1 when the photons are right-circularly polarized, and +1 when the photons are right-circularly polarized. (Note that these two variables use opposite conventions.) | ||
| + | * Cavity photon polarization | ||
| + | ** An EPICS variable giving the result of a polarization measurement on the photons exiting the cavity, in percentage points (e.g. a reading of 94.1 corresponds to 94.1% polarization). | ||
Revision as of 16:57, 1 May 2009
Background
In the Hall A Compton Polarimeter, Compton scattering occurs inside a Fabry-Perot cavity fed by an infrared (1064 nm) laser. Over the course of a run, the cavity is periodically filled with photons of a given polarization state (right or left circular), drained, and filled with photons of the opposite polarization state.
In order to compute accurate Compton asymmetries, we need a reliable, accurate way of determining the state of the Compton laser cavity during any given mps:
- Is the laser on and locked to the cavity, or unlocked? This is the difference between a few milliWatts of power and 400+ Watts.
- Are the photons in the cavity left-circularly or right-circularly polarized?
Cavity State Variables
During d2n running, two types of signals were used to keep track of cavity state data: "real-time" bits read in through the TIR module, and EPICS variables (which can be much larger), which are read into the datastream roughly every 50 MPSes. One obvious drawback of EPICS data is that, due to its less frequent readout, it lags actual changes.
The variables at our disposal are:
- Cavity power
- A "real-time" bit that is supposed to read LOW when the cavity is empty, and HIGH when the cavity is filled. This bit is buggy.
- A floating-point EPICS variable that is supposed to report the actual cavity power in Watts.
- Cavity photon polarization direction
- A "real-time" bit that is LOW when the photons are left-circularly polarized, and HIGH when the photons are right-circularly polarized.
- An EPICS variable that reads -1 when the photons are right-circularly polarized, and +1 when the photons are right-circularly polarized. (Note that these two variables use opposite conventions.)
- Cavity photon polarization
- An EPICS variable giving the result of a polarization measurement on the photons exiting the cavity, in percentage points (e.g. a reading of 94.1 corresponds to 94.1% polarization).
