Polarized Electron Target

The Møller Polarized Electron Target is placed on the beamline 17.5 m upstream of the main Hall A physics target.

Two target slots exist, called ``bottom'' and ``top''. For the Møller target at the moment a supermendur foil 12 $\mu$m thick is used, positioned in the ``bottom'' slot. The target block can be moved vertically from the center, which contains the hole for the beam, to either ``bottom'' or ``top'' position. The foil can be tilted to the beam at an angle required, in a range from $20^o$ to $160^o$. Both vertical movement of the target and its rotation is controlled by the Machine Control Center (MCC) operators. The vertical movement is controlled by 3 buttons on the Møller MEDM display, named ``center'', ``bottom'' and ``top''. Rotation is controlled by typing an angle, measured in certain units, in a ``rotary'' window and pressing RETURN. The size of the unit is defined by the end switches which stop the target rotation at low and high end points. The distance between these 2 switches is about $140^o$, and this distance is divided into 80 units. Therefore one unit is about $1.75^o$. In order to turn the target to $90^o$ one should set a value of about 40 units. The default rotary position is at 0 units and the target cannot be moved vertically being at a different position.

Before the target is moved vertically the target movement should be ``masked'' by the MCC operators, since the vertical movement may bring thick construction elements into the beam area and therefore is connected to the Fast Shutdown (FSD). The target rotation is safer and can be performed without ``masking'' the target movement and is normally performed without turning off the beam. A potentially dangerous target rotation to angles lower than $15^o$ is prevented mechanically.

The target motion is supervised using 2 TV cameras, displayed in Hall A counting house. One camera is looking from the side. A glass window in the target vacuum box allows one to see the beam area in the place where a target can be moved in. The central position (empty) of the target block is seen as an empty round hole. A target moved in is clearly visible. The second camera looks from the top at the target holder. A scale engraved on the holder shows its angle in degrees. This scale gives correct relative angles of the target. The absolute angle of the target to the beam is measured using the event rates, measured at a given target angle and at about $90^o$. At the moment the scale has a shift, that: $\theta_{target}~\approx~\theta_{scale}-2.5^o$.

The target is magnetically saturated using 2 external Helmholtz coils, providing a field of about 300 Gs along the beam axis at the target center. The coils are turned on by the Møller CODA task, during the data taking. Its polarity is reversed for each new run of data taking (one run typically takes 2-3 min).

The beam of a few $\mu$A may heat up the target locally by 20-40K, which may change slightly the target polarization. A system for cooling the target with liquid nitrogen has been built. However it is not used at the moment.

The temperature of the target holder is measured using cernox resistors.

The target magnetization can be measured in situ, by changing the field in the Helmholtz coils and measuring the voltage at pick-up coils, wound around the target foils.