Electromagnetic actuator in UHV for a SR beam viewer

Adu Verma, K J S Sawhney and R V Nandedkar

Synchrotron Utilization Division

While aligning the beam line, the synchrotron radiation beam spot has to be checked at several points, especially after each optical element.  A beam viewer has been designed and developed for this purpose.  An electromagnetic actuator controls the movement of a fluorescent screen inside the beam viewer chamber.

The schematic diagram of the beam viewer and the actuator is shown in Figure.  The main components of the beam viewer are a flipping mechanism, an induction coil and an ultra high vacuum chamber.  A fluorescent screen is fixed on to the flipper placed inside the vacuum chamber.  A fluorescent screen is fixed on to the flipper placed inside the vacuum chamber and driven by the induction coil placed outside the vacuum chamber.  The flipping mechanism rotates the fluorescent screen to an angle of 45⁰ with respect of the synchrotron radiation beam.  The centers of all parts meet to within a few mm at the center of the screen.  The fluorescent screen has well defined markings, in the form of concentric circles, which aid beam line alignment.  Once the alignment is achieved, the screen is pulled out of the beam path by switching off the induction coil.  The whole alignment procedure is carried out in ultra-high vacuum.  Such viewers are placed after each optical element.

Figure: Schematic diagram of beam viewer chamber and electromagnetic actuator.

1. Fluorescent Screen  2. Plunger  3. Induction Coil  4. UHV  Chamber.

In the flipping mechanism, the fluorescent screen is mounted on a stainless steel AISI 304 L cantilever rod (Figure).  This mechanism is moved inside UHV chamber, using an electromagnetic actuator.  The cantilever is rotated by a plunger made of a soft magnetic alloy (SOFTMAG 78A).  This plunger is placed in a cylinder made of a stainless steel AISI 304 L and moved by the electromagnetic force generated by an induction coil placed concentric, with stainless steel cylinder.  The induction coil has been designed to operate at mains voltage, so that no separate power supply is required. After optimizing the various parameters, including the consideration of overall size and heat dissipation, an induction coil of 25 mm internal diameter and comprising 12000 turns of 0.38 mm enameled copper wire was constructed.  A diode (IN4007) was used to halfway rectify the mains power supply.  Another diode (IN4007) was put across the coil to provide the discharge path in the negative half cycle of ac mains.  The magnetic force generated moves the fluorescent screen to the desired 45⁰ position here it is arrested by the mechanical limit.  The complete actuator is installed on a Conflat flange of 203 mm outer diameter, mounted onto the beam viewer chamber.  This chamber is evacuated by a turbo molecular pump to 1 x 10^-6 mbar through a gate valve.  After attaining this vacuum, the gate valve is closed.  The chamber and the actuator are then pumped by a sputter-ion pump of 140 1 scc^-1 to get a vacuum better than 10^-8 mbar after baking at 200⁰C for several hours.  The vacuum was monitored using Penning gauge.  The actuator was tested several times.  During the movement of the fluorescent screen the vacuum in the chamber did not deteriorate.