Proton Synchrotron Division

Electrostatic Septa

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Typical technical specifications

• Electrode length : 500 - 2000 mm
• gap width: variable between 10 - 35 mm
• septum thickness: 0.1 mm
• under vacuum (10-9 to 10-12 mbar range)
• Electric field strength: up to 10 MV/m
• Voltage: up to 300 kV
• septum foil made of Molybdum
• electrode made of anodised aluminium or titanium for extreme low vacuum applications
• bakeable up to 200 °C for septa in 10-12 mbar range
• power supplied by Cockroft-Walton type high voltage generators

The basic principle

Electrostatic septa provide an electric field in the direction of extraction, by applying a voltage between the septum foil and an electrode. The septum foil is very thin to have the least interaction with the beam when it is slowly extracted. Slowly means over millions of turns of the particles in the synchron. The orbitting beam generally passes through the hollow support of the septum foil, which ensures a field free region, as not to affect the circulating beam. The extracted beam passes just on the other side of the septum, where the electric field changes the direction of the beam to be extracted.
Electrostatic septa are always sitting in a vacuum tank to allow high electric fields, since the vacuum works as an insulator between septum and electrode.
To allow precise matching of the septum position with the circulation beam trajectory, the magnet is also often fitted with a displacement system, which allows parallel and angular mouvent with respect to the circulating beam.

An important feature of septa is to have a homogeneous field in the gap and no field in the region of the circulating beam. This is achieved by using the hollow support of the septum and the septum foil itself as a faraday cage. The septum separates the gap field between the electrode and the foil from the field free region for the circulating beam. Great difficulty lies in the choice of materials and the manufacturing techniques of the different components.

In the figure above a cross section of an electrostatic septum is shown. The septum foil and its support are marked in blue, while the electrode is marked in red. In the lower part of the figure the electric field E is shown as it could be measured on the axis indicated as a dotted line in the cross section. It can be seen that there is a field free region in the support of the septum foil. The field in the gap between the septum foil and the electrode is homogeneous on the axis and is equal to:
E = V/d
where V is the voltage applied to the electrode and d is the distance between the septum foil and the electrode.