The deflection unit consists of a RF cavity, operated in TM-110 mode, mounted on a pole allowing the cavity to be moved in and out of the electron beam by means of a CF63 bellows. The cavity is driven by a 15W microwave driver that also contains a feedback system which stabilizes the phase and amplitude of the fields in the cavity. This is made possible by measuring the fields in the cavity by a pickup antenna. The unit is equipped with a temperature controller which keeps the cavity at the desired frequency so that all applied RF power is absorbed. The deflection cavity is a pillbox cavity partially filled with a dielectric material with a large permittivity ε and a small tanδ, which allows a substantial reduction in size and power consumption. The symmetry axis coincides with a hole through the dielectric material, allowing the passage of the electron beam.
The deflection cavity unit can be used as an electron pulse length diagnostic tool by sweeping the electron pulse across a detection screen. It can also be used for chopping a continuous electron beam into short pulses by sweeping the beam across a slit or an aperture. A cavity with an on-axis magnetic field amplitude of 3 mT combined with a 10 μm slit at 10 cm distance from the cavity, will chop a 30 keV continuous beam into 100 fs electron pulses. Inversely the same cavity can be used to measure the pulse length of a 30 keV electron beam with 100 fs resolution using a detector with 10 μm pixels at a distance of 10 cm.
|Operating temperature||20-25 deg celcius|
|Quality factor||> 2000|
|On axis magnetic field||3 mT**|
|* Customization possible|
|** for an input power of 15W|
The deflection cavity is a compact, single-cell, power-efficient resonant microwave cavity, supporting a TM-110 mode at a resonance frequency of 3 GHz with an unloaded quality factor that can be as high as Q≈5000. The deflection cavity is a pillbox cavity partially filled with a dielectric material with a large permittivity ε and a small tanδ, which allows a substantial reduction in size and power consumption. The symmetry axis coincides with a hole through the dielectric material, allowing the passage of the electron beam. The TM-110 mode has an oscillating magnetic field oriented perpendicular to the symmetry axis, allowing periodic deflection of the electron beam passing through. Presently the ceramic ZrTiO4 is used, with ε≈36 and tanδ≈0.0002 at 3 GHz, enabling an on-axis magnetic field amplitude of 3 mT at an RF power of 15 W.
The RF cavity is driven by a 15W microwave driver with an active phase and amplitude feedback system. The driver stabilizes the phase and amplitude of the fields in the cavity which results in optimal performance. The driver measures the actual fields in the cavity by means of a pickup antenna and compares both the phase and the amplitude with the input signal that is offered to the driver. Both the phase and amplitude are then stabilized using loop electronics. The phase and amplitude errors can be monitored on a scope. In addition the amplitude and phase setpoints can be adjusted independently.
The temperature controller makes sure that our cavities stay on resonance when RF power is applied. When a RF cavity expands the resonance frequency of the cavity will change which will result in less absorbed RF power. The phase of the RF fields also changes with temperature so it is of great importance that the temperature of the cavity is actively monitored an stabilized using the temperature controller. This is done by measuring the temperature of the cavity using a Pt100 temperature sensor which is the input for the control loop that stabilizes the cavity temperature by either heating or cooling the water that flows through the copper heatsink which is directly connected to the cavity. The temperature control unit is fully integrated in our Statera microwave driver.
The cable kit contains all the necessary cables to operate the cavity. We provide phase stable cables, insulated water cooling tubing and a temperature sensor cable.