This system has been designed to serve two purposes: 1- keeping the RF cavity at a constant temperature, so that the drift of the resonance frequency is as small as possible; 2- Cooling when RF pulses are applied and heating during intervening times.
The temperature control system consists of three segments: 1- Cathode section, 2- Cell sections and 3- Input Coupler section. Each section has one cooling water channel, one Thermocoax heating element and one thermocouple temperature sensor. The temperature of the section is controlled by a Eurotherm controller. The flow of the cooling water can be adjusted by remote control of a valve. The heating rate can be controlled by remote control of power supplies connected to the heating elements.
You can see the cooling water channel of the Cathode and Cell section very clearly at the photo's of the RF cavity model. In the same pictures one thermocouple is visible at the outer diameter of the full cell. There is also a thermocouple at the outer diameter of the half cell. So, we can control the temperature of each cell individually. However, due to the fact that the RF cavity is a big peace of high conductance copper, the thermal conductivity is very high and therefore the three sections are well coupled. This is a great factor of complexity.
Nevertheless, we were able to select the settings of the Eurotherm controllers in such a way that we have reached a very good temperature stability of less than 0.1 degrees Celsius.
A special feature that we have applied in the system is a cooling bias, so that there is always a minimal flow of cooling water. Therefore, the heating elements should deliver a minimal power to get the RF cavity at a constant temperature. Now, we have chosen the minimal flow values in such a way that the ratio of the heat input in the cathode section to the cell section is roughly the same as the expected ratio of the heat dissipation of the RF field inside the cavity from full cell to half cell. We hope that in this way the transient effects will be minimized due to the switching on and off of the RF pulses. This can only be tested when the RF pulses are applied.
On top the three Eurotherm temperature controllers. Below the three power supplies feeding the heating elements. To the left all the connections to the RF cavity are visible.
