Super Conducting RF (SCRF) Cavities test and qualification at PHPMD, RRCAT

In particle accelerators RF cavities are used to generate high electric field gradient (Eacc); which then accelerates the charged particles to high energies. Copper had been the material of choice for fabrication of cavities for decades. For high duty cycle and high gradient applications the losses generated in copper cavities can be very high making these cavities impractical for use. SCRF cavities are formed using materials like Niobium/lead which becomes superconducting at very low temperatures. These cavities have very low losses and are now increasingly being used for high duty cycle/CW operation of very high energy accelerators.

Fabrication of RF cavities is a very complex procedure and requires qualification and testing of cavities at several stages. PHPMD RRCAT has developed in house room temperature test setups for 1300 MHz and 650 MHz SCRF cavities to qualify them at various stages (half cell, dumbbell, end cells) before final welding. The test setups are designed and fabricated in house and are used routinely to ensure that the resonant frequency of the final cavity is close to the design value.

For multi-cell RF cavity it is essential to ensure that not only the resonant frequency is close to design value but also that the electric field gradient inside each of the cavity cell is also equal. Bead pull method is the most commonly used technique to measure the field distribution in the Radio frequency (RF) cavity. The Bead pull test method is based on the classical Slater perturbation theory which states that if any resonant cavity is perturbed by a small bead, its resonant frequency shifts from the fundamental frequency. This frequency shift is proportional to the combination of the squared amplitudes of the electrical and magnetic fields at the location of the bead. PHPMD has in house indigenously developed a bead pull test setup to characterize multi cell cavities. The setup is shown in the image below.
Bead pull characterization of a nine cell 1300 MHz SCRF cavity at PHPMD RRCAT
Fig.1 Bead pull characterization of a nine cell 1300 MHz SCRF cavity at PHPMD RRCAT. The green curve on the computer monitor is the electric field pattern of the cavity.
After final welding, field flatness correction and surface treatments the SCRF cavities are tested at cryogenic temperatures at which they become superconducting. The purpose of these cavity tests is to determine cavity’s intrinsic quality factor (Q0), its variation with respect to average accelerating electric field (Eacc), and the ultimate gradient the cavity is able to handle before becoming normal conducting (Quenching). The result is obtained in form of a Q0 vs Eacc plot by slowly increasing the RF power and recording the values of Q0 and Eacc. The field emission radiation produced inside the cavity, the temperature of the cavity (measured using sensors attached to cavity) and the pressure inside the cryostat are also recorded during the tests. RRCAT has developed a Vertical Test Stand (VTS) for conducting cryogenic qualification of 1300 MHz and 650 MHz SCRF cavities. PHPMD RRCAT has developed the RF system for the RRCAT VTS. The RF system for the VTS consists of the LLRF control and data acquisition system which keeps the SCRF cavity under Phase Lock Loop and provide RF power with control over phase and amplitude and high power system which includes indigenously developed 500 W CW amplifiers used to provide the necessary power to the cavity during the tests. The below image shows the result of the first single cell SCRF tested at RRCAT VTS.
The result of the 1.3 GHz single cell cavity tests
Fig. 2 The result of the 1.3 GHz single cell cavity tests, the cavity has reached a peak gradient of ~36 MV/m at 1.8 K, with quality factor as high as 8E9.
Best viewed in 1024x768 resolution