Measurements of the Electrical Axes of the CeC PoP RF Cavities

Abstract

It is common knowledge that every mode in an SRF cavity has a so-called electrical axis, and only in an ideal cavity would this axis align exactly with the geometrical axis of the device. The misalignment of the electrical axis creates an additional undesirable transverse kick to the beam, which has to be corrected to achieve the designed beam parameters. In this paper we present the two methods which have been used in order to determine the electrical axes in the RF cavities of the Coherent electron Cooling (CeC) Proof of Principle (PoP) accelerator [1, 2]. The electron accelerator for the CeC PoP consists of the three main RF components: the 113 MHz SRF gun, the two normal-conducting 500 MHz bunching cavities, and the 704 MHz SRF 5-cell elliptical cavity. We discuss, in detail, the specifics of the measurement for each cavity and provide the corresponding results. In addition, we describe the influence of the field asymmetry in the 500 MHz bunchers on the beam dynamics, which was observed experimentally and confirmed by simulations. CEC POP SRF ACCELERATOR In the CeC PoP accelerator, electron bunches are generated from a CsK2Sb photocathode by the 532 nm green laser and then accelerated by the 113 MHz SRF gun up to 1.25 MV. Primary focusing of the beam is provided by the solenoid located at the minimal acceptable distance from the cathode. Two 500 MHz normal-conducting cavities and the low energy beam transport (LEBT) section are utilized to achieve a desirable ballistic compression of the beam before it is accelerated to the full energy in the 704 MHz SRF linac.The LEBT section shown in Fig. 3 consists of 5 solenoids and several pairs of dipole trims which provide a successful beam transportation. The beam line is equipped with an integrating current transformer (ICT) for the beam current measurements, two beam profile monitors (yttrium aluminium garnet (YAG) screens), three beam position monitors (BPMs), and a set of vertical and horizontal slits for the beam emittance measurements. 113 MHZ SRF PHOTOINJECTOR The 113 MHz SRF photoinjector (see Fig. 1) is based on Quarter Wave Resonator (QWR) and operates at 1.25 MV of accelerating voltage. The gun is generating high charge electron bunches (up to 10 nC per bunch) and low transverse emittances with the cathodes operating for months without significant loss of quantum efficiency. The detailed description of our gun and its successful performance can be found in [3]. Figure 1: Simplified geometry of the CeC PoP SRF photoinjector. Throughout the operation of the CeC PoP we encountered a few very specific features about the gun performance which we would like to discuss in detail. Specifically, we will address two procedures which have been used in order to determine the electrical axis of the gun and location of the cathode puck relative to the “nose” of the cavity. 0 5 10 15 20 z (cm) 0 5 10 15 20 25 E z (M V /m ) Cathode Recess 0 mm 4 mm 8 mm 12 mm 16 mm Figure 2: Electric field distribution along the gun axis for various values of the cathode recess. Cathode Location As one can see from Fig. 2, longitudinal field profile along the beam path and the peak field at the cathode surface strongly depend on the position of the cathode stalk relative to the nose of the cavity. Since the cathode is located in the area of the highest concentration of the electric field in the gun, any local changes in the geometry caused by the cathode recess will introduce a dramatic change in the field distribution, and hence, the initial focusing of the beam. In order to determine the exact location of the cathode, we performed a series of measurements to calculate the magni10th Int. Particle Accelerator Conf. IPAC2019, Melbourne, Australia JACoW Publishing ISBN: 978-3-95450-208-0 doi:10.18429/JACoW-IPAC2019-WEPRB094 MC7: Accelerator Technology T07 Superconducting RF WEPRB094 3031 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 19 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I