Bunch Length Measurements with Passive Harmonic Cavities for Uniform Fill Patterns in a 100 Mhz Rf System

The MAX IV facility includes two storage rings operated at 1.5 GeV and 3 GeV. Both rings make use of a 100 MHz RF system and are designed to operate with a uniform multibunch fill pattern as well as employ passive harmonic cavities to damp instabilities and increase Touschek lifetime. Recently, a discussion on timing modes at the MAX IV storage rings has been initiated by the user community. This implies operating the rings with other fill patterns than the originally planned multibunch mode and therefore detailed studies of the performance of the harmonic cavities are of interest. This paper presents bunch length measurements at the 100 MHz MAX II storage ring for uniform fill patterns. The purpose of the measurements was to evaluate the employed measurement method and simulation codes for future studies of fill patterns in the MAX IV storage rings. INTRODUCTION The MAX IV facility is currently under commissioning in Lund, Sweden. The facility includes two storage rings (operated at 1.5 GeV and 3 GeV). The 1.5 GeV ring has a DBA lattice, producing an emittance of 6 nm rad, whereas the 3 GeV ring employs many novel technologies, such as a MBA lattice, to achieve an emittance as low as 0.2 nm rad with insertion devices [1]. Both rings employ a 100 MHz main cavity (MC) RF system and have a design current of 500 mA [2]. They will be operated with a uniform, multibunch fill pattern with 5 nC per bunch [3]. Both rings employ passive harmonic cavities (HCs) [2] to increase Touschek lifetime by elongating the bunches and damp instabilities by enhanced Landau damping [1]. For the 3 GeV ring, the HCs are also essential for conserving the ultralow emittance at high bunch charge [4]. Simulations of collective effects for the 3 GeV ring have shown that the HC performance is of great importance for achieving the design current of the machine [5]. Recently, a discussion on timing modes at the MAX IV storage rings has been initiated by the user community [6, 7] and this raises the interest for detailed studies of the performance of the HCs. The MAX II and MAX III storage rings were shut down on December 13, 2015. Both rings were operated with a 100 MHz RF system and passive HCs. Since the design of the MAX II storage ring was similar to the MAX IV 1.5 GeV ring, MAX II was suitable for evaluating the measurement method and simulation codes to be used for future studies of fill patterns in the MAX IV rings. This paper presents measurements and simulations for uniform fill patterns in the MAX II storage ring. Measurements and simulations ∗ [email protected] for non-uniform fill patterns are presented in an additional paper [8]. MEASUREMENT METHOD AND SIMULATION CODES Bunch length measurements were performed at the D111 beamline at MAX II with an optical sampling oscilloscope. A similar setup has previously been used for bunch length measurements both at MAX II [9] and MAX III [10]. For each measurement, the synchrotron frequency with the HC tuned out and the resonance frequency of the HCwhen tuned in were measured. The measured synchrotron frequency was used to determine the MC voltage. The bunch length was then simulated using the codes described below and compared to the measured bunch lengths. Three different codes were applied for simulations, one code developed by Tavares and Andersson [10] (hereafter denoted Code 1), one code implemented by Milas [11] according to the model presented by Byrd [12] (Code 2) and mbtrack [13] (Code 3). The bunch form factor was already accounted for in Code 1 and Code 3, whereas it was added to Code 2 in the scope of these studies. The implementation is described in [8]. In the simulations a lattice model previously implemented in MATLAB Accelerator Toolbox was used [9]. The parameters given by the model are displayed in Table 1. MAX II had three 100 MHz MCs and one 500 MHz passive HC. No recent measurements of the MAX II cavity parameters had been conducted, but measurements had been performed on cavities in the MAX III storage ring [14, 15]. These cavities were very similar in design to the MAX II cavities and therefore the parameters displayed in Table 2 were used in the simulations for MAX II. Table 1: MAX II Model Parameters Energy [GeV] 1.5 Energy loss per turn [keV] 133.4 Momentum compaction 0.00382 Natural energy spread [%] 0.0701 Longitudinal damping time [ms] 3.38001 MAX II DOUBLE RF SYSTEM A detailed discussion on the dynamics of a double RF system can be found in e.g. [10,16,17]. The voltage seen by a beam in a double RF system is given by