LLRF CONTROL OF HIGH QL CAVITIES FOR THE LCLS II
The SLAC National Accelerator Laboratory is planning an upgrade (LCLS II) to the Linear Coherent Light Source with a 4 GeV CW Superconducting Radio Frequency (SCRF) linac. The nature of the machine places stringent requirements in the Low Level RF (LLRF) system, expected to control the cavity fields within 0.01* in phase and 0.01% in amplitude, which is equivalent to a longitudinal motion of the cavity structure in the nanometer range. This stability has been achieved in the past but never for hundreds of superconducting cavities in Continuous Wave (CW) operation. The difficulty resides in providing the ability to reject disturbances from the cryomodule, which is incompletely known as it depends on the cryomodule structure itself (currently under development at JLab and Fermilab) and the harsh accelerator environment. Previous experience in the field and an extrapolation to the cavity design parameters (relatively high QL 4 x 107, implying a half bandwidth of around 16 Hz) suggest the use of strong RF feedback to reject the projected noise disturbances, which in turn demands careful engineering of the entire system. The project has passed the conceptual design phase and is approaching the final design phase. LLRF prototypes are currently under construction and expected to be exercised in a cryomodule test stand at both JLab and Fermilab (using the LCLS II TESLA ILC type 1.3 GHz cavities) before the end of 2016 to validate the engineering design presented here. INTRODUCTION LCLS II [1] adds a 4 GeV L band (1.3 GHz) superconducting linac to the first 700 meters of the SLAC tunnel, increasing the repetition rate of LCLS from 120 Hz to 1 MHz (operating in CW) and an average beam current of 100mA. The SCRF linac includes 35 ILC style cryomodules (eight 9 cell cavities each) for a total of 280 cavities. LCLS II will become the world's first X ray Free Electron Laser (FEL) providing CW pulses, probing the motion of molecules, atoms and electrons on their natural time scales (femtoseconds and faster) enabling a broad range of new energy science. Accelerating cavity RF field stability affects the final FEL performance through its relationship with the longitudinal beam dynamics, namely on final beam energy, peak current and arrival time [2] Energy stability in the order of 10 5 converts to different RF stability requirements in each Linac section; 0.01* in phase and 0.01% in amplitude represents This work was supported by the LCLS II Project and the U.S. Department of Energy, Contract DE AC02 76SF00515 CSerrano@lbl.gov Figure 1: Noise requirements vs. zero dB crossing. the tightest requirement, and that for which the LLRF system is engineered. The baseline LCLS II project does not include fast beambased feedback, thus the RF stability requirement has to be satisfied by the LLRF system alone for noise integrated above 1 Hz, with beam currents up to 100 mA and a bunch repetition rate of up to 1 MHz. Enough RF power is made available by the 3.8 kW SSAs to operate the cavities at a nominal 16 MV/m, with RF budget allocated to beam loading and RF corrections (including 10 Hz peak cavity detuning due to microphonics) The LLRF system design [3] is centered around a Precision Receiver Chassis (PRC) providing low noise detection of the cavity probe signal, an RF Station (RFS) chassis implementing digital RF control in an FPGA, and two other chassis dedicated to cavity resonance control (driving fast piezos and slow stepper motors) and interlocks. FIELD CONTROL CONSIDERATIONS The very narrow bandwidth of the cavity makes it very sensitive to microphonics, where nanometers of mechanical deformation translates into tens of Hz of detuning. The nature of the disturbance (external noise sources, mechanical resonances, couplings, etc. depends on the cryomodule structure itself and the accelerator environment, which are both imperfectly known at this stage of the design. Microphonics rejection typically requires a control bandwidth in the tens of kHz range, where the optimal controller settings will be measured in the accelerator environment. The combination of high sensitivity to microphonics and the tight field stability requirements makes it necessary for the LLRF design to support high feedback gain. Proceedings of IPAC2016, Busan, Korea WEPOR042 06 Beam Instrumentation, Controls, Feedback and Operational Aspects T27 Low Level RF ISBN 978 3 95450 147 2 2765 C op yr ig ht (c) 20 16 C C B Y3. 0 an d by th e re sp ec tiv e au th or s FERMILAB CONF 16 320 AD Operated by Fermi Research Alliance, LLC under Contract No. DE AC02 07CH11359 with the United States Department of Energy Figure 2: Block diagram of the LCLS II LLRF design, including controls for two 1.3 GHz superconducting cavities. The above considerations form the basis for the engineering of the LLRF system and present different trade offs. Increasing the open loop zero dB crossing helps rejecting cavity disturbances, but (as an example) achieving a 20 kHz open loop zero dB crossing with a 16 Hz cavity bandwidth implies a proportional gain of about 1200. Any noise in the cavity field measurement chain will get amplified by the same factor and the amplified noise signal will be transferred directly into the input of the SSA. This places the critical point of the design in building a quiet sensor for the cavity field. Figure 1 shows the relationship between the noise tolerance in the cavity field detection (down conversion, digitizing, etc. in units of dBc/Hz and the choice of open loop zero dB crossing (and therefore proportional gain) The assumption is a 4% RMS (amplitude) allowed SSA drive noise, 1 ms hardware delay, 16 Hz cavity bandwidth, and 1/12T stability guideline. SYSTEM ENGINEERING DESIGN In order to build a quiet sensor, the cavity probe signals are downconverted and digitized in a separate, temperaturestabilized chassis (the Precision Receiver Chassis, PRC) described in detail later. The PRC has an FPGA board that acquires the digitized cavity signals and sends them over a digital optical fiber link to the RF Station, implementing the feedback algorithm. Figure 2 shows a block diagram of the LLRF design where 4 out of the 8 cavities in a cryomodule are represented along with the controls for two cavities through one penetration. Note the physical separation of the forward and reflected signals from the cavity probe, responding to the engineering needs discussed above. PROTOTYPE CHARACTERIZATION Prototypes for all the components in the LCLS II LLRF system [3] have been built and tests with the LCLS II cryomodules are expected before the end of 2016. Figure 3 shows a block diagram of the characterization exercise for the up and downconverters (designed at Fermilab) and the digitizer boards (designed at LBNL) The FPGA board is the common platform used in the RF stations, precision receivers, interlocks and resonance control chassis illustrated in Figure 2. A 1320 MHz local oscillator provides a frequency reference as in the production system. A 203/264 (non binary) NCO drives the DAC in the digitizer board at a 188.6 MHz clock (derived from the 1320 MHz LO with a divide by 7) This produces 145 MHz, which is mixed up to 1300 MHz and then back down to 20 MHz (the Intermediate Frequency, IF) The ADCs receives 20 MHz with a clock of 94.3 MHz (derived using a divide by 14 output of the same divider chip) Note that 20 MHz 7/33 the ADC clock. The use of two independent ADC channels allows for computing correlations and separating the noise contributions form the DAC upconverter chain from the downconverter ADC chain as shown in Figure 3 [4] If we call the ADC signals x and y respectively, and we convert the time series signals to frequency domain, we can compute the autocorWEPOR042 Proceedings of IPAC2016, Busan, Korea ISBN 978 3 95450 147 2 2766 C op yr ig ht (c) 20 16 C C B Y3. 0 an d by th e re sp ec tiv e au th or s 06 Beam Instrumentation, Controls, Feedback and Operational Aspects