LOW-LEVEL RADIO-FREQUENCY SYSTEM INTEGRATED WITH FEED-FORWARD CONTROL AND VECTOR MODULATION*

To provide a more accurate and stable Radio-Frequency (RF) signal in conditioning and processing test progress, it is necessary to design a Low-Level Radio-Frequency (LLRF) control system which can provide high precision RF driving signal based on meeting the amplitude and phase stabilization requirement. Through Feed-Forward operation, accurate phase adjustment and amplitude adjustment are realized inside the pulse, to realize the precision and automation of phase-inversion, amplitude stabilization, phase stabilization, and waveform adaptation matching. An LLRF System integrated with feed-forward control and vector modulation output was designed and built, and the long-term working stability of the LLRF system was verified during a new 50MW S-band Klystron conditioning progress.


INTRODUCTION
A test stand based on a 50 MW Klystron, shown in Fig. 1, was newly built on Hefei Advanced Light Source (HALS) campus.The construction of the S-band high power test stand includes the RF system, timing system, interlocking system, vacuum system, mechanical system, and radiation protection system, which can help to realize the start-up and operation of the S-band high power test stand [1].Finally, the S-band high-power microwave source (Klystron) will be operated and tested on this platform for a long time to study the long-term operating performance of the product, provide user operation data, and provide user experimental proof for product performance upgrading.
In the test stand, the LLRF system should fulfill the function of generating stable and high precision RF signal [2,3], and the whole LLRF system contains a Local Oscillator

SCHEMATIC DESIGN OF S-BAND LLRF CONTROL SYSTEM
The schematic of the LLRF control system is shown in Fig. 2

CLOSE-LOOP CONTROL DESIGN OF S-BAND LLRF CONTROL SYSTEM
When the precise mathematical model of the controlled object or the system parameters varies with time, PI control can be suitable for control [4].The discrete form of the algorithm is in Eq. ( 1) is the control quantity provided by the current system, () is the error between the measured value and the set value, ()is the arithmetic sum of all the errors in the past and is the two control parameters.The schematic of the control loop is shown in Fig. 5.
Amplitude FF

Amplitude and Phase Loop
Figure.5: Schematic of control loop In the LLRF system, there are two 512 ×16 bit registers called feed-forward (FF) table, which respectively de-scribes the amplitude and phase of the LLRF pulse waveform.The LLRF system will obtain the amplitude and phase information of the current output pulse through sampling and data processing, calculate the deviation between the set value and them, then input the deviation into the amplitude and phase feedback closed loop to calculate the control quantity [5].Finally, the LLRF would generate the pulse waveform in the feed-forward table after the control vector is rotated [6], which is described in Eq. ( 2), where Io and Po are the output of integration and proportion parameters separately, and the   and   are the offset parameters.The amplitude phase closed loop control embedded in the LLRF adopts the PI control algorithm, which can maintain a long-time stable status [7].

MEASUREMENT AND S BAND KLYS-TRON CONDITIONING
The measurement and conditioning schematic are shown in Fig. 6.The VM output was split into two signals, one to drive the Solid-State Amplifier for conditioning and the other connected to one of the DAC channels for stability test.The conditioning process for the S-band high-power Klystron must follow the principle that when a breakdown or Vacuum break happens, the beam voltage for the modulator has to be decreased (Fig. 8) to protect the Klystron from damage [8].
Fig. 8 The process of conditioning.
After 4 weeks of conditioning process, the microwave output power of the S-band Klystron had reached 51 MW with 2.5 us pulse width and 5 Hz repetition rate under the beam voltage of 320 kV.

CONCLUSION
A 4-channel LLRF system with feed-forward control and vector modulation has been designed, fabricated, and tested in HALS.The results demonstrate that the LLRF system has high stability both in amplitude and phase.The close-loop control has been proven to compensate the drift of amplitude and phase caused by temperature changes and vibrations.Further studies should be conducted to add more channels and adopt higher sampling frequency to the LLRF system.
(LO) generator to provide 2830.5 MHz LO signal, a clock generator to provide 102 MHz clock signal, a 4-channel front-end down converter card to down-convert the pickup signals, and a digitizer card with Artix 7 FPGA on it.

Figure 1 :
Figure 1: Whole view of the S band test stand.

Figure 2 :
Figure 2: Schematic of the LLRF control system.

2 )
Templates are provided for recommended software and authors are advised to use them.Please consult the individual conference help pages if questions arise.

Figure 6 :
Figure 6: Whole layout of measurement A 5-hour continuous test was conducted and 2 LLRF output pulses were recorded per second to characterize system stability using the average value of the pulses.As shown in Fig.7(a) and Fig.7(b), during the long-time stability test, the RMS stability of Amplitude and Phase was 0.03% and 0.03 degree respectively under open loop status.In the closed-loop working state, the drift of amplitude and phase caused by temperature changes and vibrations during the long-term operation had been removed in Fig.7(c) and Fig.7(d).

14th
International Particle Accelerator Conference,Venice, Italy JACoW Publishing ISBN: 978-3-95450-231-8 ISSN: 2673-5490 doi: 10.18429/JACoW-IPAC2023-MOPL149 MC1.A08: Linear Accelerators 891 MOPL: Monday Poster Session: MOPL MOPL149 Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022).Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI.This is a preprint -the final version is published with IOP (a) Open-loop phase stability of VM (b) Open-loop amplitude of VM (c) Close-loop phase stability of VM (d) Close-loop amplitude stability of VM Figure 7: Measurement of the LLRF system.