Spallation neutron source low level rf control system
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EPICS Meeting 2005, SLAC. Spallation Neutron Source Low-Level RF Control System. Kay-Uwe Kasemir, Mark Champion. April 2005. SNS Linac LLRF. RF Reference Line, Timing System. Klystron. Cavity. LLRF. [mW]. [MW]. Fwd. Ref.

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Spallation neutron source low level rf control system

EPICS Meeting 2005, SLAC

Spallation Neutron SourceLow-Level RF Control System

Kay-Uwe Kasemir, Mark Champion

April 2005

Sns linac llrf

RF Reference Line, Timing System








  • LLRF produces the Milliwatt-level “RF drive”: Pulsed @ 60Hz, 1.3ms

  • 96 cavities of various types,15 warm (RFQ, MEBT, DTL, CCL), rest superconducting (SCL);some cavities at 402.5, some at 805 MHz.

  • RF Reference, forward, reflected and cavity data is down-converted to50 MHz, then sampled at 40 MHz,resulting in data streams of I, Q, -I, -Q, … (re. & im. of complex data)

  • At 40 MHz, a fully digital control system (FPGA & CPU) computes output data stream which gets up-converted to 402.5 resp. 805 MHz.

Llrf hardware

VXI Crate with


SNS Utility board

Time of day, events, etc.via SNS real-time data link

SNS Timing board

TTL timing pulses

“High Power Protection Module”

Monitors power levels,arc detectors, …for fast shutdown

“Field Control Module”

40MHz FPGA-based control loop

SC Linac uses IOCs with 2xHPM, 2xFCM.Total of about 55 IOCs.

We use VXI as if it was VME

No need for VXI “slot-based” addressing

8-bit VME instead of 16-bit VXI interrupt vectors.

VME CPU on extender board cheaper than VXI slot-zero controller.

LLRF Hardware

Hpm high power protection module
HPM - High Power Protection Module

  • Fast RF Shutdown

  • Monitors 8 RF channels,-50…+10 dBm (10nW…10mW),for the duration of the RF gate.

    • Per-channel threshold and time limit.Trips when RF above threshold longer than time limit.

  • Monitors 14 arc detector inputs.

  • Computed/software interlocks.

  • Everything can be enabled/disabled,except one hardware input for “Vacuum OK” signal.

  • 2 history buffers capture 2ms of data (2s resolution).User selectable for any RF channel, arc detector,or misc. other internals of the HPM.

  • Trip information is sent to the FCM via redundant VXI backplane lines. FCM then cuts the RF drive.

Fcm ancestor sns generation 1 2 llrf

LBNL MEBT LLRF Control Box(Larry Doolittle, )

“nanoEngine”check-card computer,runs Linux & EPICS R3.14.

Xilinx Spartan FPGAXC2S150for 40MHz control loop.

Mini-Circuits components.

OK, but at its limit:6kB of history buffer,no floating-point CPU.

FCM Ancestor: SNS “Generation 1 & 2” LLRF

Fcm generation 3

VXI Carrier Board

Analog Front End

Replaces Mini-Circuits.

Digital Front End


Xilinx Virtex II XC2V1500 (2xI/O, 4xlogic cells over gen. 1,2)

RF Output

AFE & RFOspecific to 805vs. 402.5 MHz

Firmware (VHDL)translated fromGen. 1/2 LLRF(Verilog),then extended.

FCM - “Generation 3”

Fcm firmware vs cpu software
FCM Firmware vs. CPU Software

  • FCM Firmware

    • 40 Mhz PI controller.

    • History Buffers: 5 x 512 x 16 bit x {I, Q}with configurable zoom & pan.

    • Feed-Forward Buffer: 4096 x 8bit x {I,Q},200ns or 400ns granularity.

  • CPU Software

    • ‘Arm’ waveform updates synchronously with SNS event link.

    • Decode raw buffer data intoI, Q, Phase, Amplitude waveforms.

    • Resonance Error Calculation.

    • Adaptive Feed-Forward.

    • Automation.

Example sns llrf screen for one cavity
Example SNS LLRF Screen for one cavity

Cavity amplitude & phase controls

Calls up Auto-Run sequencer page

Calls up FCM “expert” pages including Adaptive FeedForward controls

The auto run sequence simplifies turn on of an rf station
The Auto-Run Sequence simplifiesturn on of an RF station

  • Ramp RF drive level open-loop in frequency-tracking mode until desired cavity gradient is reached.

  • Warmup the cavity in frequency-tracking mode until acceptable resonance error is reached.

  • Close the feedback loop after setting loop phase.

  • Monitor the status of the HPRF system throughout the sequence.

Adaptive feedforward beam compensation
Adaptive FeedForward Beam Compensation

Fig. 1 Beam loading in DTL6 with ~40 us, 20 mA beam induced error of 2.7% and 2 deg in amplitude and phase.

Fig. 2 Beam loading eliminated by means of Adaptive FeedForward.

Sns llrf status

  • Installation of IOCs completed,maybe 80% of the cavities connected.

  • Working on improved

    • Interlocks

    • Closed loop for superconducting section

    • Adaptive feed forward

    • Firmware:Larry Doolittle wrote new firmware in Verilog which synthesizes for Generation 1, 2 & 3 hardware.Debug this ‘portable’ firmware and use it,or stay with proven ‘FCM-only’ firmware?