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LCLS X-Band RF System The LCLS beam is run off crest on the RF in order to set up a particle position verses energy correlation. The correlation is used to compress the bunch when it is run through a chicane. The RF waveform is a sine shape and the correlation set up by running the bunch off crest has a large second order component, it’s not very linear. The X-band RF station, with higher frequency, has a much larger curvature in RF voltage verses time. The beam is run on the decelerating crest in order to remove most of the second order, nonlinear, part of the correlation. The required 22MV of peak accelerating gradient will require a klystron power output of 24MW. More information can be found in the CDR Chapter 7, by Paul Emma. Results of the beam energy vs. position correlation and how the X-Band station takes out the curvature are shown in figure 1. Figure 1. X-Band Station to Linearize Energy Position Correlation - P. Emma Accelerator Structure The accelerating structure is the H60VG3N-C structure from NLC. The structure is a 60cm long 5π/6 which has been tested at NLCTA to 65MV/m. LCLS operation requires 32MV/m with a peak input power of 21MW. The structure has a dual feed at the input and output. The output of the structure goes to two independent RF loads one of which has a WR90 directional coupler in between the structure and load. This is used to measure the output power and RF phase of the structure. The input feeds come from each arm of a magic-T. There is a WR90 direction coupler at the input to the magic-T to measure the input power and RF phase to the structure. Water flow through the accelerator is about 0.3GPM. The average power into the structure is only 126W at 30pps 200nS. The structure's temperature coefficient is 36 degX/degC and the structure is tuned for 45degC. The nominal water settings at 30Hz is 111degF for accelerator water in the gallery. The water flow required to raise 1.1degC with 126W of power input is 126W/(4.2J/(cc-degC)x 1.1degC)= 30cc/sec = 1.8l/min = 0.47GPM High Power Waveguide The XL4 klystron is powered by modulator 21-2 in the klystron gallery. There is about 6ft of WR90 at the output of the klystron before it reaches a window assemble. The window assembly uses 2 mode converters to go from WR90 to circular waveguide where the window is mounted and then back to WR90. The WR90 is routed above penetration 21-3 where a mode converter changes to WC293. This over-moded circular waveguide runs 35feet through penetration 21-3 into the tunnel. In the tunnel a mode converter changes from WC293 to WR90. The WR90 is routed over to the accelerator structure. A diagram of the system is shown in figure X. Figure X, X-Band waveguide layout. XL4 Klystron and modulator The XL4 klystron was designed to output over 50MW of X-Band power. The beam power to achieve this is 410kV at 350A, 144MW. XL4 klystrons have been run reliably at NLCTA for thousands of hours at over 50MW of output at a 1.6uS pulse width and 60Hz. LCLS requires 24MW at 200nS and 120Hz. The tube is expected to run reliably at these levels. The standard 5045 15:1 pulse transformer was changed to a 17:1 transformer to achieve higher voltages at the tube from a standard 5045 modulator. The 5045 modulator PFN was redesigned for a short pulse. The pulse shape from the modulator is Gaussian with the 200nS RF pulse at the top. The modulator has an SCR front end control and uses a dequing circuit to regulate the PFN voltage. As of August 2008, the klystron has been in operation at 30Hz, 22MW at 200nS for the last 2 commissioning runs. The klystron beam is at 372kV at 250A, 93MW. XL4 Klystron Magnet Power Supplies There are three coils in an XL4 klystron, as shown in figure X, a upper and lower focus coil and a bucking coil. The upper and lower focus coils each take 2 power supplies in parallel to get up over 300 amps. The current for each circuit is run through shunts which are connected to meters with interlocking capability. Windows are set for each circuit. The modulator is shut off through the MKSU if the current goes outside the window. There are Klixons and water circuits that will shut off the magnet power supplies if they trip. Figure X, Klystron Magnet Power Supplies XL4 Klystron Water Circuits Flow rates for the three klystron water circuits follow: 1. Klystron Body, 2GPM, trip at 1GPM 2. Klystron Collector, 8 to 10GPM, trip at 6GPM 3. Klystron Tank and Magnet, 8 to 10GPM, trip at 6GPM The water circuits interlock the modulator through the MKSU. The klystron tank and magnet water circuit shuts off the magnet power supplies if tripped. Station Vacuum Interlocks The new station gauge interlocks and an ion pump PLC interlock is run into the interlocking summing chassis. The output of interlock summing chassis connects to the MKSU station gauge vacuum interlock. The interlocking summing chassis takes inputs from the vacuum gauges before and after the X-Band window assemble. The PLC monitors ion pump power supplies and is connected to the interlocking summing chassis to be able to turn off the modulator if ion pump currents are above a set threshold. The EPICS L1X vacuum panel show a diagram in figure X. The black bar between W200 and W220 is the X-Band window assembly. Figure X, EPICS L1X Vacuum panel. Drive Amplifier Saturated drive levels for different XL4s are listed in table 1. LDF2-50, 3/8 inch Heliax, is used to go from the drive amplifier to the tube input. Ten feet of LDF2-50 Heliax has a loss of 1.5dB. The drive power coupler, KRYTAR 1824, has an insertion loss of 0.7dB. An HP X362A Low Pass Filter, loss less than 1dB, is at the input of the klystron. Total loss from the drive amplifier to the klystron is about 3.2dB. 400W of power at the klystron requires 840W of power out of the drive amplifier. If lower loss is required the Heliax can be replaced with WR90 waveguide. At 11.424GHz 10ft of copper WR90 has a loss of about 0.04dB. This would reduce the drive amplifier power requirement from 840W to 600W. XL4 Tube Number Saturated Drive Power 2C 362 3A 400 5D 145 6A 400 7B 800 8A 400 9A 400 12A 400 13A 400 Table 1. Saturated Drive Powers for XL4 Tubes. Data taken from tube folders. LLRF Control System A diagram for the X-Band RF system is shown in figure X. 25.5MHz from RF HUT 2830.5MHz LO Gen LKG-2821-J5 PAD 380-208-60 TUNNEL J5 LKG-2827 J26 AccOut 2856MHz from 380-208-51 +2dBmJ24 CH1 LO RF Reference 380-208-50 X-Band J22 AccIn LKG-2730-J10 4 X Multiplier LO Generator J16 CH0 +2dBm J8 16dBm CLK 6dBm In 11398.5MHz 9dBm J3 16dBm J6 -2dBm 11424MHz CH3 6dBm J7 9dBm LKG-2836 16dBm J4 LKG-2830 J9 +2dBm J27 Monitor J5 J15 380-208-21 Panel J12 J14 CLK Distribution LKG-2608 LKG-2718 J7 MKSU KF-21-2B28 +2dBm J24 J13 J9 J12 J23 KLYS BEAM LO PAD CH2 Klystron Klystron Reflected 16dBm J16 380-208-60 J27 Drive Forward Power CLK CH3 Control KF-21-2B15 X-Band Coupler J26 J8 J22 CH0 +2dBm CH1 Chassis ?dB KF-21-2B33 380-208-56 FP J3 Monitor J7 Filter 10dB Couplers Attenuator Diode detectors 13dBm J13 -20dB J5 J10 J11 MIXER Monitor CLK TWT Amplifier IN PAC OUT LO RF 714-107 J15 J14 IF -2dBm 380-208-40 13dBm KF21-2B39 ?dB KF21-2B36 10dB KLYSTRON Klystron Station 21-2 RF IN 1 RF OUT 2 Figure X, X-Band system diagram X-Band 4X Multiplier The X-Band reference RF is generated in the RF Hut. The X-Band 4X Multiplier chassis, figure X, uses 2856MHz from the RF reference system and multiplies it by 4 to get 11424MHz. The 11424MHz outputs of the multiplier chassis feeds the X-Band LO Generator, the PAC at linac station 21-2, and is monitored by channel 3 of the X-Band PAD in the RF Hut. 2856MHz Mon FP N J1 4 X Multiplier Midwest Microwave 2856MHz IN Marki AQA-1933K 10dB 0dBm CPL-5215-10-SMA-79 RP N J3 20dBm 10dB Coupler 11.4GHz RP N J5 +5dBm In +10dBm Out 10dB Coupler 11.4GHz RP SMA J6 Pulsar Pulsar +5VDC 200mA 11424MHz BPF CIAO CA1112-343 PS2-19-450/8S CS20-05-436/10 Gain = +20dB P1dB = +27dBm NF = 5dB 11.4GHz Mon RP SMA J4 IP3 = 35dBm Pulsar 11.4GHz Mon FP N J2 +15VDC 600mA PS2-19-450/8S Figure X, X-Band 4X Multiplier Chassis SD-380-208-50-C0 X-Band LO Generator The X-Band LO Generator uses the 11424MHz from the multiplier and the 25.5MHz from the S- Band LO Generator chassis to Single Side Band, SSB, generate 11398.5MHz LO frequency for the Phase and Amplitude Detectors, PADs. A diagram for the X-Band LO Generator Chassis is shown in Figure X. REAR PANEL N J7 25.5MHz 2 +13dBm out of 2830.5LO chassis S 1 25.5MHz REAR PANEL N ZFSC-2-2 ZFSC-2-2 REAR PANEL N J6 2 +7dBm J5 25.5MHz In +23dBm S F5 SLP-30 1 +20dBm CAP PI NETWORK +11dBm +16dBm 90 0 0 JSPQ-80 DC Block +V 15V4 1 2 J4 3 + - 15V 4mA -V REAR PANEL N -15V 11.424GHz In 17dBm 25.5MHz DIPLEXER Balancing Hybrid 10dB 10dB Marki IQ0714LXP FRONT PANEL N J1 0dBm 11.3985GHz +10 to +13dBm REAR PANEL N IF LO RF -6dBm 11.3985GHz J9 13dBm PULSAR REAR PANEL N J8 CS10-12-435/1 LO RF 15V3 PULSAR 11.3985GHz IF CAIO CS10-12-435/1 CA1112-441 PULSAR 11.424GHz to 11.3985GHz SSB IQ Modulator PS2-19-450/8S DIODE DETECTOR DC 0 to ? Volt J3 CIAO CA1112-441 PULSAR FRONT PANEL BNC Gain = +30dB CS10-12-435/1 In Diode Det P1dB = +30dBm NF = 3.4dB FRONT PANEL N +15VDC 900mA 11.424GHz J2 Figure X. X-Band LO Generator Chassis X-Band Coupler Chassis The X-Band Coupler Chassis is used to interface control of the drive power to the MKSU. It is also used to couple down high power RF signals for monitoring. Detector diodes change the klystron forward power, reflected power, and drive power into video signals which are connected to the MKSU for monitoring by the control system. The coupler chassis and connections are shown in figure X. DRIVE FORWARD REFLECTED PAC MKSU FP N To PAD CH1 FP N test FP N test OUT IPA Coupler 13dBm J13 J3 J2 J1 Connector to PAD X-Band -20dB CH0 RP BNC -20dB Drive -20dB Couplers LP Filter 10kHz 6dBm J5 Chassis KRYTAR 0-28mA Attenuator 17dB 0-200mA Model 1824 10dB -20dB 16dBm 33dBm 380-208-56 -20dB ?dB FP BNC Test VideoAmp -20dB J4 M8-0412NZ MIXER 17dBm BNC to 239-025 MKSU J6 -20dB -20dB RF LO IF 0dBm J7 J8 J9 J10 J11 J12 10dB N to BNC to BNC to PAD MKSU MKSU CH3 IN 10dBm MAX -20dB HP X362A LPF TWT OUT KLYSTRON To Accelerator KRYTAR -53dB 714-107 Coupler XL4 Model 1824 -20dB Figure X, X-Band Coupler Chassis 380-208-56 and connections. X-Band PAC The X-Band PAC chassis uses the same control board as the other PAC chassis. The control board puts out 2 preset waveforms on a trigger pulse to drive I and Q of the IQ Modulator. The diagram of the PAC board is in figure X. TRIGGER EXTERNAL CLOCK Monitor TTL TRIGGER SSSB SSSB 119MHz FP BNC 120Hz 60nS NIM Gate Monitor Chassis RP N FP BNC RP 15 Pin D RF OUTPUT RP BNC RF INPUT RF OUTPUT To TWT Amp Monitor Monitor RP N I MONITOR Q MONITOR FP N FP N +13dBm Out FP BNC FP BNC H9 H10 J2 H7 J3 SSSB H6 P5 RF Module Trig TTL VLFX-80 Low Pass 17 to 30uS 10dB -10dB Couplers Pulsar CS10-12-435/1 16bit DATA -10dB 16 bit 3 ETHERNET 11.424GHz Ref I&Q MODULATOR Amp I I DATA Amp 1 CLK XILINX CONTROL / RP N -10dB 2 RF LO J5 MAX5875 -2dBm In 2 X 16 bit DAC SPARTAN 3 Arcturus uC5282 -10dB Q 119MHz Clock 16bit DATA FPGA Microcontroller Module (1MHz to 200MHz) CS/ IQ0714MXP with 10/100 Ethernet 4 10dB VLFX-80 Low Pass Q CLK 13 to 16dBm CLK ETHERNET RAW J4 AD8099 Diff Amp Control Control CIAO CA1112-343 COM Gain = +20dB P1dB = +27dBm Temperature NF = 5dB Monitor IP3 = 35dBm Temperature SLOW ADCs t Monitor +15VDC 600mA H12 PAC Temp t IQ Temp SSSB Temp SSSB P-FWD RF INPUT SSSB P-RFL Monitor SSSB PWR +5V Control Board Diode -12V FP BNC Figure X, X-Band PAC Chassis FS-380-208-40-C0 X-Band PAD The PAD Chassis down mixes the 11424MHz with 11398.5MHz to a 25.5MHz IF frequency on 3 channels, 0, 1, and 3. Channel 2 is connected to a coupler and an input transformer before being digitized. The coupler, Minicircuits ZFDC-10-6-S+, 0.005MHz to 20MHz limits the bandwidth of the signal before it reached the digitizer board. On the digitizer board, the standard input transformer, Minicircuits TC4-1T, 0.5 to 300MHz, is replaced by Minicircuits TT1-6- KK81, 0.004MHz to 300MHz to give a total bandwidth of 0.005MHz to 20MHz. This signal is used to measure the beam voltage to the klystron from the MKSU. CHAN 0 25.5MHz IF FP BNC - J1 4 X 16 bit ADC Control Board 102MHz Clock CHAN 0 TEST LTC2208 RF INPUT MIXER M80412NZ IF Board Transformer Coupled Inputs RP N 16bit DATA 16 bit LO RF IF IF Chan. 0 FIFO DATA OUT 10dB WCLK 64k words CONTROL / J15 COM VLFX-80 Minicircuits FILTER 25.5MHz BP CHAN 1 16bit DATA CS/ Arcturus uC5282 714-114-50-R3 25.5MHz IF FIFO CLK Microcontroller Module Chan. 1 +12VDC FP BNC - J3 WCLK 64k words CHAN 1 with 10/100 Ethernet RF INPUT J14 ETH1 CIAO CA1112-343 16bit DATA Gain = +20dB RP N FIFO Chan. 2 P1dB = +27dBm TEST WCLK 64k words NF = 5dB IP3 = 35dBm 1 MIXER M80412NZ IF Board +15VDC 600mA 16bit DATA 2 10dBm FIFO LO RF 3 IF Chan. 3 LO INPUT 4 IF OUT WCLK 64k words 11.4GHz +2dBm 5 10dB VLFX-80 Minicircuits FILTER 25.5MHz BP Control RAW J13 ETH2 RP N +15VDC 714-114-50-R3 +12VDC CPLD 5VDC QSPI LO1 CLK IN CLK OUT 0.8A x 2 Analog TRIG TEST PORT 5VDC FP N - J9 CLOCK IN CLOCK Mon 20 pin ribbon CHAN 2 0.5A Digital TRIG In TRIG Mon Test 102MHz 102MHz 120Hz FP BNC - J12 FP BNC - J5 RP N FP N - J11 RP BNC QSPI CHAN 2 RF INPUT RP N ZFDC-10-6-S+ 24Bit 0.005 to 20MHz 11.3dB coupling CHAN 3 Analog Test FP BNC - J7 Input CHAN 3 RF INPUT Board RP N TEST ANALOG IN ANALOG IN MIXER M80412NZ IF Board PAD Usage Ch0 - LO Power LO RF IF IF OUT Ch1 - RF Head Temp 10dB Ch2 - ADC BRD Temp VLFX-80 Minicircuits FILTER 25.5MHz BP Ch3 - +12VDC 714-114-50-R3 Ch4 to Ch7 external temps +12VDC Figure X, X-Band PAD Chassis FS-380-208-60-C0 The PAD chassis in the RF Hut measures the input and output RF for the accelerator structure on channel 0 and channel 1 respectively. Channel 2 is not used and Channel 3 measures the reference RF from the multipler. The PAD chassis at the klystron station measures the PAC output on channel 0, the drive amplifier output on channel 1, the klystron beam voltage on channel 2, and the klystron output on channel 3. The RF Hut X-Band PAD panel is shown in figure X. Since the fill time of the structure is only 100nS and the RF pulse width is only 200nS the window to look at the structure is only 12 points, or 118nS at 102MSPS. Figure X, RF Hut X-Band PAD Panel PAC Calibration, Set-up, and Operation There are 2 EPICS panels used for PAC control. One is for operation (destination), PAC L1X, and one for calibration, PAC CAL L1X. The two panels are shown in Figure X. Figure X PAC Destination (Operation) Panel and PAC Calibration Panel. Calibration: To enter calibration mode press the "Calib Rqst" button on the PAC CAL L1X panel. In calibration mode there are 2 waveform selection which can be made. Both load a cosine waveform into I. One of the calibration wavforms loads a sine waveform into Q while the other load a sin+pi waveform into Q. These waveforms will make a Single Side Band SSB modulator out of the IQ modulator driven by the PAC. The two selections of waveforms make either an upper side band modulator or a lower side band modulator. The FPGA "Trigger mode", bit 0 and 1 of the Control Register (2000h) is set to "Internal Trigger", 01 for calibration mode. No external trigger is required for calibration. There is also an internal and external clock mode. The mode used depends on if the PAC is a Slow PAC, SPAC, or normal PAC. The internal clock is used for SPACs and the external clock is used for normal PACs. This uses bit 0 of the Clock Select reg (200Eh). Bit 0 set is internal clock, bit 0 cleared is external clock. The software selects the correct clock mode. Once calibration mode is entered the "SLOW update of I waveform in FPGA" should show a cosine wave and the "SLOW update of Q waveform in FPGA" should show a sine wave. The "Amplitude of Calib WF" is a gain factor applied to both waveforms. The default value for this is 16384. To calibrate the PAC a spectrum analyzer is connected to the output of the unit. The front panel RF Out Mon J2, figure X, is a good place to connect the analyzer. The center frequency of the spectrum analyzer is set to the frequency passing through the PAC. The span should be set to about 500kHz. This will enable viewing of upper and lower sidebands when the PAC turns into a SSB modulator. Offset Calibration: To calibrate the offset the I Gain and Q Gain are both set to 0. This sets the output of the DACs to zero. To remove offsets in the system the fundamental frequency needs to be suppressed by adjusting I Offset and Q Offset. Start by moving in 100 count changes and go back and forth between I and Q to minimize the fundamental. Fine adjustments are made by changing the offsets by 10. The fundamental should be suppressed by 60dB by adjusting the offsets. Gain Calibration: Once the offsets are calibrated enter values of 32000 in I Gain and Q Gain. This should change the PAC into a SSB modulator. The fundamental should decrease and one side band come up to about the fundamental power level. The opposite side band will be down by at least 10dB to start. Lower either I Gain or Q Gain initially by increments of 1000 and then by increments of 100 to reduce the level of the lower sideband. The ratio of the two side bands gives an estimate of the linearity of the system. Since there is not phase correction on the modulator, the ratio of the two sidebands will likely be between 25dB and 40dB based on the accuracy of the 90 degree phase difference between I and Q in the modulator. Once the I Offset, Q Offset, I Gain and Q Gain values are set, the calibration of the modulator is complete. The Calib Done button can now be pressed. The PAC is now running and phase and amplitude can be changes by entering the I Adjust and Q Adjust values on the PAC L1X panel. The I Adjust is the diagonal elements of a rotation matrix and Q Adjust are the off diagonal elements of the matrix. Figure X, X-Band PAC Chassis Feedback Operation The VME L1X feedback panel is shown in figure X. Figure X, VME L1X feedback panel. The initial I and Q readings from the PAD are rotated by Phase Offset corrections and displayed at the top of the panel. The Phase Offset corrections are entered on the "Adjust scale factors & offsets" panel. The Phase Offset corrections are set so that a corrected reading of zero is the phase at which the beam has maximum energy gain through the accelerator structure. The Voltage Scale Factor is set so that the energy gain of an electron passing through the accelerator structure at zero phase is displayed. These parameters are set by the operators using beam based measurements. Either or both of the 2 channels, L1X In and L1X Out, can be used in the phase or Amplitude feedback. Weighting factors as to the fraction of each input which is used in the phase and amplitude feedbacks are set independently with the CH0 Weighting Factor and CH1 Weighting Factor. These weighting factors are typically set for equal weighting of each channel. The weighted average is then passed on to the phase and amplitude feedbacks. The Phase feedback looks at the difference between the entered "Desired" phase and the "Wt average" phase calculated from the PAD readings. For the feedback to calculate a correction, three conditions must be met, the amplitude must be larger than the Minimum Amplitude, the difference, error, between the Desired and Wt averge must be larger than the Minimum Correction, and the Local Phase FB button must be in an On state. If those three conditions are met, the absolute value of the error signal is clamped by the Maximum Correction. The clamped error is then multiplied by the Smoothing factor and subtracted from the Previous set point to generate a new set point. The new phase set point is then used along with the new amplitude set point to calculate I Adjust and Q Adjust which get sent to the PAC. The Amplitude feedback divides the Desired value by the Wt average. As with the phase feedback, there are three conditions which allow the feedback correction to be calculated, the Local Amplitude FB button must be in an on state, the fractional error must be larger than the Minimum Correction, and the Wt average must be larger than the Minimum Amplitude. If the three conditions are met, the fractional error is clamped by the Maximum Correction multiplied by the smoothing factor and used to scale the previous set point to get a new set point. The new set point is then limited to a range between lower and upper Ampl Setpt limits. Feedback setup The lower and upper amplitude set point limits are set so the feedback does not move far from the desired operating point. The upper set point limit is set to about 3% above the operational voltage level. The PACs are vary non-linear due to saturation in the drive amplifiers and klystrons. The lower limit needs to be set high so the power out does not go below the Minimum Amplitude required for feedback operation and should be about 3% below the operating point. The Minimum Amplitude is set to prohibit feedback response to dropouts and should be set to about 5% below the operating point. The smoothing factors are currently set to 0.4 so the feedbacks will correct fast enough for software running routines which require large phase and/or amplitude movements.
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