Linac RF System Design Options Y. Kang RAD/SNS/NScD/ORNL Project – X Collaboration Meeting April , 2011

2 Managed by UT-Battelle for the Department of Energy Outline  RF System options considered mainly for cost reduction in construction  The amplitudes and the phases at the cavity inputs (through RF power distribution) need to be controlled precisely  RF System Options –One generator powers one cavity –One generator powers multiple cavities using high power vector modulators –One generator powers multiple cavities using reactive matching –Multiple amplifiers power one cavity using solid-state amplifiers

3 Managed by UT-Battelle for the Department of Energy RF Amplifiers  Vacuum Tube Amplifiers –Klystrons (and Multi-Beam Klystrons)  long pulse ~ 10 MW, Short pulse >100 MW, CW in MW range –Other Vacuum Tubes  IOT, tetrode, triode –High power density compared to SSA –High voltage power supply / modulator / transformer needed –Reduction of construction cost using fan-out distribution (a very high power klystron to feed multiple cavities)  Solid-State Amplifiers –Latest SS device advancement: Si LDMOS, GaN, SiC –Power output: 300 W – 1kW (CW or pulsed) –Power Gain: Usually 10 – 20 dB –Efficiency: 50 – 70% –Low junction voltage => low voltage power supply –Output protection with isolator (~ 0.1 – 0.2 dB loss)  Power Combining/splitting –Power combining becomes more important for high power >100 kW  Insertion loss: ~ 0.1 dB per stage; 3 stages can have ~ 10% loss  Isolation between input ports  Input power splitter loss is less critical compared to the output power combining –Radial combiner –Wilkinson type combiner –Hybrid junctions, directional couplers

4 Managed by UT-Battelle for the Department of Energy  Easy to setup with repeating identical subsystems  Usually the construction cost is most expensive  SNS is employing this system –A cavity is driven by an amplifier –With klystrons, high voltage power supplies (or HVCM) are needed –Each cavity has independent RF control One Generator to One Cavity

5 Managed by UT-Battelle for the Department of Energy RF Power Combining with SSA

6 Managed by UT-Battelle for the Department of Energy  RF power splitting network is fixed  The power splitting ratios are determined for supplying the maximum required power to each cavity  With a single amplifier input, the individual high power RF vector modulator controls the power delivered to the cavity –The input power to each vector modulator is fixed –Unused power at the cavity will be dissipated at the vector modulator load or at the circulator load One Amplifier to Multiple Cavities

7 Managed by UT-Battelle for the Department of Energy Vector Modulator Construction 180  Hybrid 90  Hybrid 0/90  Hybrid 180/90  Hybrid ● Traveling wave ● Standing wave –Isolation required between RF generator and VM –2 x peak voltage develops  Vector modulator is constructed with two hybrids and two phase shifters  Output amplitude and phase can be varied with the two phase shifters

8 Managed by UT-Battelle for the Department of Energy High Power RF Phase Shifters  Ferrimagnetic –YIG or microwave Ferrites can be used with magnetic field biasing (permeability varies) –Magnetic biasing (inductive) can be relatively slow for several hundred MHz application (<100 usec) –Low loss can be achieved with spin wave mode operation (tan  ~ )  Ferroelectric –BST (Barium Strontium Titanate) or similar materials show Ferroelectric property (permittivity variable with applied electric field) –Very high permittivity (  r >200) –Relatively high dielectric loss factor (tan  ~ 0.001) –Limited high average power handling with loss –Limited peak power handling with dimensions –Electric field biasing (capacitive) can be faster 5 th CW and High Average Power RF Workshop 2008

9 Managed by UT-Battelle for the Department of Energy 5 th CW and High Average Power RF Workshop 2008 Vector Modulator Test ●With two  phase shifters −Amplitude can vary 0 – 100% of the input −Phase can vary ± 90 

10 Managed by UT-Battelle for the Department of Energy  A special RF power splitting and control arrangement can provide high power efficiency while allowing the fan-out distribution  Using fast high power phase shifters, delivering required RF voltage vector to individual cavity can be possible  The number of phase shifters are basically identical to the power splitting system using vector modulators  The transmission line circuit works as an variable power splitter One to Multiple Cavities (RF Power Distribution with Maximum RF Power Efficiency)

11 Managed by UT-Battelle for the Department of Energy Realization of One Generator to Many Cavities  Using fast high power phase shifters, delivering the power to individual cavity with RF vector control can be possible –Use a phase shifter between cavities (as a variable length transmission line section ) –Use phase shifter with short circuit to provide reactive loading  The system works as a single system to deliver the RF amplitude and phase in each cavity as needed  Only the power needed by each cavity is delivered  For reduction of construction cost, using a very high power klystron as the RF power generator is favored

12 Managed by UT-Battelle for the Department of Energy System Equation (I) Consider an array of N-cavity loads connected to a transmission-line network. Let [V P ] be the port voltage vector of a set of specific cavity excitations for an optimum operation. where the short-circuit terminal admittance matrix of the whole system [Y P ] = port admittance matrix for the cavities, [Y T ] = short circuit admittance matrix of the transmission line network, and [Y L ] = load admittance matrix. The relation between the terminal currents [I S ] and the terminal voltages [V P ] is

13 Managed by UT-Battelle for the Department of Energy System Equation (II) The transmission line admittance matrix The reactive load admittance matrix and the cavity port admittance matrix are: The input impedance is found by selecting the element Z ii in impedance matrix [Z s ] If the i-th terminal is feeding with the generator, only I i =1 in the current matrix

14 Managed by UT-Battelle for the Department of Energy Solution Procedure where voltage reflection coefficient  (z) is related as This voltage standing wave in the transmission line section between the cavity and the input port is The voltage vector can be defined for standing wave with the measured reflection coefficient  (z) (for no power V i = 0.) The above equations can be solved for a specified voltages V P : the transmission-line characteristic impedances, Y S m-1, Y S m, and cavity spacing, d m-1 (d m ) are given so that the phase delays d m (d m-1 ), and reactive loads B m are found. The phase shifts are related to the lengths of the transmission line sections and the reactive loads as

15 Managed by UT-Battelle for the Department of Energy One-to-Many RF Distribution Matching with Redundancy  Superposition of several one-to-many schemes can be made to several-to- many for added redundancy  Any RF power generators can be shut off  Any load (cavity) can be disconnected or disabled −Any of the load voltages can be set to zero  All control is done with one control system the high power RF phase shifters to deliver RF power with almost no waste

16 Managed by UT-Battelle for the Department of Energy Cost Comparison for Powering a Cavity 800 MHz, 500 kW, 8% duty cycle (10 MW) Amp:CavityN:11:1 (SNS)1:N (VM)1:N Amplifier $1,800,000$180,000$100,000 (Solid-state, 600 x $3k) (Klystron) (Klystron, $2M/20) Power Supply, Modulator, Control integrated$120,000$100,000 Waveguide, Splitter/Combi ner $50,000$40,000$30,000 Circulatorintegrated$45,000 $10,000 ($200k/20) $10,000 ($200k/20) Vector modulator, Phase shifter $0 $40,000 Transmitter$50,000$140,000$70,000$40,000 LLRF$15,000$20,000$10,000 Water utilities$10,000$250,000$20,000$10,000 RF Cost / Cavity$1,925,000$795,000$380,000$340,000 Operation Power120%100%130%105%

17 Managed by UT-Battelle for the Department of Energy Summary  With device technology advancement and cost reduction of SSA, powering cavities with SSA may be feasible –Low voltage power supplies, hot swappable system, etc.  Fan-out Distribution –The fan-out system using vector modulators is feasible with power overhead –The fan-out system with reactive matching can work as a whole to deliver the exactly required RF amplitudes and phases at the cavities only with phase shifters –The phase delays and reactive loadings are realized by using high power fast phase shifters –Low-level RF control system that can be used with the fan-out systems are being developed