RF Power Generation I

RF Power Generation I Gridded Tubes and Solid-state Amplifiers Professor R.G. Carter Engineering Department, Lancaster University, U.K. and The Cockcroft Institute of Accelerator Science and Technology Overview • High power RF sources required for all accelerators > 20 MeV • Amplifiers are needed for control of amplitude and phase • RF power output – 10 kW to 2 MW cw – 100 kW to 150 MW pulsed • Frequency range – 50 MHz to 50 GHz • Capital and operating cost is affected by – Lifetime cost of the amplifier – Efficiency (electricity consumption) – Gain (number of stages in the RF amplifier chain) – Size and weight (space required) <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">2</li></ul>General principles • RF systems – RF sources extract RF power from high charge, low energy electron bunches – RF transmission components (couplers, windows, circulators etc.) convey the RF power from the source to the accelerator – RF accelerating structures use the RF power to accelerate low charge bunches to high energies P  P  PRF out  Heat <ul style="display: flex;"><li style="flex:1">RF in </li><li style="flex:1">DC in </li></ul><ul style="display: flex;"><li style="flex:1">P</li><li style="flex:1">P</li></ul><ul style="display: flex;"><li style="flex:1">RF out </li><li style="flex:1">RF out </li></ul>• RF sources Efficiency  P  P P<ul style="display: flex;"><li style="flex:1">DC in </li><li style="flex:1">RF in </li><li style="flex:1">DC in </li></ul>– Size must be small compared with the distance an electron moves in one RF cycle <ul style="display: flex;"><li style="flex:1"></li><li style="flex:1"></li></ul>PRF out Gain dB 10log   10  <ul style="display: flex;"><li style="flex:1"></li><li style="flex:1"></li></ul>– Energy not extracted as RF must be disposed of as heat PRF in <ul style="display: flex;"><li style="flex:1"></li><li style="flex:1"></li></ul><ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">3</li></ul>RF Source Technologies <ul style="display: flex;"><li style="flex:1">• Vacuum tubes </li><li style="flex:1">• Solid state </li></ul><ul style="display: flex;"><li style="flex:1">– High electron mobility </li><li style="flex:1">– Wide band-gap materials (Si, </li></ul>GaAs, GaN, SiC, diamond) – Low carrier mobility – Small size – Large size – High voltage – High current • Tube types – Low voltage – Gridded Tubes (Tetrodes) – Inductive output tubes (IOTs) – Klystrons • Single Transistors – Si LDMOS: 450 W at 860 MHz – GaN: 180W at 3.5 GHz; 80 W at 9.6 GHz – Gyrotrons – Magnetrons (locked oscillators) – GaAs: 65 W at 14 GHz <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">4</li></ul>SOLEIL 352 MHz amplifier •••Output power 180 kW 726 × 315 modules in 4 towers 2 × Si LDMOS transistors per module in push-pull ••53dB Gain Overall efficiency ~ 50% Images courtesy of Synchrotron SOLEIL <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">5</li></ul>Solid State <ul style="display: flex;"><li style="flex:1">Advantages </li><li style="flex:1">Disadvantages </li></ul>• No warm-up time • High reliability • Low voltage (<100 V) • Air cooling • Complexity • Losses in combiners • Failed transistors must be isolated • Electrically fragile <ul style="display: flex;"><li style="flex:1">• High I2R losses </li><li style="flex:1">• Low maintenance </li></ul><ul style="display: flex;"><li style="flex:1">• High stability </li><li style="flex:1">• Low efficiency </li></ul>• Graceful degradation <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">6</li></ul>Tetrode construction Cathode Control grid Screen grid Anode Images courtesy of e2v technologies <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">7</li></ul>Tetrode characteristics n Vg 2 Va Ia  C Vg1    2 a • n = 1.5 to 2.5 • DC bias conditions relative to cathode – Anode voltage positive – Screen grid positive (~ 10% Va) – Control grid negative • Anode current depends – Strongly on control grid voltage – Weakly on anode voltage • In practice Anode is at earth potential Graph courtesy of Siemens AG <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">8</li></ul>Tetrode common grid connection • Grids held at RF ground isolate input from output • Input is coaxial • Anode resonant circuit is a reentrant coaxial cavity • Output is capacitively or inductively coupled <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">9</li></ul>Class B operation •Control grid bias set so that anode current is zero when no RF input ••Conduction angle = 180° Resonant circuit makes anode voltage variation sinusoidal 2I2  I0 V2  0.9V0 ••Va > Vg2 always Theoretical efficiency ~70% 1 0.9 P  I2V2  I0V0 2<ul style="display: flex;"><li style="flex:1">2</li><li style="flex:1">4</li></ul><ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">10 </li></ul>Classes of amplification Class A Conduction angle Maximum theoretical efficiency <ul style="display: flex;"><li style="flex:1">Gain increasing </li><li style="flex:1">Harmonics </li></ul>increasing <ul style="display: flex;"><li style="flex:1">360° </li><li style="flex:1">50% </li></ul>AB B 180° – 360° 180° 50% - 78% 78% <ul style="display: flex;"><li style="flex:1">C</li><li style="flex:1">< 180° </li><li style="flex:1">78% - 100% </li></ul>• All classes apart from A must have a resonant load and are therefore narrow band amplifiers • Class AB or B usually used for accelerators <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">11 </li></ul>CERN 62 kW 200 MHz amplifier • RS 2058 CJ tetrode – Siemens (now Thales) • Class AB operation – Assume Class B for illustration • Efficiency - 64% Photo courtesy of e2v technologies Photo courtesy of CERN <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">12 </li></ul>Tetrode amplifier design •••Choose Va = 10 kV, Vg2 = 900 V Choose Va = 1.5 kV when Ia = Ipk Theoretical Class B efficiency 0.85 th   67 % 462 P   92.5 kW 00.67 88.6 I0   9.25 A 10 ••Assume Ipk = 4I0 (theoretically π I0) Ipk  37 A and draw the load line Graph courtesy of Siemens AG <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">13 </li></ul>Calculation of performance (1) Angle <ul style="display: flex;"><li style="flex:1">Va </li><li style="flex:1">Ia </li></ul>•Find Va and Ia in 15° steps <ul style="display: flex;"><li style="flex:1">(deg) (kV) </li><li style="flex:1">(A) </li></ul>Va V0 V2 cos <ul style="display: flex;"><li style="flex:1">0</li><li style="flex:1">1.5 </li></ul>1.8 2.6 4.0 5.7 7.8 10.0 37 35 30 20 8.5 3 15 30 45 60 75 90 •Find I0 and I1 by numerical Fourier analysis 1P  V2I2 P  I0V0 2020P2V2   R2  V0 = V2 = P0 = 10.0 kV 8.5 kV 96 kW I0 = I2 = <ul style="display: flex;"><li style="flex:1">9.6 </li><li style="flex:1">A</li></ul>P0I2 16.2 A ••Initial estimate of Ipk/I0 was too high Iterate for self-consistent solution P2 = R2 = 69 kW <ul style="display: flex;"><li style="flex:1">72 </li><li style="flex:1">%</li><li style="flex:1">524 </li></ul>1<ul style="display: flex;"><li style="flex:1">I  (0.5I  I </li><li style="flex:1"> I </li><li style="flex:1"> I </li><li style="flex:1"> I </li><li style="flex:1"> I </li></ul>) 0a0 a15 a30 a45 a60 a75 12 1I  (I 1.93I a0 <ul style="display: flex;"><li style="flex:1">1.73I </li><li style="flex:1">1.41I </li><li style="flex:1"> I </li></ul>a45 a60  0.52I ) 2<ul style="display: flex;"><li style="flex:1">a15 </li><li style="flex:1">a30 </li><li style="flex:1">a75 </li></ul>12 <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">14 </li></ul>Calculation of performance (2) • Grounded grid operation <ul style="display: flex;"><li style="flex:1">Calculated </li><li style="flex:1">Measured </li></ul>10 V0 I0 <ul style="display: flex;"><li style="flex:1">10 </li><li style="flex:1">kV </li></ul>AV  80  240  320 V 19.6 69 9.4 P2 P1 Gain <ul style="display: flex;"><li style="flex:1">62 </li><li style="flex:1">kW </li></ul>kW dB %I1  I2  Ig1 RF  I2 2.6 14.3 72 1.8 V115.4 64 R   20  1I1 1P  V I  2.59 kW <ul style="display: flex;"><li style="flex:1">1</li><li style="flex:1">1 1 </li></ul>2W.Herdrich and H.P. Kindermann, “RF power amplifier for the CERN SPS operating as LEP injector”,   P2  P<ul style="display: flex;"><li style="flex:1">Gain 10log10 </li><li style="flex:1">14.3 dB </li></ul>CERN SPS/85-32, PAC 1985 1<ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">15 </li></ul>Tetrode input and output circuits • Input and output circuits are coaxial b  Z0  60ln    a  • a = inner diameter, b = outer diameter • Characteristic impedance is very low (~ few ohms) • Careful design of matching is essential R  20  Xkg1  5.7  in Rout  524  Xg 2a  20  <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">16 </li></ul>Cooling and protection <ul style="display: flex;"><li style="flex:1">Anode Cooling </li><li style="flex:1">Protection </li></ul>•••Air ••••Coolant flow <ul style="display: flex;"><li style="flex:1">Water </li><li style="flex:1">Coolant temperature </li></ul><ul style="display: flex;"><li style="flex:1">Tube temperature </li><li style="flex:1">Vapour phase </li></ul>Anode, screen and grid overcurrent •Anode voltage (fast) Switch-on sequence ••Heater voltage Grid bias (pause) •••Anode voltage Screen grid voltage RF drive Image courtesy of e2v technologies <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">17 </li></ul>Combining tetrode amplifiers CERN SPS 200 MHz, 500 kW, amplifiers Photo courtesy of CERN <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">18 </li></ul>Tetrode limitations •••Cathode current density Anode dissipation Transit time •••Anode length << λ0 Anode diameter RF screen grid dissipation Thales Diacrode® reduces this –Va least when Ia greatest •Voltage breakdown Image courtesy of Thales Electron Devices <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">19 </li></ul>Inductive output tube (IOT) Differences from tetrode • Electron flow axial – Requires axial magnetic field to prevent beam spreading • Anode voltage is constant – Electron velocity is high • Bunched beam induces current in output cavity • Separate electron collector – Large collection area • Increased isolation between input and output <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">20 </li></ul>IOT output gap interaction • Beam current class AB or B like a tetrode • At resonance electric field in the gap is maximum retarding when bunch is in the centre of the gap • Effective gap voltage reduced by transit time effects • Effective gap voltage less than ~0.9V0 to allow electrons to pass to the collector • Theoretical efficiency ~ 70% 14P  I2Vg,eff  0.9I0V0 22<ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">21 </li></ul>UHF IOT for TV broadcasting Frequency Power 470 - 810 MHz 64 kW Beam voltage Beam current Gain 32 kV 3.35 A 23 dB <ul style="display: flex;"><li style="flex:1">Efficiency </li><li style="flex:1">60% </li></ul>Photos courtesy of e2v technologies <ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">22 </li></ul>Examples of IOTs for accelerators Frequency Beam voltage Beam current RF output power Efficiency 267 67 500 1300 MHz 40 3.5 90 25 1.0 16 kV <ul style="display: flex;"><li style="flex:1">A</li><li style="flex:1">6.0 </li></ul>280 70 kW <ul style="display: flex;"><li style="flex:1">%</li><li style="flex:1">>65 62 </li></ul><ul style="display: flex;"><li style="flex:1">>22 21 </li><li style="flex:1">Gain </li><li style="flex:1">22 </li><li style="flex:1">dB </li></ul><ul style="display: flex;"><li style="flex:1">June 2010 </li><li style="flex:1">CAS RF for Accelerators, Ebeltoft </li><li style="flex:1">23 </li></ul>

RF Power Generation I

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