Measurements and potential improvements of the amplifier for the 10 MHz cavities G.Favia, V.Desquiens, M.Morvillo

Overview ①The 10MHz RF system ②The 10MHz feedback system ③The 10MHz system PSPICE model ④The 10MHz system upgrade program (possible options) ⑤Concluding remarks

PS HIGH LEVEL RF SYSTEMS The PS machine contains cavities operating at different frequencies. The 10 MHz cavities are the most important because they accelerate the bunches to the desired energy and perform beam gymnastic. The 10 MHz cavities (10+1 double gap cavities tuneable from 2.8 to 10 MHz) are driven by amplifiers based on electron tubes, for reasons of radiation hardness and power dissipation.

RF amplifier adopting: 3x YL1056 tubes for predriver and driver 1xRS1084 in the final stage Housed in the cavity base Ferrite loaded cavity Gap voltage up to 10kVp (20kVp per cavity) Frequency range 2.8 – 10 MHz Tuning via bias current that drives the ferrite to partial saturation Shunt resistance of equivalent lumped circuit(due to ferrite losses): 3 MHz 10 MHz 10 MHz SYSTEM

①The 10MHz RF system ②The 10MHz feedback system ③The 10MHz system PSPICE model ④The 10MHz system upgrade program (possible options) ⑤Concluding remarks

BEAM INSTABILITIES The impedance of the cavity as observed by beam is several kΩ per gap due to ferrite losses and final tube anode resistance. This high impedance would lead to strong beam instabilities such as transient beam loading during bunch splitting manipulations CURE: WIDEBAND NEGATIVE FEEDBACK PERTURBATION DUE TO THE BEAM CAVITY CURRENT PROVIDED BY THE POWER RF AMPLIFIER FEEDBACK THE CAVITY IMPEDANCE IS REDUCED TO 305Ω/gap AT 3MHz WITH THE AMPLIFIER CONNECTED OBJECTIVE: INCREASE OF 6 dB

AMPLIFIER DESCRIPTION LOAD IMPEDANCE TRANSFORMATION FROM 50Ω TO 200Ω(1:4) FREQUENCY TUNING VIA A DC CURRENT 180° PHASE SHIFT FOR THE LOCAL FEEDBACK

①The 10MHz RF system ②The 10MHz feedback system ③The 10MHz system PSPICE model ④The 10MHz system upgrade program (possible options) ⑤Concluding remarks

PSPICE MODEL FOR PSC10 SYSTEM Several PSPICE model have been developed by D.Grier in order to reproduce the behaviour of the 10MHz system. The actual PSPICE model of both tubes YL1056 and RS1084CJ presents some differences from the datasheet due to the linear approximation of the curves. For this reason some simulations deviate from the measurements.

*PS AMP:RS1084CJ approx. with 1500v Ug2.SUBCKT RS1084CJ-1500VS ;(CATH GRID ANODE SCREEN) *I sources for cath.,screen,grid,anode as fn(Vg,Va) Gk value = {v(2,10)*(V(1,10)+(V(30,10)/V(3,10)))};cathode Gs value = {v(4,10)*v(5,10)*v(2,10)};screen I Gg value = {v(6,10)*1};grid I Ga value = {v(2,10)*(i(vmk)-i(vms)-i(vmg))};anode I * *tables(construct from constant I curves) *E1=sum Is+Ia+Ig (=Ik) as fn Vg at Va= E table {V(20,10)} = (-290,0) (-250,0.5) (-210,2) (-175,5) (-140,10) (-85,20) (-40,30) (5,40) (45,50) (85,60) (130,70) (175,80) * *E3=gives slope of Ik with Va for different Vg when substit. in Gk----- E table {V(20,10)} = (-500,50meg) (-320,200k) (-300,25K) (-240,6k) (-220,4400) (-160,2000) (-140,1500) (-40,1300) (130,1100) * *E2 truncates all I for -ve Va E table {V(30,10)} = (30,0) (100,1) * *E4 & E5 decribe Is as fn(Va,Vg=0v) & (Is as multip f(Vg,Va=2kV) E table {v(30,10)} = (1.1k,12) (1.2k,8) (1.7k,4) (2.3k,2) (3.3k,1) (4.8k,0.5) (10k,0.2) E table {v(20,10)} = (-290,0) (-270,0.025) (-250,0.0625) (-210,0.125) (-170,0.25) (-100,0.05) (0,1) (100,1.5) (220,2.67) (280,4) * *E6=grid I as fn(Vg) E table {v(20,10)} = (0,0) (10,0.5) (20,1) * *Electrode capacitances Ckg pF Cka pF Cag pF Cg1k pF Cg1g pF Cag pF * Dummy resistors across table nodes Rd MEG Rd MEG Rd MEG Rd MEG Rd MEG Rd MEG Rd Rd MEG * I meas.voltmeters Vms Vmk Vma Vmg ENDS TUBES PSPICE MODEL ANOMALIES

TUBES PSPICE MODEL IMPROVEMENT * PSpice Model Editor - Version SUBCKT YL VS ;(CATH GRID ANODE SCREEN) *I sources for cath.,screen,grid,anode as fn(Vg,Va) Gk value = {v(2,10)*(V(1,10)+(V(30,10)/V(3,10)))};cathode Gs value = {v(4,10)*v(5,10)*v(2,10)};screen I Gg value = {v(6,10)*1};grid I Ga value = {IF(v(20,10)>-38, +(V(100,10)+ + V(101,10)*V(20,10)+ + V(102,10)*V(20,10)*V(20,10)+ + V(103,10)*V(20,10)*V(20,10)*V(20,10)),0)} ;anode I * tables of coefficient for the Anode current fitte with 3th degree polinomial curve * the parameter is the Anode voltage the variable is the grid voltage E table {v(30,10)}= (200,2.79)(300,2.762) (400,2.732) +(600,2.821) (2500,3.002) * E table {v(30,10)}=(200,0.2195)(300,0.1974) (400,0.1769) +(600, )(2500,0.1859) * E table {v(30,10)}= (200, )(300, ) (400, ) +(600, ) (2500, ) E table {v(30,10)} = (200,4.861e-05)(300,2.815e-05) +(400,1.131e-5)(600,1.131e-5)(2500,1.012e-05) * Rd Rd Rd Rd *E1=sum Is+Ia+Ig (=Ik) as fn Vg at Va= E table {V(20,10)} = (-50,0) (-41,.2) (-32,0.6) (-26.5,1) (-23,1.5) (-18,2) (-7.5,4) (2,6) (10,10) *E3=gives slope of Ik with Va for different Vg when substit. in Gk----- E table {V(20,10)} = (-50,300K) (+20,100K) *E2 truncates all I for -ve Va E table {V(30,10)} = (30,0) (100,1) *E4 & E5 decribe Is as fn(Va,Vg=0v) & (Is as multip f(Vg,Va=.5kV) E table {v(30,10)} = (300,.8) (450,.7) (600,.6) (700,.58) (1.2K,.45) (1.4K,.4) (2.2k,.3) (3k,.25) (5K,.2) E table {v(20,10)} = (-20,.15) (-15,.25) (-12,.5) (-10,.6) (-7,.7) (-5,.83) (0,1) (5,1) *E6=grid I as fn(Vg) E table {v(20,10)} = (0,0) (10,.5) (20,1) *Electrode capacitances Ckg pF Cka pF Cag pF Cg1k pF Cg1g pF Cag pF * Dummy resistors across table nodes Rd MEG Rd MEG Rd MEG Rd MEG Rd MEG Rd MEG Rd Rd MEG * I meas.voltmeters Vms Vmk Vma Vmg ENDS

①The 10MHz RF system ②The 10MHz feedback system ③The 10MHz system PSPICE model ④The 10MHz system upgrade program (possible options) ⑤Concluding remarks

INCREASED GAIN OF PREDRIVER AND DRIVER STAGEI anodeV anodeGain Predriver0.7 A800 V13.3 dB Driver1.2 A1200 V PRESENT CONFIGURATION: CHANGE OF TUBES WORKING POINT: STAGEI anodeV anodeGain Predriver0.9 A600 V14.63 dB Driver1.8 A800 V TOTAL INCREASE OF GAIN: ~4dB FOR THE FIRST TWO STAGES

INCREASED GAIN OF THE FINAL STAGEI anodeV anodeOPEN LOOP CAVITY Gain Final1.5 A15 kV PRESENT CONFIGURATION: INCREASE OF ANODE BIAS CURRENT: STAGEI anodeV anodeOPEN LOOP CAVITY Gain Final1.5 A15 kV72.84 dB Final2 A15 kV74 dB Final2.5 A15 kV74.31 dB Final3 A15 kV74.61 dB HIGHER DISSIPATED POWER A NEW COOLING SYSTEM FOR THE AMPLIFIER IS REQUIRED

OTHER IMPROVEMENTS Replace the final grid resonator by three devices in order to separate the functions of impedance transormation, variable inductance and phase inversion RESONATOR MADE BY FERRITE TOROIDS BIASED BY A MAGNETIC FIELD GENERATED BY A DC CURRENT IN THE TUNING WINDINGS VARIABLE INDUCTANCE 1:4 TRANSMISSION LINE TRANSFORMER THAT REDUCES THE LEAKAGE INDUCTANCE AND STRAY CAPACITANS OF THE ACTUAL TRANSFORMER IMPEDANCE TRANSFORMATION Take the feedback from the cavity gap in order to avoid resonance on the anode of final. The cavity is proven to be a good bandpass filter.

①The 10MHz RF system ②The 10MHz feedback system ③The 10MHz system PSPICE model ④The 10MHz system upgrade program (possible options) ⑤Concluding remarks

CONCLUSIONS The new PSPICE model based on Grier’s one will be realized in order to reach results closer to real measurements. That requires:  Revision of the entire actual model  Replacement of the actual tubes model  Measurements on tubes to verify if datasheet specifics follow real tubes behaviour

CONCLUSIONS As observed a higher amplifier gain can be reached without re-designing the entire system. That requires:  Change of working point for YL1056 amplifier keeping the same dissipated power  Moving the anode bias current of the final tube and realization of a new cooling system Using the addictional gain will probably be possible to increase the overall feedback of 3 dB. Other modifications will be necessary:  Development and implementation of a new grid resonator  Move the overall feedback pickup from the anode to one cavity gap IF THESE TESTS WILL NOT SHOW SIGNIFICANT IMPROVEMENTS A NEW SOLUTION WILL BE ADOPTED, USING FOR EXAMPLE SOLID-STATE AMPLIFIERS

Thank you for your attention