RF test benches for electron cloud studies

Transcription

1 RF test benches for electron cloud studies Fritz Caspers 1, Ubaldo Iriso Ariz 2, Jean-Michel Laurent 2, Andrea Mostacci 1 1 PS/RF Group, 2 LHC/VAC Group

2 1. The Traveling Wave multiwire chamber 1.1. Introduction: the TW chamber and the need of a Ring Resonator 1.2. Improvements on the original TW chamber 1.3. Coupler requirements 1.4. Calculation and design of strip-line type coupler circuit 1.5. The final Ring Resonator 1.6. Conclusions and outlook 2. The Standing Wave single conductor coaxial chamber 2.1. Introduction: Why a Standing Wave coaxial chamber? 2.2. Experimental set-up 2.3. Multipacting in a resonant TEM structure 2.4. Scrubbing effect in samples and for the total chamber 2.5. Conclusions and outlook 2

3 1.1. Introduction: the TW chamber RF load ø60 ø !Short pulses travel along 6 inner wires simulating the proton bunches and producing multipacting in the chamber. 70 cut-off tub e 1500!The pulses are dumped in a load placed at the end of the vacuum chamber. 235!Lay-out voltage limited by the wideband 100 W power amplifier: Vpk-pk MAX ~150V " E electron_max ~75eV T1, T2, Uin (for stainless steel) Signal generator Wideband Power amplifier (100 W) +Vpol 3

4 Multipacting limits in Secondary Emission Yield: SEY S TAINLESS S TEEL S ECONDARY ELECTRON YIELD (S EY) MEASURED BY Y. BOJKO CERN-LEP-VAC ST. STEEL AFTER BAKE-OUT 350ºC Electron Energy (ev)! Before bombardment, E e =75eV " SEY~1.6 Courtesy of N.Hilleret! After bombardment, SEY(E e =75eV) < 1.3 " no multipacting! GOAL: E e for LHC: 250 ev " new lay-out for increasing V in a factor ~3-4. 4

5 Ring Resonator (RR) principle 90 deg. phase shifter RF load Wideband directional coupler Button e-meter!the pulsed power is not dumped into the load, but re-circulated again inside the chamber by using a directional coupler. Signal Pulse Generator Wideband Power amplifier (100 W) 5

6 Motivation for the RR: loop power Gain G - power gain c = 1 db c = 20dB loop power gain c ranging from 1 to 20 db G = α c c = coupling factor (db) α = set-up attenuation G = total power Gain c a - one-way attenuation (db) REFERENCE: Microwave filters, impedance-matching networks, and coupling structures. G.L.Matthaei, L/ Young, E.M.T.Jones α~0.6 db c~10 db " G power ~8 db (voltage factor~2.5). 6

7 1.2. Original TW chamber requirements 0-2 Attenuation in transmission. Att (db) Freq (MHz) Initial status Current status! Total losses along the RR must be < 1 db (according to plot before)! Finally, a chamber < 0.3 db up to 600 MHz (after improvements) 7

8 1.3. Coupler requirements G power =8 db Bandwidth: MHz Coupling factor c=10 db DC isolation up to 1 KV Very low transmission losses at each arm ( ) db SOLUTION: l/4 symmetric 9 sections coupler! Examples for multiple section l/4 strip-line coupler characteristics REFERENCE: Strip-line circuit design, Harlan Howe, JR In our design, the relative Bandwidth=18, ripple= 1 db Since f center =300 MHz " l/4 = 25 cm " total coupler length = 2.25 m! 8

9 Reasons for the choice of the presently used coupler layout Not easy to find on the market a coupler with such a characteristics: c=10 db, BW=2 decades, standing about 1 kv between the strip-lines and to ground Not a printed version because metallic losses increase due to the presence of a dielectric (increase of current density) Shielding box not bigger than roughly 10x10 cm in cross section: limit due to propagation of wave-guide modes (cutoff around 1 GHz) That s why we decided to build it ourselves!! 9

10 1.4. Calculation and design of strip-line circuit l/4 Z1 Z2 Z3 Z4 Z5 Z4 Z3 Z2 Z1 Each section has to be designed according to tabulated impedance values in odd and even mode.! Design was done by SuperFish (2-d Electrostatic computer code)! The design was tested and measured for each section in a test bench (50 cm long) using VNA. 10

11 1.5. The final Ring Resonator coupler multiwire chamber Ring Resonator and does it really work? 11

12 Main coupler characteristics: Odd and Even mode impedances Z(W) Theoretical values Odd mode impedance measurements Even mode impedance measurements time(ns) 12

13 Main coupler characteristics: coupling factor 40 MHz 0 20 MHz -10 db Theoretical coupling factor Calculated coupling factor Measured coupling factor f (GHz) * Thanks to C. Deibele for having carried out numerical calculations for the coupler parameters. 13

14 Ring Resonator power enhancement: coupler direct ch db E E E E E E E+08 f(hz)! Every 40 MHz (LHC beams), the minimum increase is 6 db 14

15 1 Phase offset deg phase offset in the freq domain. Constant amplitude No phase offset in the freq domain. Constant amplitude time(ns)! Better use of the amplifier power limiting characteristics by applying a 90 deg phase shifter: bipolar to polar Gaussian pulse gives a gain of ~ 3 db 15

16 1.6. Conclusions and outlook Preliminary tests conclude that the coupler (in fact, the Ring Resonator) works reasonably well Expected increase of the Traveling Wave voltage by about a factor of 3: 2 (= 6 db measured) from the coupler, 1.5 from the phase shifter and amplifier limitations properties. The difference in electron energy (Ee): original set-up: E e =75 ev current set-up: E e =225 ev Modeling and scaling of 2-point type multipacting (as produced in accelerators) Characterize of scrubbing effect for different surface coatings or treatments 16

17 The second RF test bench: 2. The Standing Wave single conductor coaxial chamber 2.1. Introduction: Why a Standing Wave coaxial chamber? 2.2. Experimental set-up 2.3. Multipacting in a resonant structure 2.4. Scrubbing effect for a chamber and for samples 2.5. Conclusions and outlook 17

18 2.1. Why a Standing Waves coaxial chamber? High electric fields are easily obtained by using standing waves with the limited power available High surface electron bombardment dose Scrubbing effect for different surface treatments in a whole chamber (not just on a sample) is suitable But Sinusoidal fields " multipacting is one-point type, not like in particle accelerators 18

19 2.2. Experimental set-up Signal Pulse Generator 50 W Pow er am plifier Incident wave Transmitted wave Coaxial TEM line shorted at each end RF Power inferred via a adjustable magnetic coupler Electron pickup E l/2 configuration in TEM mode: E max always in the center and perpendicular to wall surface Dext=100 mm Dint=32 mm Bombardment dose measured by integrating the electron current in the pick-up Pumping port The set-up allows testing multipacting in samples. 19

20 Samples testing BNC connector ZEFT CF Sample samp le Ø 30 Ø x 21 Ø 35 CF Electron p ickup with grid Electron pick-up with grid. CF 16 ø12 Pickup samp le holder Pick-up sample holder. 20 Since E is always maximum in the center of the chamber, by placing a sample glued to the pick-up we can trigger multipacting in the sample, if the multipacting level for the sample is lower than the one for the stainless steel. Ferrite and amorphous carbon (a-c) has been tested in this set-up. 20

21 2.3. Multipacting in a Resonant TEM structure One point type multipacting: 2ary e - cosmic ray 1ary e - e - avalanche Outer wall E(z) zhml B(z) B(z) zhml E(z) Multipacting takes place when E is high enough to produce the e - avalanche 21

22 Multipacting signatures (1)!Pressure increase during multipacting 1.E-06 E = 6.5 kv/m E = 7 kv/m P (mbar) 1.E-07 1.E-08 E = 4 kv/m E = 4 kv/m P evolution during multipacting for stainless steel time (min)! Multipacting level is defined as the minimum electric field E to trigger multipacting. For stainless steel, multipacting level before bombardment is set in 5.8 kv/m. 22

23 Multipacting signatures (2) Data taken modulating E amplitude around the multipacting level.! Electron current (I e ): No I e is detected before multipacting level. When multipacting level is exceeded, I e increases with E.! Cavity detuning: 1) Transmitted wave level off 2) Reflected wave increase 3) Electron current detected! Electron cloud detunes the cavity! 23

24 2.4. Scrubbing effect (1) Scrubbing effect for a stainless steel chamber Stainless steel as received Exposed dose 0.34 mc/mm2 Exposed dose 1.20 mc/mm2 Ie(µA) E (V/m)! Multipacting level increases with the exposed dose 24

25 2.4. Scrubbing effect (2) Scrubbing effect for a ferrite sample Multipacting level for stainless steel 10 8 Ie(µA) Multipacting level for ferrite st measure after venting Exposed dose 0.06 mc/mm2 Exposed dose 0.17 mc/mm E (V/m)! Multipacting is first produced in the center of the chamber (where the ferrite sample is placed) and then in the other parts (stainless steel). 25

26 2.4. Scrubbing effect (3) Scrubbing effect for a NEG coating before activation NEG as received Exposed dose = mc/mm2 Exposed dose = mc/mm2 I(mA) NEG= Non Evaporable Getter NEG thickness film: 1 mm E (V/m)! Scrubbing effect is remarkable even before activation 26

27 NEG after activation! No I e is detected, no changes in Reflected Wave, no changes in Transmitted Wave but P increases. 1.0E-6 after activation before activation, after bombardment before activation and bombardment P(mbar) 1.0E-7 1.0E-8 Increase of P at 10.6 kv/m, roughly twice the m.l. for stainless steel; or 5 times for ferrite. 1.0E E (V/m) 27

28 2.5. Conclusions Suitable set-up to study scrubbing effect not only for samples, but also for surface coatings or treatments. Space charge due to electron cloud detunes the cavity For a NEG coating, preliminary results show no evidences for electron presence after activation, but P increases. and Outlook Details study of NEG coating behavior Determine the E e both with computer code (collaboration with G.Rumolo) and measurements Measure rise time for electron cloud build-up Test for other surface treatments: TiN and ArGD 28