Christopher Nantista ILC 1 st Baseline Assessment Workshop

Christopher Nantista ILC 1 st Baseline Assessment Workshop September 7, 2010

Outline 1. Concept and Layout 2. Experimental Program 3. Experimental Results 4. Future Experiments 5. ILC Operation With KCS (Power Requirements) 6. ILC Operation With KCS (Power Flow)

Concept and Layout

Klystron Cluster System Layout surface rf power cluster building surface • service tunnel eliminated shaft ~1. 06 km upstream • underground heat load greatly reduced downstream accelerator tunnel CTO ~1. 06 km TE 01 waveguide

Shafts Each main linac is composed of 280 three-cryomodule rf units* KCS would best be accommodated by moving the main shaft locations from the RDR upstream as indicated and adding two additional shafts, for a total of 5, each serving two klystron clusters, one feeding upstream and one downstream. ~1. 064 km 28 28 28 ~684 m 28 28 28 ~380 m 28 28 beam -- main shaft -- additional KCS shaft Each KCS: 28 rf units, 30 klystrons worth of power, 34? klystrons *additional CM’s in e- linac to compensate undulator loss will require extension of one system. The RTML will use underground klystrons, not KCS.

Nominal Parameters # of shafts per main linac # of KCS systems per main linac # of rf units (tap-offs) per system # of cryomodules per system 5 × 2 systems/shaft 10 28 (1. 064 km) × 3 cryomodules/rf unit 84 × 8 2/3 cav. ’s/cryomod. # of cavities per system 728 # of klystrons/modulators per system ~32 -34* 18* 295 -310 148 -155 peak rf power per system (MW) -2 × 10 MW/klystron ½ beam power * Two extra are included to allow for one failure. One or two more may be needed as overhead for LLRF control.

Combining and Distributing Power Couplings ranging from ~1 to 1/28 to the TE 01 (low loss, no surface E-field)mode are required. CTO (Coaxial Tap-Off) “ 3 -port” coupler 1 3 2 determines coupling For combining, the tap-offs are installed backwards. Proper phase and relative amplitude needed for match (mismatched power goes to circulators). First and last CTO’s are 3 d. B units reversed relative to the others with port 1 shorted at the right phase. Power Combining: 2 2 1 3 3 -3 d. B l Power Dividing: -4. 8 d. B 1 2 3 2 1 -3 d. B 3 2 1 -4. 8 d. B 1 -3 d. B 2 3 2 1 -6 d. B 3 3 1 … -7 d. B 3 -3 d. B 2 1 l

Tunnel Cross-Section Layout (Preliminary ) Waveguide footprint will be reduced! KCS Option American Version KCS main waveguide and CTO (smaller along most of tunnel, as suggested by red circle) Can be made more compact by: RDR Layout • not combining outputs • removing pump ports from between CTO and windows • redesigning CTO w/ one output (10 MW window? ) courtesy of Tom Lackowski

Local RF Power Distribution Scheme Power from each CTO is locally distributed along 3 cryomodules containing 26 cavities. The cavities are mostly fed in pairs to eliminate the need for circulators. Distribution is tailored to accommodate measured gradient limits of cavities. CTO customizable coupler feed 1 feed 2 load hybrid 38 m

Prototype Local Distribution Systems (for Fermilab NML Facility) For power division control, split and recombine through different phase lengths. Version 1: VTO’s For ILC, eliminate circulators. Version 2: motorized Ubend phase shifters and folded magic-T’s For ILC, eliminate circulators and replace T w/ magic T or hybrid.

New Local Distribution Idea Version 2 (motorized U-bend phase shifters) is suitable where remote control is needed. Version 1 (VTO’s) is suitable where occasional manual adjustment is foreseen. For one time customization, the following version seems most economical and compact. Changing widths of upper and lower spacers oppositely changes coupling while maintaining phases. One phase shifter is eliminated, locking cavity phases for reflection cancellation. PARTS LIST (2 cavities): 3 magic-T’s 2 H-plane U-bends 2 H-plane bends 1 E-plane bend 1 phase shifter 2 loads 2 bi-directional couplers 2 semi-flex guides 2 WR 650 stubs (1/4 wavelength different) 4 customized spacers 1 waveguide run to next unit 25 gaskets + bolts

Experimental Program

Goals Clearly, a full KCS demonstration is not feasible within the scope of pre-project commitments and funding. The KCS R&D program at SLAC is aimed at establishing confidence in the scheme as a viable single-tunnel option for powering an ILC. The current (hardware) program is focused on: • • designing required components (rf & mechanical design) prototyping components testing functionality of components and assemblies high power performance testing (evacuated and pressurized)

Goals More specifically, our immediate goals are to efficiently transmit highpower rf through a ~10 m run of the circular TE 01 mode main waveguide using CTO’s and also to resonate the line to demonstrate robustness under high field levels such as it would see with full KCS power. Transmission Line 4. 5 MW 10 m of WC 1890 0 d. B ± 14. 9 psig ~4. 5 MW <1 MW back-shorted tap-in ~-25 d. B Resonant Line 10 m of WC 1890 ± 14. 9 psig ~75 MW (300 MV equiv. peak fields)

Progress To date, we’ve made: • Four 2. 44 m sections of WC 1890* • Vacuum pumpout spool for WC 1890 • A ¼ wave WC 1890 spacer • Two 3 -d. B CTO’s • Two tapers between CTO and WC 1890 • CTO shorting caps for transmission and resonant coupling • Four WR 650 pumpout spool/ flange adaptors for connecting CTO ports to vacuum windows * circular waveguide w/ 18. 90” (0. 48 m) diameter

Main Waveguide 2. 438 m (8’) 0. 480 m (18. 9”) Four 8’ sections (9. 75 m total) of 0. 48 m-diameter waveguide (WC 1890). Fabricated from formed aluminum sheets, welded and machined. (1 mm radius tolerance) one-side double grooved flanges: vacuum/pressure seal – Viton® O-rings rf seal – Bal Seal® canted coil contact spring

Coaxial Tap-Offs (CTO’s) The CTO (coaxial tap-off) is essentially a 3 -port device which couples power from its central circular TE 01 waveguide through a gap into a coaxial region. From here, power is coupled out into a wrap-around waveguide and split between two radial WR 650 ports. Variation of the gap allows different coupling values. In reverse, the CTO can be used for combining, given the correct power ratios and phases. By shorting one circular port, it can be made into a launcher. We have two welded aluminum 3 -d. B CTO’s.

CTO Auxiliary Parts NOTE: not final depths 13. 75” → 18. 90” Two circular step tapers to connect to WC 1890 main waveguide. Shorting end caps for launching and resonant coupling. launcher (not final depths) coupler

Vacuum Pumping SLAC L-band vacuum window crushed indium wire WR 650 pressure vacuum 20 l/s ion pumps will be used on vacuum ports built into window adaptors on either side of each CTO. Two 100 l/s ion pumps will pump through a special WC 1890 pump-out insert in the middle of the large circular waveguide run.

Experimental Assemblies CTO cold tests transmission tests input assembly 12. 894 m 9. 990 m resonant line tests Location: Roof of NLCTA bunker Power source: SNS modulator and Thales “ 5 MW” klystron

Experimental Results

Program Installation RF from klystron Pump-out CTO output ports with loads WC 1890 8 -ft sections CTO input ports

Indium Seal Vacuum Test In preparation for system assembly, we high-power tested, under vacuum, two WR 650 vacuum window adaptor/pumpout spools s back-to-back between two windows. The adaptors were connected with a grooved flange insert with crushed indium wire sealing. The dual purposes of the run were to test the indium seal and to test running aluminum WR 650 under vacuum. Power to Waveguide (k. W) We ran into a problem running above 200 -300 k. W which appears to be aluminum multipacting, unrelated to the indium. We will try repeating the experiment after cleaning and etching the parts, which may not have been properly prepared.

Initial CTO Cold Test S Parameters (d. B) This plot made by combining 4 -port cold test measurements for a 2 -port response of two CTO’s back-to-back, shorted with transmission “caps”. The latter were made deliberately too shallow with the intention, not yet realized, of tuning them by shims and remachining. red – reflection blue – transmission yellow – loss @1. 300 GHz: @1. 3036 GHz: R = -20. 43 d. B (0. 906%) R = -27. 84 d. B (0. 164%) T = -0. 0843 d. B (98. 08%) T = -0. 0421 d. B (99. 04%) Missing: 1. 02% Missing: 0. 800% These reasonable preliminary results demonstrate that the CTO basically works as expected. Since these measurements represent two at a given spacing, the match of a single CTO may be better or worse. Cap tuning and further characterization of the CTO’s was deferred due to (BAW driven) schedule reprioritization.

Resonant Test Setup Due to the approaching workshop, we decided to proceed as expeditiously as possible directly to the pressurized resonant test. We hoped to put power in and generate high fields in the 15 psig pressurized assembly before BAW. If KCS can work pressurized, there is no need to pursue the vacuum tests, and we can eliminate vacuum pumps from the envisioned ILC KCS system. ceramic block pressure windows substituted for pillbox vacuum windows end plate replacing taper and machinable tuning “cap”

Establishing a TE 01 n Resonance Near 1. 3 GHz Big pipe shorted w/ an end plate (no end taper or cap). Measured Resonances: 1. 27893 1. 27983 1. 28512 best 1. 28763 1. 29544 sharp 1. 3035 1. 30587 small 1. 31356 1. 31625 1. 32310 1. 32443 Analytic TE 01 n resonances for 11. 73 m of WC 1890 Analytic TE 01 n resonances (effective length for pipe, taper, CTO and shorting cap for 11. 73 m+0. 0711 m (lg/4) of WC 1890: chosen to fit data): 1. 2749 1. 2851 1. 2954 1. 3058 1. 3162 1. 3266 Murphy’s Law: TE 01 n resonances straddle 1. 3 GHz as best they can. 1. 2699 1. 2801 1. 2903 1. 3006 1. 3109 1. 3213 spacer added here Measured Resonances w/ lg/4 spacer: 1. 28006 1. 28250 1. 28756 1. 28907 1. 29026 1. 300565 1. 30531 1. 31094 1. 32131 Outsmarting Murphy w/ a lg/4 spacer we’d made for cold testing: TE 0, 1, 83 within a MHz of 1. 3 GHz.

S 11 (d. B) Characterizing the Waveguide Resonator Imaginary Q circle fit fr = 1. 300586 GHz (→ 1. 30023 GHz @ 14 psig) QL = 33, 787 b = 3. 3221 → Dfr. FWHM = 38. 5 k. Hz Q 0 = 146, 028 tc = 2 QL/w = 8. 27 s

Needed Input Power Calculation (WC 1890) 0. 593 c (WC 1375) SW energy density: u(z) = 2 Pt/vg(z), where Pt is the traveling wave power in either direction To produce field levels equivalent to 300 MW TW, one only needs ¼, or 75 MW, in Pt. Pi = 566 k. W U = 7. 19 J Pd = 402 k. W C = -27. 25 d. B

Interlock Field Probe As a safety interlock, a (non-directional, uncalibrated) probe of the WC 1890 rf field was included, so that power could be shut off in the case of field collapse due to rf breakdown. A loop antenna inserted about ½ way through a perforation hole in the WC 1890 pumpout spool. NWA showed ~-64 d. B coupling from the input port. For 566 k. W input, it should see 225 m. W.

Future Experiments

Further Work Further work can be done toward establishing the feasibility of the Klystron Cluster Scheme, including: • • demonstrating the matched tap-off function with a third CTO demonstrating power combining designing a very high-power TE 01 mode bend demonstrating transmission efficiency of such a bend demonstrating power (field) handling of such a bend developing a TE 01 mode diagnostic directional coupler studying the combining scheme/efficiency studying coupling errors, mode conversion, line resonances… ?

Overmoded Bends To get from the surface cluster building to the linac, we must transport ~310 MW L-band rf pulses around 3(? ) bends receiving and restoring the circular. TE 01 waveguide mode. Designing, prototyping and testing an appropriate bend is a primary focus of our next phase of KCS R&D. Below are examples of approaches taken at X-band. TE 20 General Atomics profiled curvature bend in corrugated waveguide Finned waveguide, breaking TM 11 degeneracy (Nantista et al. PAC ‘ 93) TE 20 TE 01 SLAC compact bend in 40. 6 mm circular waveguide tapering to and from rectangular TE 20 mode

Resonant Ring (2012) To test waveguide, bends, and CTO’s at full traveling wave power, a resonant ring could be built, incorporating a tap-off/tap-in section. This would require development of a CTO based directional coupler not required in ILC. It would also need a diagnostic coupler that could monitor power flow in either direction. Such an undertaking would presumably be beyond the scope of the current Technical Design Phase. phase shifter directional coupler 160 m of WC 1890 310 MW diagnostic bi-directional coupler tap-off tap-in

ILC Operation With KCS (Power Requirements)

Main Waveguide Power Transmission Loss a(a=0. 24 m) = 5. 671 10 -5 nepers/m, WC 1890 main waveguide a(a=0. 1746 m) = 2. 014 10 -4 nepers/m WC 1375 CTO waveguide For a 38 m rf unit that’s roughly 1. 5 m WC 1375 and 36. 5 m WC 1890, we have: [1. 5 m*2. 014 10 -4 nepers/m + 36. 5 m*5. 671 10 -5 nepers/m] =. 00237 nepers → power transmission = 0. 9953 (0. 47% loss/rf unit in main waveguide) Peak Power Flow in Main Waveguide / 10 MW For unit power at the 28 th rf unit, we need 29. 874 units at the first and 29. 945 at the shaft. That’s 6. 95% extra that gets attenuated in the main waveguide.

Cavity Gradient Limit Distribution While the design gradient for the ILC remains 31. 5 MV/m, in order to increase the yield of usable cavities, the acceptable range of limiting gradient has been increased to within ± 22. 2% of this value. From the following data plots, the drop-off of cavity yield with gradient would seem to be roughly linear over the range of interest, indicating a flat distribution of limits. 2007 2010 2 nd pass Cavity Yield 1 st pass Gradient (MV/m) Yamamoto/Ginsburg LCWS 2010

Cavity Pairing To eliminate the need for circulators at most of the cavities, they are powered in pairs through a hybrid or magic-T, with the reflected and discharged power combining into a load on the fourth port. This requires running the cavities in each pair at the same gradient, the lower of the two limits, resulting in a loss in average gradient. To minimize this effect, some degree of sorting of cavities to closely match gradient limits is called for. In our calculations, we assume an rf unit’s worth of cavities (26) is received and sorted between the 3 cryomodules as follows. Sorting Recipe: • Order 26 random cavities by gradient limit. • Pair the first, last, or middle 24 in order, whichever minimizes gradient loss. • Assign pairs alternately to each 13 -cavity feed, alternating between beginning and end of sort (i. e. of 12 ordered pairs, assign [1 12 3 10 5 8] and [2 11 4 9 6 7]. ) • Assign last 2 so as to minimize power requirement difference between feeds.

Effect on Average Gradient raise cavity Cavity Gradient Limit Distribution: distribution to flat from 24. 652 MV/m to 38. 739 MV/m (31. 6955 MV/m ± 22. 2%) compensate. Distribution Cavity Gradient Limit Distribution: flat from 24. 5 MV/m to 38. 5 MV/m (31. 5 MV/m ± 22. 2%) Mean: 31. 306 MV/m Std: 790. 5 k. V/m 0. 62% gradient degradation due to pairing Mean: 31. 502 MV/m Std: 798. 8 k. V/m

Achieving Flat Gradient Flat acceleration across the beam pulse is important for maximizing gradient. With control of power, Qe, and fill time, a steady state match can be achieved at the desired gradient in a cavity, giving constant acceleration with no power is reflected during the beam pulse. With KCS, 748 cavities are locked to a common source timing. With power and Qe adjustable, a steady state constant gradient is still achievable, but there will generally be some level of power reflected. Power Needed for Flat Acceleration Blue: timing set for zero reflection at center gradient Red: timing set for zero reflection 5% below center gradient Normalized to 31. 5 MV/m × 1. 0377 m × 9 m. A = 294. 2 k. W Mean: 1. 0533 Mean: 1. 0426 20% spread example For a given distribution of gradients, the timing can be set so as to minimize the average reflected power, not necessarily at the point where the center gradient is matched.

Achieving Flat Gradient (2) Below are the power needed and coaxial coupler setting for optimal common timing across the gradient range we are considering. reflected power timing set for match at 29. 8 MV/m eam b o t er in pow Average ~6. 0% above the nominal 294. 3 k. W. Qe, max/Qe, min = 3. 359 QL / 106 Power Needed for Flat Acceleration Gradient Distribution: flat 24. 652 - 38. 739 MV/m (31. 6955 MV/m ± 22. 2%) Lowering the match point also reduces the range of QL adjustment required. For upstream(downstream) KCS systems, the effective spread in fill time is ~7. 64(0. 40) s. This is roughly 1. 28%(. 07%) of the nominal fill time and has little effect on efficiency.

Needed Power at Cavities With the given distribution of cavities and sorting prescription, there will still be some statistical spread in how much power each rf unit needs. Since the CTO’s will not be adjustable, excess power will have to be provided to accommodate most (97. 5%? ) units. With no adjustment to the split between halves of an rf unit, the statistics are over 13 cavity feeds, or 1/2 rf units. The difference is marginal. Mean: 5. 89% over nominal (294. 3 k. W) Mean+2 std (97. 5%): 12. 49 % over (12. 15% w/ feed division adjustment) 5. 9% reflected, 6. 6% into end loads mean + 2 std (97. 5%) = 8. 5807 MW 2 (mean+2 std) = 8. 607 MW (0. 307% more) Mean: 8. 1018 MW Std: 239. 45 k. W Mean: 4. 0509 MW Std: 126. 30 k. W

RF Unit Layout end load for excess power w/o feed adjustment CTO adjustable coupler feed 1 feed 2 load hybrid 38 m w/ feed adjustment CTO adjustable coupler feed 1 feed 2 load hybrid

Loss Tally 294. 3 k. W × 1. 059 × 1. 062 × 26 ÷ 0. 95 × 28 ÷ 0. 935 ÷ 0. 983 ÷ 0. 993 ÷ 0. 965 ÷ 0. 977 294. 77 MW (nominal to beam per cavity = 31. 5 MV/m 1. 038 m 9 m. A) (for flat gradient w/ cavity gradient spread and common timing) (for statistical spread in feed/rf unit requirements w/ fixed couplings) (cavities/rf unit) (~5% local distribution losses) = 9. 06 MW/rf unit @ CTO (rf units) (6. 5% main waveguide losses) = 271. 3 MW @ beginning of linac run (shaft and bends) (combining CTO circular waveguide losses) ~8% (input circulator and WR 650 losses) klystron-to-tunnel (CTO coupling/klystron amplitude mismatches) from klystrons

Klystrons Needed per KCS The calculation/estimate suggests we need 294. 8 MW worth of klystron power. At 10 MW each, 30 klystrons would give us 300 MW (1. 7% to spare). However, we want to be robust against a single klystron failure per system. With N sources combined in a passive network, failure of one source leaves combined the equivalent of (N-1)2/N sources. With 32 klystrons, we have 320 MW (8% to spare). With one failure, and 31 on of 32, we have 300. 31 MW available (1. 8% to spare). However, we also need 5% overhead for LLRF compensation of such effects as beam current (loading) variation and Lorentz force detuning. This overhead could be harnessed via phase control of the rf drives, oppositely dephasing the klystrons in pairs, as illustrated. Out of phase components will Vtot deflect power to loads, while the in phase components contribute to combined power reduced as P = Pmax cos 2 f. To avoid the asymptotic loss of control as f→ 0, we might allow f to vary from 14. 96˚ (93. 3%) to 8. 13˚ (98%). The real power overhead needed for LLRF is then ~7% and the maximum power requirement rises to 294. 8 MW ÷ 0. 933 = 315. 8 MW, (of which up to 310 MW would be used) With 33 klystrons and one failed, we have 310. 3 MW (1. 8% low). With 34 klystrons and one failed, we have 320. 3 MW (1. 4% to spare). V 2 n f -f V 2 n-1

Summary There are multiple reasons for the increase in required klystrons per 28 rf units: klystrons: 28 equivalent to the RDR requirement, 1 per rf unit. +2 7% more for long range distribution for eliminating service tunnel. +2 for redundancy (allowing one failure). In the RDR, such failures had to be covered by including additional rf units. +2 to recover enough for 5(7)% LLRF overhead after a 12. 5% hit due to cavity gradient spread (flat gradient w/ common timing and feed statistics). Most of this hit would exist in the RDR scheme. 34 NOTE: The preceding calculations could well be off by ~2 -3 percent, depending on actual cavity distribution and error margins in loss estimates.

ILC Operation With KCS (Power Flow)

KCS Average Power Diagram* 4. 395 MW flow (MW) “wall plug” tunnel * 28 rf units, full power 4. 395 power supplies (. 92) heat loads 351. 7 k. W 4. 043 modulator (. 95) 202. 1 k. W 3. 841 klystron collector (. 60? ) 1. 5366 MW 2. 305 klystron waveguides 80. 9 k. W 2. 224 combining loads 51. 0 k. W 2. 173 combining waveguide (CTO’s) 15. 3 k. W 2. 157 shaft & bends 36. 5 k. W 2. 121 main KCS tunnel waveguide 138. 0 k. W 1. 983 local distribution waveguide 98. 9 k. W 1. 884 distribution end loads 109. 8 k. W 1. 774 cavity reflection loads 736. 2 k. W 1. 038 beam 1. 038 MW (31. 5 MV/m 1. 038 m 9 m. A 0. 969 ms 5 Hz 26 cav/unit 28 units)

Cavity Reflected Power (normalized) Main Waveguide Heat Load (W/m) Tunnel Heat Load Notes average: ~130 W/m fill discharge matched case Power dissipation along tunnel in the KCS main waveguide is non-uniform, declining roughly linearly from its peak at the shaft to ~1/30 this vale at the end of the KCS. Because ILC is pulsed, not CW, there is nominal power reflected from cavity into load (during fill and discharge) of ~38% of cavity input power. This is in addition to all attenuations and mismatch reflections. Additional heat load associated with the rf comes from the electronics crates in the tunnel. Preliminary estimates set this at ~200 -300 k. W per KCS.