PEP Super-B High Power RF Peter McIntosh SLAC Super-B Factory Workshop in Hawaii April 2005 University of Hawaii

Stanford Linear Accelerator CenterOutline RF Requirements Cavity Limitations Voltage Voltage Power PowerKlystrons 1.2 MW 1.2 MW 2.4 MW 2.4 MWCirculators HVPS System System Configurations Conclusions

Stanford Linear Accelerator Center RF Requirements 3 cavity solutions being investigated: R/Q = 5, 15 and 30  (see S Novokhatski’s talk). R/Q = 5, 15 and 30  (see S Novokhatski’s talk). The RF power required for L = 7e35 and 1e36 varies as a function of cavity option as the R/Q impacts primarily the HOM losses: As R/Q goes up  cavity HOM losses go up! As R/Q goes up  cavity HOM losses go up! The R/Q also impacts the cryogenic losses which affect the Total AC power required: As R/Q goes up  cavity cryogenic losses go down! As R/Q goes up  cavity cryogenic losses go down! For the cavity options being investigated, the net difference in Total AC power is almost zero! Assuming the cavity to be used lies somewhere between 5 – 30  we can see that ……

Stanford Linear Accelerator Center RF and AC Power (5  )

Stanford Linear Accelerator Center RF and AC Power (30  ) Increased Reduced

Stanford Linear Accelerator Center RF and AC Power Summary To define the number of cavities required, have assumed that 1 MW can be supplied to each RF cavity (see later). For L = 7e35 using R/Q = 5  cavity: LER = 21.7 MW LER = 21.7 MW HER = 16.2 MW HER = 16.2 MW For L = 7e35 using R/Q = 30  cavity: LER = 22.1 MW LER = 22.1 MW HER = 16.2 MW HER = 16.2 MW For L = 1e36 using R/Q = 5  cavity: LER = 39.8 MW LER = 39.8 MW HER = 24.9 MW HER = 24.9 MW For L = 1e36 using R/Q = 30  cavity: LER = 42.0 MW LER = 42.0 MW HER = 25.0 MW HER = 25.0 MW Cavity HOM losses increase by 2.2 MW in the LER at 1e36. Total AC cryogenic power however reduces considerably for the 30  cavity by 50% for both luminosity options compared to the 5  cavity. Net AC power difference is comparable (to within 2%) for each cavity option at each luminosity.

Stanford Linear Accelerator Center R/Q=15  Solution

Stanford Linear Accelerator Center Cavity Limitations - Voltage Practical achievable voltage/cell depends upon: Cavity Q o Cavity Q o Niobium purity Niobium purity Cryogenic operating temperature Cryogenic operating temperature Cryogenic load Cryogenic load For the R/Q = 5, 15 and 30  cavities: Required voltage per cell V c = 1.25 MV, requiring Q o = 3e9, 1e9 and 1e9 respectively. Required voltage per cell V c = 1.25 MV, requiring Q o = 3e9, 1e9 and 1e9 respectively. For feedback stability R/Q = 5  preferable  lowest detuning (see D. Teytelman’s talk) For feedback stability R/Q = 5  preferable  lowest detuning (see D. Teytelman’s talk) For cryogenic reasons R/Q = 30  preferable (see later). For cryogenic reasons R/Q = 30  preferable (see later). Number of cavities required is the same for L =7e35. Number of cavities required is the same for L =7e35. At L = 1e36, the cavity HOM losses in the LER require more RF cavities (2) at R/Q = 30  At L = 1e36, the cavity HOM losses in the LER require more RF cavities (2) at R/Q = 30  What cavity voltage can we expect to reach …. What cavity voltage can we expect to reach ….

Stanford Linear Accelerator Center Voltage for R/Q = 5  Cavity

Stanford Linear Accelerator Center Voltage for R/Q = 30  Cavity

Stanford Linear Accelerator Center Voltage for R/Q = 15  Cavity

Stanford Linear Accelerator Center Cavity E pk and H pk Parameters S Novokhatski

Stanford Linear Accelerator Center Voltage Overhead (for 30  ) Theoretical Quench Limit for Nb (Hpk = 1700 Oe or kA/m) Field Emission Onset (Epk > 10 MV/m)

Stanford Linear Accelerator Center Voltage Overhead (for 5  ) Theoretical Quench Limit for Nb (Hpk = 1700 Oe or kA/m) Field Emission Onset (Epk > 10 MV/m)

Stanford Linear Accelerator Center Voltage Overhead (for 15  ) Theoretical Quench Limit for Nb (Hpk = 1700 Oe or kA/m) Field Emission Onset (Epk > 10 MV/m)

Stanford Linear Accelerator Center Cavity Limitations - Power To minimize the number of RF cavities per ring: Based on what has been achieved at ~ 500 MHz for both KEK-B and CESR: Based on what has been achieved at ~ 500 MHz for both KEK-B and CESR: 1 MW total RF input power per cavity has been chosen! Cavity will employ dual RF feeds, each providing up to 500 kW. RF breakdown investigations need to be performed to identify a system that can meet this power requirement at 952 MHz. Coaxial coupler arrangement  more compact. Is this power level realistically achievable?

Stanford Linear Accelerator Center Cavity Input Couplers KEK-B (f RF = 508 MHz): Biased coaxial coupler Operate typically up to 350 kW For Super-KEKB hope to reach 500 kW Tested up to 800 kW (through) CESR (f RF = 500 MHz): Aperture waveguide coupled Operate typically up to 300 kW Operated up to 360 kW (through)

Stanford Linear Accelerator Center Klystrons – 1.2 MW SLAC already produces 1.2 MW tubes at 476 MHz for PEP-II. Each powered by a 2.5 MVA DC HVPS. Tube operates at 83 kV and 24 A with perveance of Maintaining these beam parameters for 952 MHz would enable the same HVPS system to be used. Scale the cavity frequencies, drift tube spacing, gap lengths, drift pipe and beam radii. Magnetic field increases by factor of 2  existing 476 MHz tube focus coil adequate.

Stanford Linear Accelerator Center 1.2 MW Klystron – Small Signal

Stanford Linear Accelerator Center 1.2 MW Klystron – Large Signal

Stanford Linear Accelerator Center 1.2 MW Klystron Specification ParameterValue Frequency (MHz) 952 Beam Voltage (kV) 83 Beam Current (A) 24 Perveance1.004 Bandwidth (MHz)  10 Gain (dB) 47 Efficiency (%) Collector (Full power) Gun RF Output (WR975) Accelerating Cavities

Stanford Linear Accelerator Center Klystrons – 2.4 MW Doubling in RF power means that the existing 2.5 MVA HVPS can no longer be used  now need a 4 MVA HVPS. Beam power characteristics increase up to 125 kV and 29.2 A with drop in perveance to Higher beam voltage increases cavity spacing and gap lengths  accelerating section ~ 20% longer than the 1.2 MW tube. Magnetic field comparable to that of the 1.2 MW tube. Thermal loading of the output circuit requires more detailed investigation. Suspect will most likely require a dual output to minimize thermal loading at the RF windows.

Stanford Linear Accelerator Center 2.4 MW Klystron – Large Signal

Stanford Linear Accelerator Center MW Klystron Specification ParameterValue Frequency (MHz) 952 Beam Voltage (kV) 125 Beam Current (A) 29.2 Perveance (A/V 3/2 ) Bandwidth (MHz)  8* Gain (dB) 49.8 Efficiency (%) 70 Collector (Full power) Gun RF Output (WR975) Accelerating Cavities * Needs further optimization

Stanford Linear Accelerator Center Klystron Option Footprints MHz 83 kV and 24 A Perveance = MHz 83 kV and 24 A Perveance = MHz 125 kV and 29.2 A Perveance =

Stanford Linear Accelerator Center 2.4 MW AFT Circulator Layout

Stanford Linear Accelerator Center Circulators Spec 1.7% Full Reflection! 1 dry load, 1 water load x 4 increase c.f. 1.2 MW 476 MHz unit Klystron would see 2.4 kW in beam abort

Stanford Linear Accelerator CenterHVPS Originally designed for a depressed collector klystron. Existing 2.5 MVA HVPS has a primary SCR-controlled rectifier operating at the existing site-wide distribution voltage of 12.47kV: control provides for fast voltage adjustment and fault protection. control provides for fast voltage adjustment and fault protection. Rectifier configuration prevents the dump of filter capacitor stored energy into the klystron in the event of a klystron arc kV enters the circuit breaker and manual load disconnect switch and provides a safety lock and tag disconnect for maintenance. Remote turn-on and turn-off is by a full, fault-rated vacuum breaker used as a contactor. A 12-pulse rectifier reduces power line harmonic distortion to industrial standards.

Stanford Linear Accelerator Center PEP-II/SPEAR3 2.5 MVA HVPS

Stanford Linear Accelerator Center Super-B HVPS Options 1.2 MW Klystron: Existing 2.5 MVA HVPS system compatible. Existing 2.5 MVA HVPS system compatible. No development overhead. No development overhead. 2.4 MW Klystron: Same 2.5 MVA HVPS design, with larger transformers to reach 4 MVA: Same 2.5 MVA HVPS design, with larger transformers to reach 4 MVA: Applicable transformers are commercially available. Higher voltage required (125 kV): Higher voltage required (125 kV): Makes HV connections more difficult/expensive. Anticipate a 20 – 30% size and cost increase over the existing 2.5 MVA unit. Anticipate a 20 – 30% size and cost increase over the existing 2.5 MVA unit.

Stanford Linear Accelerator Center System Configuration MW Klystron 1.2 MW Circulator Single 952 MHz RF Cavity WR975 Waveguide

Stanford Linear Accelerator Center 2.4 MW Klystron 2.4 MW Circulator Dual 952 MHz RF Cavities WR975 Waveguide System Configuration 2

Stanford Linear Accelerator Center 2.4 MW Klystron 1.2 MW Circulator 1.2 MW Circulator Dual 952 MHz RF Cavities System Configuration 3

Stanford Linear Accelerator CenterConclusions RF requirements for L=7e35 and L=1e36 identified  need up to 190 MW site AC power! Low R/Q cavities needed for stability control. Cavity voltage and RF power limits identified  how far can we push these?!? High power klystrons (> 1 MW) at 952 MHz look to be achievable. High power circulators appear to be available from industry. HVPS systems for Super-PEPII klystrons are available now at 1.2 MW, but require development at 2.4 MW. Watch this space!

Thank You

Stanford Linear Accelerator Center RF Parameters Summary