Milorad Popovic On behalf of helical cooling channel design group (Katsuya Yonehara) Accelerator Physics Center, Fermilab 8/10/13NuFact 13, M. Popovic1

8/10/13NuFact 13, M. Popovic2 Emittance evolution CoolingHeating “Ionization Cooling”, A. Chao edt., Physics and Engineering 1. Lost energy by ionization process in absorber 2. Energy is recovered in accelerator 3. Smaller angular distribution after acceleration → Transverse emittance cooling

8/10/13NuFact 13, M. Popovic3 Dipole magnet Wedge absorber Homogeneous absorber filled dipole magnet Homogeneous absorber filled helical dipole+quadrupole magnet Red: Reference orbit Blue: Beam envelop p p-δp p p+δp

8/10/13NuFact 13, M. Popovic4 Ya Derbenev & R.P. Johnson, PRSTAB (2005) End view Design helical orbit Helical quadrupole component is needed to generate stable phase space (in pure solenoid field shown in red)

 Advantage: A continuous magnet structure  Fast cooling in a short channel length  Single pass ▪ No critical injection & extraction problem ▪ No temperature growth in an absorber heated by beam  No beam resonance in an HCC lattice  No electric breakdown limit due to external magnetic fields in GH 2 -filled RF cavity (show later)  Challenge: Require technology development 8/10/13NuFact 13, M. Popovic5

8/10/13NuFact 13, M. Popovic6

8/10/13NuFact 13, M. Popovic7 ν = GHz λ = 1.0 – 0.8 m ν = 0.65 GHz λ = 0.5 – 0.3 m ν = 1.3 GHz λ = 0.3 m 100 z = 0 m 92 z = 40 m 86 z = 49 m 73 z = 129 m 66 z = 219 m 60 z = 303 m GH2 pressure = 160 atm Analytical helical field Idea pillbox cavity 60 μm Be RF window E ~ 27 MV/m Goal phase space Study2a K. Yonehara et al., IPAC 2010, MOPD076

ν = to 0.65 GHz ν = 0.65 to 1.3 GHz 8/10/138NuFact 13, M. Popovic ν = to 0.65 GHz ν = 0.65 to 1.3 GHz There is a particle loss due to mismatching when RF freq. is changed Matching segment is introduced to fix this issue (shown next) 0.3 mm rad 1 mm 60 % with decay loss (shown in previous plot)

NuFact 13, M. Popovic9 Longitudinal Matching Design  Rather than using cavities with an intermediate frequency, we used an appropriate mix of cavities running at the upstream and downstream frequencies*: 59% of the upstream frequency (325 MHz) and 41% of the downstream frequency (650 MHz). In practice, we used a mix of 60%/40%. Note that if higher gradients are allowed for higher frequencies, then voltage, frequency, and cos(φ s ) should be adjusted to preserve the geometric mean of their product*: *See backup slides for derivation. 8/10/13 C. Yoshikawa, IPAC 13, pp. 1484

10NuFact 13, M. Popovic Δt (nsec) ΔE (MeV) Seg4 End Seg4a 1.25 m (quarter synchrotron oscillation) 8/10/13

z = 0 m λ HCC = 0.8 m 11 Transverse Matching Design Radial matching from λ HCC = 0.8 m to λ HCC = 0.5 m is linear over a longitudinal distance of 4m (1 full synchrotron oscillation length). NuFact 13, M. Popovic Δx(mm) Δp x (mm) z(m)024 σ x (mm) σ Px (MeV/c) σ x σ Px (mm-MeV/c) σ y (mm) σ Py (MeV/c) σ y σ Py (mm-MeV/c) Δx(mm) Δp x (mm) Δy(mm) Δp y (mm) Δy(mm) Δp y (mm) z = 4 m λ HCC = 0.5 m z = 2 m λ HCC = 0.65 m Transmission with log/tran. matching is almost 100 % 8/10/13

NuFact 13, M. Popovic12

8/10/13NuFact 13, M. Popovic13 HCC RF cell

electron-gun Final assembly Assembly without magnet inside cryo-module 2 segments Highlights: RF power all enters at end of Cryostat- minimizing penetrations and thus easing magnet/cryo design Magnet and RF thermally independent 8/10/1314NuFact 13, M. Popovic

8/10/13NuFact 13, M. Popovic15 HCC magnet consists of helical solenoid (HS) coil and a straight solenoid Helical solenoid (HS) cell Made NbTi, NbSn 3 & YBCO HS coils Experimentally verified simulation Development of winding HS coil Design mechanical support Development of quench protection G. Flanagan et al., IPAC 2012, THPPD041

Unlike our previous NbTi and YBCO helical solenoids (2004,2011) we are moving away from the offset pancake models (they are fine for NbTi and YBCO). This time we are looking at smooth mandrel to ease transitions (which can be problematic with Nb3Sn in a HS configuration). This will also improve field quality. Discrete pancakes Smooth mandrel design 8/10/1316NuFact 13, M. Popovic RF cell

 A continuous absorber is required in HCC  Continuous HCC RF system is considered  Proposed hydrogen gas filled RF cavity  Gas acts as a beam cooling material  Gas also suppresses focusing dark current by an external magnetic field → RF cavity is insensitive to the magnetic field effect  Proposed dielectric loaded RF cavity  Compact 8/10/13NuFact 13, M. Popovic17

 Beam passes through a gas filled RF cavity and generates ion pairs  A huge amount of RF power is loaded by ionized particles  We call it as “plasma loading”  Especially, electrons are dominant in the process  Electrons can be removed in a very short time by doping an electronegative gas in hydrogen  We used Dry Air (DA; it contains 20 % of oxygen) and SF 6  We demonstrated above process in experiment 8/10/13NuFact 13, M. Popovic18

19 Measured RF envelops Magenta: No beam Blue: With beam (0 < t < 10 μsec) Green, Black, Red: Plasma loading with electronegative gas (DA: dry air) No magnetic field effect as we expected H 2 gas pressure 20 to 100 atm at 300 K RF frequency 800 MHz RF amplitude 5 to 50 MV/m Beam current protons/s 8/10/13NuFact 13, M. Popovic B. Freemire, IPAC 13, pp. 1496

8/10/13NuFact 13, M. Popovic20 Capture rate is proportional to the concentration of dry air It also proportional to the pressure of gaseous hydrogen This means that the capture process is NOT taken by two-body reaction but three-body one It makes that the capture rate can be square of gas pressure Measurement shows that Expected capture time is order of 0.1 nsec that is sufficiently short to remove electrons in one RF cycle M. Chung, IPAC 13, pp. 1463

 Make a compact RF system by loading a dielectric material in a RF system  Critical issue is a Surface Breakdown  Ceramic has relatively large secondary electron emission yield (SEY > 1)  Primary electron generates more than one electron  It induces a cascade process  Similar as a multipactor process  Fill a buffer gas to eliminate the surface breakdown 8/10/13NuFact 13, M. Popovic21 Electron cascade Ceramic E

8/10/13NuFact 13, M. Popovic22 Old cavity test RF gradient at highest point (convex of electrode) RF gradient on ceramic rod Open blue circle: peak E at electrode Close blue circle: surface E Compare old result (orange) At p < 200 psi, observed peak E (open circle) shows gas BD At p > 200 psi, observed peak E shows a plateau This limit seems to be determined by the surface E since it is close to the dielectric strength L. Nash et al., IPAC’13 TUPFI068

 Demonstrate HCC cooling performance in a numerical simulations  Demonstrate gas-filled RF with beam  Plasma loading should be manageable by adding an electronegative gas  Demonstrate proof of principle of dielectric loaded gas-filled RF cavity  Promote technology development of HCC magnets and cavities  Progress  RF power source (magnetron), Cryostat (LN2 chiller), Mechanical analysis of pressure vessel, Design 6D cooling demonstration channel, etc 8/10/13NuFact 13, M. Popovic23