Recent progress of RF cavity study at Mucool Test Area Katsuya Yonehara APC, Fermilab 1

Ionization cooling channel NuFact'11 - K. Yonehara2 Perpendicular momentum before cooling absorber Perpendicular momentum after cooling absorber becomes smaller due to ionization energy loss process μ beam Absorber RF cavity Magnet After π → μ decay & μ collection Longitudinal momentum is regained by RF cavity RF cavity is embedded in strong B field (> 2 T) Beam envelop Achievable smallest transverse beam phase space is determined by focus strength (β ⊥ ) and Z & A of cooling absorber

Problem: B field effect on RF cavity NuFact'11 - K. Yonehara3 Gradient in MV/m Peak Magnetic Field in T at the Window >2X required field X A. Bross, MC’11 Required E in cooling channel Data were taken in an 805 MHz vacuum pillbox cavity

Mucool Test Area (MTA) & work space NuFact'11 - K. Yonehara4 Multi task work space to study RF cavity under strong magnetic fields & by using intense H - beams from Linac Compressor + refrigerator room Entrance of MTA exp. hall MTA exp. hall SC magnet 200 MHz cavity Workstation 400 MeV H - beam transport line

Illustrated “Standard model” of RF breakdown event NuFact'11 - K. Yonehara5 1. An “asperity” emits a surface electron RF cavity wall 2. Electron gains kinetic energy from E RF cavity wall 3. High energy e - smashes on cavity wall and generates secondary e - 4. Electron heats up cavity wall 5. Repeat heating and cooling wall material induces wall damage Show just dominant process 6. Some amount of wall material is taken off from wall and generates dense plasma near surface B field confines an electron beam and enhances breakdown process as shown in slide 3

Material search High work function & low Z element can be a good material for cooling channel –Beryllium & Aluminum are good candidate NuFact'11 - K. Yonehara6 M. Zisman, Nufact’10 Beryllium button assembled 805 MHz pillbox cavity Simulated max grad in an 805 MHz RF cavity with Be, Al, and Cu electrodes Test will be happened in this summer & fall

Special surface treatment By treating cavity surface by using superconducting cavity technique a field enhancement factor significantly goes down In addition, we propose a very thin coat on the cavity wall by using Atomic Layer Deposition (ALD) method to reduce a field enhancement factor Or, apply E × B force on the wall surface to defocus dark current –Test has been done –Investigation & analysis are on going NuFact'11 - K. Yonehara7

RF R&D – 201 MHz Cavity Test Treating NCRF cavities with SCRF processes  The 201 MHz Cavity – 21 MV/m Gradient Achieved (Design – 16MV/m)  Treated at TNJLAB with SCRF processes – Did Not Condition  But exhibited Gradient fall-off with applied B NuFact'11 - K. Yonehara8 1.4m A. Bross, MC’11

Fill up dense gas to slow down dark current NuFact'11 - K. Yonehara9 Maximum electric field in HPRF cavity Schematic view of HPRF cavity 805 MHz High Pressure RF (HPRF) cavity has been successfully operated in strong magnetic fields Metallic breakdown Gas breakdown: Linear dependence Governed by electron mean free path Metallic breakdown: (Almost) constant Depend on electrode material No detail study have been made yet Gas breakdown Operation range (10 to 30 MV/m) P. Hanlet et al., Proceedings of EPAC’06, TUPCH147

Study interaction of intense beam with dense H2 in high gradient RF field NuFact'11 - K. Yonehara10 Beam signal (x8) (8 μs) RF power is lost when beam is on RF power is recovered when beam is off RF pulse length (80 μs) p + H 2 → p + H e - Ionization process 1,800 e - are generated by incident K = 400 MeV Does intense beam induce an electric breakdown? → No! RF power reduction due to beam RF power reduction due to RF breakdown RF breakdown light Beam induced light By comparing RF power reduction and light intensity in beam induced plasma with these at real RF breakdown, beam induced plasma density must be very thin. Observed plasma density in RF breakdown = cm -3 Estimated beam induced plasma density = cm -3 ν= 802 MHz Gas pressure = 950 psi Beam intensity = /bunch

Preliminary estimation of plasma loading effect in HPRF cavity for cooling channel NuFact'11 - K. Yonehara11 From RF amplitude reduction rate, RF power consumption by plasma can be estimated E = 20 MV/m Hence, energy consumption by one electron is (including with initial beam intensity change) Joule t = 200 ns Muon collider: n e per one bunch train = μ × 10 3 e = electrons → 0.6 Joule Neutrino Factory: n e per one bunch train = μ × 10 3 e = electrons → 0.06 Joule A 201 MHz pillbox cavity stores 8.5 Joule of RF power > For MC, 0.6/8.5 of RF power reduction corresponds to 4 % of RF voltage reduction > For NF, 0.06/8.5 of RF power reduction is negligible Plasma loading effect in higher frequency pillbox RF cavity will be severe since the cavity stores less RF power > Need some technique to reduce plasma loading effect ν= 802 MHz Gas pressure = 950 psi Beam intensity = /bunch

Improve performance of HPRF cavity NuFact'11 - K. Yonehara12 Doping electronegative gas (SF6, NH3) This test will be done soon. Other possible improvement: Large charge capacitive RF cavity Plasma loaded RF cavity has a big impedance change > Modify Klystron (ex. multiple RF power injection) to match the impedance Plasma loading in denser gas tends to be smaller > Simply fill denser gas in the cavity to reduce plasma loading effect Induce plasma instability by E × B force Local electric field due to plasma oscillation Apply B ⊥ E to induce E×B force (ex. Lifetime of wakefield plasma is O(fs))

Summary MTA is a multi task working space to investigate RF cavities for R&D of muon beam cooling channel –Intense 400 MeV H - beam –Handle hydrogen (flammable) gas –5 Tesla SC solenoid magnet –He cryogenic/recycling system Pillbox cavity has been refurbished to search better RF material –Beryllium button test will be happened soon E × B effect has been tested in a box cavity –Under study (result seems not to be desirable) 201 MHz RF cavity with SRF cavity treatment has been tested at low magnetic field –Observed some B field effect on maximum field gradient –Further study is needed (large bore SC magnet will be delivered end of 2011) HPRF cavity beam test has started –No RF breakdown observed –Design a new HPRF cavity to investigate more plasma loading effect NuFact'11 - K. Yonehara13

Additional comments/corrections from my presentation Slide 11: There were assumptions to estimate power deposition in an electron swarm –Use the same cavity length as an 805 MHz HPRF test cavity: m –Electron production ratio (10 3 electrons/muon) is estimated from 950 psi H2 room temperature –For NF, this number will be fine –For MC, we will put 2940 room temperature –Then, the number of electrons in the cavity will be electrons/muon –E field drops 89 % E amplitude reduction can be managed by tuning RF frequency –We do not use crest for cooling channel –Acceleration field can be tuned by adjusting RF frequency (ex MHz → MHz for 60 ns muon beam pulse length) NuFact'11 - K. Yonehara14