Multiple-cell cavity for high mass axion dark matter search 3 rd Workshop on Microwave Cavities and Detectors for Axion Research π , %&& = π %&& π . , ππ·π π πΆ 1 π % π %&& * β = π %&& ππΊπΊ πΏ πΏ β Junu JEONG, Sungwoo YOUN Korea Advanced Institute of Science and Technology (KAIST) Institute for Basic Science/ Center for Axion and Precision Research (IBS/CAPP) J. Jeong et al, Phys. Lett. B, 777, 412-419 (2018)
Outline β’ Motivation of the Multiple-cell cavity β’ Frequency tuning mechanism β’ Phase-matching of the Multiple-cell cavity β’ Demonstration of experimental feasibility β’ JANIS He-3 system β’ Summary
Motivation of the Multiple-cell cavity β’ Axion under a strong magnetic field converts to a RF photon, and resonates in microwave cavities. β’ Exploring higher frequency regions π , %&& = π %&& π . requires smaller size of cavities. , ππ·π π πΆ 1 π % 67 898 = π 2.4048 π %&& π 2π cavity radius * β = π %&& ππΊπΊ πΏ β’ Multiple cavity system can compensate πΏ β for the reduced volume. β’ However, it is still inefficient in volume. β’ We want to maximize volume usage. Single large cavity Single small cavity Multiple small cavities 3
Motivation of the Multiple-cell cavity How to make Multiple-cell cavity 1. Single cylindrical cavity fitting into the magnet bore π 2. Split by metal partitions placed with equidistant , %&& = π %&& π . , ππ·π π πΆ 1 intervals π % π %&& 3. Introducing a narrow hole at the center of the * cavity β = π %&& ππΊπΊ πΏ πΏ β β’ Multiple-cell cavity provides more effective way to increase volume. β’ Resonant frequency increases with the cell multiplicity. β’ Same frequency tuning mechanism as multiple cavity system can be employed. β’ A single RF antenna extracts the signal out of the cavity. Single large cavity Multiple-cells Multiple-cell cavity 4
Motivation of the Multiple-cell cavity Multiple cavity system vs. Multiple-cell cavity Magnet bore: 100 mm / cavity height: 200 mm / wall thickness: 5 mm Quad-cavity Quad-cell Sext-cell Configuration Volume [L] 0.62 1.08 1.02 Frequency [GHz] 7.30 5.89 7.60 Q (room temp.) 19,150 19,100 16,910 Form factor 0.69 0.65 0.63 * Conversion power 1.00 1.65 1.32 * Scan rate 1.00 2.72 1.98 * Conversion power and scan rate is relative value to that of quad-cavity system 5
Frequency tuning mechanism Frequency tuning mechanism employs the same concept as for conventional cylindrical cavity detectors. β’ Altering the field distribution of TM 010 -like mode by moving a dielectric or metal rod, inserted into each cell. Using a single magnet, we can scan a wide mass range using various multiple-cell cavities, increasing # of cells. Scan rate (A.U.) vs. Relative frequency 30 25 8 6 4 Scan rate (A.U.) 20 2 15 10 5 0 0 0.5 1 1.5 2 2.5 3 3.5 Relative Frequency single cylindrical cavity multiple-cell cavities 6
Condition for phase-matching Since the individual cells are spatially connected, the relative Form factor vs. Misalignment rod angle rod position in a cell affects the entire field distribution. 0.6 β’ Asymmetric field distribution, localization of the field, 0.5 generates huge reduction in the form factor. 0.4 Form factor Frequency tuning mechanism requires that the field 0.3 distribution in individual cells is identical each other. 0.2 β’ We refer to such condition as βphase-matchingβ 0.1 0 -15 -10 -5 0 5 10 15 Misalignment angle [deg] Symmetric Asymmetric 7 (Phase-matched) (Field localized)
Phase-matching of the Multiple-cell cavity E field distributions The simplest case, double-cell cavity. β’ TM 010 -like mode = in-phase β’ TM 110 -like mode = 180ΒΊ out of phase β’ When the two rods are aligned symmetrically, the field distributions of modes are also symmetric. β’ Once phase-matched, the electric field strength at the center of the cavity becomes zero for the TM 110 -like mode. After phase-matched, the higher TM n10 -like modes have zero TM 010 -like TM 110 -like electric field at the center of the cavity. Electric field strength [A.U.] E field profiles TM 010 -like TM 110 -like Phase-matched Phase-unmatched 8 cut line length [mm] cut line length [mm]
Experimental confirmation of phase-matching Relative power vs. Threshold β’ The center electric field of modes can be measured by 1 monopole antenna located at the center of the cavity. Field center 0.99 Wall β’ When rods are symmetrically aligned, the resonant peak 0.98 0.97 of the higher TM n10 -like modes in S-parameter fades away. Relative power 0.96 0.95 β’ Phase-matching is achieved by aligning the tuning rods 0.94 until the higher mode peaks vanish. 0.93 0.92 0.91 β’ The coupling strength for higher modes to be less then 0.9 -1 -0.8 -0.6 -0.4 -0.2 0 0.05dB at the sacrifice of power loss less than 1%. Higher mode depth [dB] (when lower mode is critically coupled) TM 010 -like mode TM 010 -like mode remains TM 110 -like mode TM 110 -like mode fades away 9 Arbitrary rod configuration and arbitrary coupling Symmetric rod configuration and arbitrary coupling Symmetric rod configuration and critical coupling
Demonstration of experimental feasibility Demonstration was performed using a double-cell cavity at RT. β’ Inner diameter = 90mm β’ Inner height = 100mm β’ Split cavity design β’ 99.5% alumina for tuning rods β’ Q L = 2,200 at room temperature Linear piezo actuator Monopole antenna Center gap Network analyzer Tuning rod Piezo controller Piezo rotator Computer Cavity system 10
Demonstration of experimental feasibility Demonstration of the tuning mechanism using a double-cell cavity 1. Two resonant modes are featured by the two reflection peaks. 2. Phase-matching is assured by vanishing of the higher frequency peak and the corresponding circle. 3. Critical coupling is characterized by the maximum depth of the remaining reflection peak and the corresponding circle passing through the center of the smith chart. TM 010 -like mode TM 110 -like mode TM 010 -like mode TM 010 -like mode TM 110 -like mode fades away critical coupling 1. Phase unmatched, arbitrary coupled 2. Phase matched, arbitrary coupled 3. Phase matched, critical coupled 11
Demonstration of experimental feasibility Repeated 200 times with large frequency shift β’ Good linear behavior of the target frequency with step β’ Phase-matching (critical coupling) are already satisfied more than 90% (50%) of the time after tuning. β’ Less than 2 seconds are required to complete the tuning process. β’ Frequency shift + Phase-matching + Critical coupling 3940 Frequency [MHz] Entries 3920 Stable tuning 120 Phase-matching Frequency 3900 shift 100 Critical coupling 3880 Phase-matching+ 80 3860 Critical coupling 3840 60 Time for tuning < 2 sec 3820 40 3800 ~1 MHz/Step 3780 20 DAQ ~ order of minutes 3760 0 0 20 40 60 80 100 120 140 160 180 200 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 12 Step Time spent [s]
JANIS He-3 system Inner vacuum can JANIS system w/ 9T SC magnet was installed at one of LVPs at CAPP β’ Cryogenic and SC magnet system utilizing charcoal Charcoal sorption pump sorption pump β’ 1K pot lowers the temperature by pumping LHe-4 to condense the He-3 into the He-3 pot β’ He-3 pot evaporatively cooled to maintain the base 1 K pot temperature at 300mK He-3 pot Charcoal Charcoal 9T SC magnet at 45 K at 4 K 13 release He-3 absorb He-3
JANIS He-3 system: SC Magnet β’ SC magnet generates 9T magnetic field in persistent mode β’ Field distribution was well modeled with FEM (finite element method) simulation in CAPP. β’ The test operation of 9T SC Magnet was performed and confirmed operating in persistent mode without quench. Property Specification Manufacturer Cryomagnetics Superconductor NbTi B max 9 T (4.2 K) Magnetic field distribution Inner bore 125 mm Ramping Persistent mode Ramping Height 235 mm up down Operating current 81 A LHe 20 liter per day 14 Operation mode Persistent
JANIS He-3 system: Cryogenic system Continuous operation β’ Test operation confirmed the desired specifications. β’ Cavity temperature is maintained at 2K while continuous operation with fully energized magnet. T charoal ~ 45 K T 1K pot ~ 1.9 K T He-3 pot ~ 1.7 K Charcoal sorption pump 1 K pot w/ fully energized magnet He-3 pot HEMT amps T 1K pot ~ 2.1 K T N : 1~2 K T He-3 pot ~ 1.8 K T Cavity ~ 2.1 K Linear actuator Cu 2-cell Cavity I.D. = 110 mm I.H. = 220 mm Q 0 = 18,000 Tuning rotator 15
Target sensitivity (10 KSVZ) 2-cell 4-cell 8-cell Average magnetic 7.8 field [T] 2.8 ~ 3.3 3.8 ~ 4.5 5.8 ~ 7.0 Frequency [GHz] ( π L%M β π LOP [GHz]) (0.5) (0.7) (1.2) 2-cell 60,000 51,000 51,000 4-cell Q 0 (RRR=9) 8-cell 0.45 0.45 0.40 Form factor 2.0 1.9 1.7 Volume [L] 0.5 DAQ efficiency 2.1+2.0 2.1+3.0 2.1+4.0 T sys [K] Scan rate [GHz/year] 5.4 4.8 5.0 for 10 KSVZ 20 Year 2018 2019 20 Geometry Quarter 1 2 3 4 1 2 3 4 1 Double-cell Quadruple-cell 16 Octuple-cell
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