[PPT] - Multiple-cell cavity for high mass axion dark matter search 3 rd PowerPoint Presentation

SLIDE 1

Multiple-cell cavity for high mass axion dark matter search

Junu JEONG, Sungwoo YOUN

Korea Advanced Institute of Science and Technology (KAIST) Institute for Basic Science/ Center for Axion and Precision Research (IBS/CAPP)

3rd Workshop on Microwave Cavities and Detectors for Axion Research

𝑏 𝛿∗ 𝛿 𝑕%&& ℒ = 𝑕%&&𝑏𝐺𝐺 * 𝑄

%&& = 𝑕%&& ,

𝜍. 𝑛% 𝐶1

,𝑊𝐷𝑅

J. Jeong et al, Phys. Lett. B, 777, 412-419 (2018)

SLIDE 2

Outline

Motivation of the Multiple-cell cavity
Frequency tuning mechanism
Phase-matching of the Multiple-cell cavity
Demonstration of experimental feasibility
JANIS He-3 system
Summary

SLIDE 3

Motivation of the Multiple-cell cavity

Axion under a strong magnetic field converts to a RF photon,

and resonates in microwave cavities.

Exploring higher frequency regions

requires smaller size of cavities. 𝑔

67898 = 𝑑

2𝜌 2.4048 cavity radius

Multiple cavity system can compensate

for the reduced volume.

However, it is still inefficient in volume.
We want to maximize volume usage.

𝑏 𝛿∗ 𝛿 𝑕%&& ℒ = 𝑕%&&𝑏𝐺𝐺 * 𝑄

%&& = 𝑕%&& ,

𝜍. 𝑛% 𝐶1

,𝑊𝐷𝑅

Single large cavity Single small cavity Multiple small cavities

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SLIDE 4

Motivation of the Multiple-cell cavity

How to make Multiple-cell cavity 1. Single cylindrical cavity fitting into the magnet bore 2. Split by metal partitions placed with equidistant intervals 3. Introducing a narrow hole at the center of the cavity

𝑏 𝛿∗ 𝛿 𝑕%&& ℒ = 𝑕%&&𝑏𝐺𝐺 * 𝑄

%&& = 𝑕%&& ,

𝜍. 𝑛% 𝐶1

,𝑊𝐷𝑅

Single large cavity Multiple-cells Multiple-cell cavity

Multiple-cell cavity provides more effective

way to increase volume.

Resonant frequency increases with the cell

multiplicity.

Same frequency tuning mechanism as

multiple cavity system can be employed.

A single RF antenna extracts the signal out of

the cavity.

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SLIDE 5

Motivation of the Multiple-cell cavity

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

Multiple cavity system vs. Multiple-cell cavity

Magnet bore: 100 mm / cavity height: 200 mm / wall thickness: 5 mm * Conversion power and scan rate is relative value to that of quad-cavity system

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SLIDE 6

5 10 15 20 25 30 0.5 1 1.5 2 2.5 3 3.5

Scan rate (A.U.) Relative Frequency

Scan rate (A.U.) vs. Relative frequency

Frequency tuning mechanism

Frequency tuning mechanism employs the same concept as for conventional cylindrical cavity detectors.

Altering the field distribution of TM010-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.

6 single cylindrical cavity multiple-cell cavities

2 4 6 8

SLIDE 7

Condition for phase-matching

Since the individual cells are spatially connected, the relative rod position in a cell affects the entire field distribution.

Asymmetric field distribution, localization of the field,

generates huge reduction in the form factor. Frequency tuning mechanism requires that the field distribution in individual cells is identical each other.

We refer to such condition as “phase-matching”

Symmetric (Phase-matched) Asymmetric (Field localized)

7 0.1 0.2 0.3 0.4 0.5 0.6

15
10
5

5 10 15 Form factor Misalignment angle [deg]

Form factor vs. Misalignment rod angle

SLIDE 8

Phase-matching of the Multiple-cell cavity

The simplest case, double-cell cavity.

TM010-like mode = in-phase
TM110-like mode = 180º out of phase
When the two rods are aligned symmetrically, the field

distributions of modes are also symmetric.

Once phase-matched, the electric field strength at the

center of the cavity becomes zero for the TM110-like mode. After phase-matched, the higher TMn10-like modes have zero electric field at the center of the cavity.

8 cut line length [mm] cut line length [mm] Electric field strength [A.U.]

E field distributions TM010-like TM110-like E field profiles TM010-like TM110-like Phase-matched Phase-unmatched

SLIDE 9

Experimental confirmation of phase-matching

The center electric field of modes can be measured by

monopole antenna located at the center of the cavity.

When rods are symmetrically aligned, the resonant peak
f the higher TMn10-like modes in S-parameter fades away.
Phase-matching is achieved by aligning the tuning rods

until the higher mode peaks vanish.

The coupling strength for higher modes to be less then

0.05dB at the sacrifice of power loss less than 1%.

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TM010-like mode TM110-like mode TM010-like mode remains TM110-like mode fades away

Arbitrary rod configuration and arbitrary coupling Symmetric rod configuration and arbitrary coupling Symmetric rod configuration and critical coupling

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1

1
0.8
0.6
0.4
0.2

Relative power Higher mode depth [dB]

(when lower mode is critically coupled)

Relative power vs. Threshold

Field center Wall

SLIDE 10

Demonstration of experimental feasibility

Demonstration was performed using a double-cell cavity at RT.

Inner diameter = 90mm
Inner height = 100mm
Split cavity design
99.5% alumina for tuning rods
QL = 2,200 at room temperature

10 Piezo rotator Linear piezo actuator Center gap Tuning rod Monopole antenna Cavity system Piezo controller Computer Network analyzer

SLIDE 11

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.

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TM110-like mode TM010-like mode TM110-like mode fades away TM010-like mode TM010-like mode critical coupling

1. Phase unmatched, arbitrary coupled
2. Phase matched, arbitrary coupled
3. Phase matched, critical coupled

SLIDE 12

Demonstration of experimental feasibility

Repeated 200 times with large frequency shift

Good linear behavior of the target frequency with step
Phase-matching (critical coupling) are already satisfied more than

90% (50%) of the time after tuning.

Less than 2 seconds are required to complete the tuning process.
Frequency shift + Phase-matching + Critical coupling

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Step 20 40 60 80 100 120 140 160 180 200 Frequency [MHz] 3760 3780 3800 3820 3840 3860 3880 3900 3920 3940

~1 MHz/Step

Time spent [s] 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Entries 20 40 60 80 100 120

Phase-matching Critical coupling Critical coupling Phase-matching+

DAQ ~ order of minutes

Frequency shift

Time for tuning < 2 sec Stable tuning

SLIDE 13

JANIS He-3 system

JANIS system w/ 9T SC magnet was installed at one of LVPs at CAPP

Cryogenic and SC magnet system utilizing charcoal

sorption pump

1K pot lowers the temperature by pumping LHe-4 to

condense the He-3 into the He-3 pot

He-3 pot evaporatively cooled to maintain the base

temperature at 300mK

Charcoal sorption pump 1 K pot He-3 pot 9T SC magnet Inner vacuum can

Charcoal at 45 K Charcoal at 4 K release He-3

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absorb He-3

SLIDE 14

JANIS He-3 system: SC Magnet

SC magnet generates 9T magnetic field in persistent mode
Field distribution was well modeled with FEM (finite element method)

simulation in CAPP.

The test operation of 9T SC Magnet was performed and confirmed
perating in persistent mode without quench.

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Property Specification Manufacturer Cryomagnetics Superconductor NbTi Bmax 9 T (4.2 K) Inner bore 125 mm Height 235 mm Operating current 81 A Operation mode Persistent

Persistent mode Ramping up Ramping down

LHe 20 liter per day Magnetic field distribution

SLIDE 15

JANIS He-3 system: Cryogenic system

Test operation confirmed the desired specifications.
Cavity temperature is maintained at 2K while

continuous operation with fully energized magnet.

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Tcharoal ~ 45 K T1K pot ~ 1.9 K THe-3 pot ~ 1.7 K

Continuous operation

T1K pot ~ 2.1 K THe-3 pot ~ 1.8 K TCavity ~ 2.1 K

w/ fully energized magnet

Cu 2-cell Cavity

I.D. = 110 mm I.H. = 220 mm Q0 = 18,000

HEMT amps

TN: 1~2 K Charcoal sorption pump 1 K pot He-3 pot

Tuning rotator Linear actuator

SLIDE 16

Target sensitivity (10 KSVZ)

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2-cell 4-cell 8-cell

Average magnetic field [T]

7.8

Frequency [GHz] (𝑔

L%M − 𝑔 LOP [GHz])

2.8 ~ 3.3 (0.5) 3.8 ~ 4.5 (0.7) 5.8 ~ 7.0 (1.2)

Q0 (RRR=9)

60,000 51,000 51,000

Form factor

0.45 0.45 0.40

Volume [L]

2.0 1.9 1.7

DAQ efficiency

0.5

T

sys [K]

2.1+2.0 2.1+3.0 2.1+4.0

Scan rate [GHz/year] for 10 KSVZ

5.4 4.8 5.0

Geometry

2-cell 4-cell 8-cell

Year 2018 2019 20 20 Quarter 1 2 3 4 1 2 3 4 1 Double-cell Quadruple-cell Octuple-cell

SLIDE 17

Summary

1. New design of cavity, multiple-cell cavity, is introduced.

Multiple-cell cavity provides more effective way to increase volume.
A single RF antenna extracts the signal out of the cavity.

2. Frequency tuning mechanism and criteria for the phase-matching is introduced.

Frequency tuning mechanism employs the same concept as for conventional cylindrical cavity detectors.
After phase-matched, the higher TMn10-like modes have zero electric field at the center of the cavity.
Phase-matching is achieved by aligning the tuning rods until the higher mode peaks vanish.

3. Demonstration of experimental feasibility was performed.

There is a good linear behavior of the target frequency with step.
Less than 2 seconds are required to complete the tuning process.

4. JANIS He-3 system installation in CAPP has been completed.

JANIS system with 9T SC magnet was installed at one of LVPs at CAPP.
We are targeting 10 KSVZ sensitivity of high mass axion using the multiple-cell cavities at IBS/CAPP for

the next two years.

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