Upgrade Status of S-band Accelerating Structure

Hiroyasu EGO
on behalf of the accelerating-structure and positron group
KEK Injector Linac division

KEKB-ARC March, 25, 2024

Contents• Problems in stabilizing beam acceleration• Upgrade work and countermeasures against the problems‣Accelerating structure‣Pulse compressor• Remaining problems• Summary
KEKB-ARC March, 25, 20242

Problems in stabilizing beam acceleration : Accelerating structure
A lot of S-band accelerating structures of PF typefabricated about 40 years ago as the injector for the PF ring
don’t work well

Mean RF parameters of five kinds of the PF-type

ra [MΩ/m]τ [neper]Tf [us]Va/P1/2 [MV/MW1/2]
Pin [MW]

57.8
0.335
0.51
7.29
40
KEKB-ARC March, 25, 20243

Problems in stabilizing beam acceleration : Accelerating structure
Problems : Water leaks, power reflection, discharge limiting the operation of the beam acceleration
Recently 3 structures every year

Water leak in February 2024

Unavailable to beam acceleration

KEKB-ARC March, 25, 2024

4

Problems in stabilizing beam acceleration : Accelerating structure
Problems : Water leaks, power reflection, discharge limiting the operation of the beam acceleration

Limit power operation and lower accelerating voltage

KEKB-ARC March, 25, 2024

5

Upgrade : Accelerating structure• 21/142 structures suffering from these problems• Severe problems beyond repair• No sound spareCountermeasures : New S-band accelerating structureswith following upgrades !

• Higher accelerating gradient• Simple and sure fabrication• Reduction in discharge• Suppression for long-range wake instabilities

KEKB-ARC March, 25, 2024

6

Upgrade : Accelerating structure
Higher accelerating gradientRF parameters

PF-typera [MΩ/m]

61.857.8

τ [neper]

0.36

0.335

Tf [us]

0.57

0.51

Va/P1/2 [MV/MW1/2]

7.8
KEKB-ARC March, 25, 2024

7% up

7.29
7

Upgrade : Accelerating structure
Simple and sure fabrication

Fabrication by 2-step brazing of simply stacked cells
1) Au-brazing for coupler assembly
2) Ag-brazing for coupler-assembly, cylinders and discs stacked alternately

KEKB-ARC March, 25, 2024

8

Upgrade : Accelerating structure
Reduction in discharge

1:2 oval ﬁllet of disc iris
reducing the strength of peak surface
ﬁeld by 20%No sharp edge inside of the structure

KEKB-ARC March, 25, 2024

9

Upgrade : Accelerating structure
Suppression of long-range wake instabilities for 2-bunch operation

HEM11-π（standing wave-like）with highest transverse impedance
11000

Z [Ohm/mm]

10000

5000

00

5000

10000

15000

20000

25000

30000

35000

frequency [MHz]Transverse impedance distribution
KEKB-ARC March, 25, 2024

HEM11

Dispersion diagram
10Upgrade : Accelerating structure
Suppression of long-range wake instabilities for 2-bunch operation
Linear-detuning of the HEM11-π mode to make beat oscillation
by setting adequate disc-iris aperture distribution

HEM11-π modes
with band-width of 122.3 MHzW(s) [/V/pC]

840-4-8096.29 ns

1st bunch

10t [ns]~1/1000

2nd bunch

2nd bunch

KEKB-ARC March, 25, 2024

11

Upgrade : Accelerating structureHigh-power operation

New structure in the test bench

Successful operation of 100 MW

All fabricated structures were installed in the beam-line after conditioning
and accelerate beams stably now
KEKB-ARC March, 25, 202412

Problems in stabilizing
Upgrade beam: Acceleratingacceleration
structure: Pulse compressor• SLED-type pulse compressors (KEK-SLED) in operation about 30 years• No spare• Expensive fabrication cost over 1/3-million dollar

Countermeasures : New pulse compressor
with following features• Low-cost fabrication• Pulse-compressing performance same as KEK-SLED’s• Compatible with KEK-SLEDKEKB-ARC March, 25, 2024

13

Upgrade
Upgrade: Accelerating: Pulse compressor
structureLow-cost Fabrication

Simple structure required
New

Spherical-cavity-type pulse compressor (SCPC)

KEK-SLED

Approximately half cost of SLED-type
KEKB-ARC March, 25, 2024

14

Upgrade
Upgrade: Accelerating: Pulse compressor
structurePulse compressing performance
Spherical single-cell cavity
2-fold degenerative spherical TE112 mode
High Q : 100,000
Coupling β : 6.425mm
63-port waveguide polarizer
Rectangular-to-cylindrical mode converter
Exciting the rotational TE11 mode in the cylinder port

Pulse compression

x 6.15E-field distribution

KEKB-ARC March, 25, 2024

15

Upgrade
Upgrade: Accelerating: Pulse compressor
structure Figure 6: Waveforms in the high-power test of the input(blue), output (orange), and calculation with numericallyFigure 4: Photographs of (a) the installed SCPC, (b) highoptimized cavity parameters (black). (up) RF powers,power rf loads and (c) a schematic view of high-power test
Pulse
compressing
performance(down) phases.Proceedings
The output phase
3.1 sMeeting
arises of Pof thejump
18thatAnnual
setup; (1) a signal generator, (2) a phase shifter, (3) afrom the difference between theAugust
resonance
frequency
9 - 12,2021, of
QST-Tak
klystron, (4) SCPC, (5) high-power rf loads, (6a-6d) rf
PASJ2021
WEOB06SCPC andthe operating frequency of the klystron.
pickups before/after SCPC, (7) an IQ demodulator.

SCPC in the test bench

Successful 80 MW peak operation in the test bench[5]

波数は、今回の試験において出力最大時には設計値の2856 MHz より 58 kHz だけ低周波側であった。高電力試験中に、装置は設計値 30℃を上回る 31.6 ℃になっており、銅の熱膨張に従って 77 kHz 低周波数側にシフトしているはずである。また、製作時のチューニングとして、30℃真空相当での共振周波数が 2856 MHz より 50 kHz
高 周 波 側 に チ ュ ー ニ ン グ し た [9] こ と か ら 、 総 合 し て
31 kHz の誤差が見えていることになる。この周波数差は、チューナーにより、50 kHz 程度の共振周波数の変化が可能であることから、問題ないと判断した。
空洞のチューニングは高電力特性を見極めてからと考えており、現時点でチューニングはしていない。そこで、最大電力入力時の SCPC の共振周波数に一致した RFを入力することによって周波数が一致しているときの波形を測定することにした。
入力 波形として平均 パワー 14 MW、ピークパ ワー
20 MW に対して、圧縮された出力波形はピークパワー
80 MW、ピークパワーゲイン 5.74 であった。Figure 7 は高電力試験時の SCPC の出力波形を表す。出力波形は、
3-week test
入力クライストロンからの RF 波形に対する応答を計算しFigure 5: Summary of high-power
test. RF power output
Figureた結果とよく一致していることが確認できた。低電力試験
7: The input (blue) and output (orange) waveformof SCPC (orange) and of klystron (blue). Pressure
when 時の数値計算結果のパワーゲイン
the operating frequency of the klystronis modulated6 には到達しなかった
measurement by cold cathode gauges located afterが、これはクライストロン出力が理想的な矩形パルス波形
to SCPC
resonance frequency.
(up)44-B
RF powers, (down)
Currently available peak output
power
ofby150MW(green),in theunit ofklystronseparated
rf window
closebeam-accelerating
to SCPC
ではなく、位相反転のスピードが有限であることなどに起phases.
(red), and high-power rf load (blue).
因する。
Figure 6(down) において、位相反転時（ 3.1 s）に16SCPC 出力波形の位相に飛びが見られた。この位相差
KEKB-ARC March, 25, 2024

[6]
[7]

[8]

[9]

Remaining issues to be overcome
Over-time deteriorations of high-power components
Bursts of RF absorbers in dummy loads

Discharges on waveguide flanges

Water leakage
3 structures / year

KEKB-ARC March, 25, 202417

R&D plan
New 3-year plan

5-year upgrade plan completed
2018

2019

2020

2021

New S-band structure development

2022

2023

2024

✔

2025

Today

installation

Design４prototypes

Tests

installation
Fabrication of 12 structures
Tests
Fabrication of more 12 structures
Procurement

New pulse compressor development✔

Fabrication

New dummy load

DesignPrototype fabrication

Tests
Fabrication #1

installation
Tests

Fabrication #2

LS1KEKB-ARC March, 25, 2024

18

Summary• 5-year upgrade development completed‣16 accelerating structures✴

Higher accelerating gradient✴

Simple and sure fabrication✴

Reduction in discharge✴

Suppression for long-range wake instabilities‣Pulse compressor of new type✴

Simple fabrication for low-cost✴

Pulse-compressing performance same as KEK-SLED’s✴

Compatible with KEK-SLED• Remaining problems‣New 3-year plan of the structure fabrication against water leakage‣Development of new dummy load to carry out the stable high-power operation
KEKB-ARC March, 25, 2024

19

Thank you

BPAC Feb.
5, 2024
KEKB-ARC
March,25, 202420