prototype mebt buncher cavity test - pxie · 2015. 8. 25. · 7 jim steimel | prototype mebt...

Prototype MEBT Buncher Cavity Test

Jim SteimelPIP-II Colloquium18 August 2015

• Buncher cavities interspersed through MEBT to maintain longitudinal focus through the MEBT line.

• They are not meant for acceleration, only bunch structure maintenance.

• Blue blocks in MEBT block diagram represent locations of cavities.

• Solid model of MEBT design shown in picture to the left.

MEBT Design

8/25/2015Jim Steimel | Prototype MEBT Buncher Cavity Test2

BuncherCavities

Picture of the prototype bunchercavity test setup in PXIE enclosure.

Solid model of the MEBT bunchercavity identifying the different ports.

MEBT Buncher Cavity


• Purpose of the test is to verify all, non-beam related cavity specifications.

• Primary purpose is to examine cavity response to thermal issues (water cooling temperature and high power drive).

CavityFrequency, MHz 162.5Operating mode CWOperating temperature, °C 35Nominal accelerating voltage at beta=0.067, MV 0.07Maximum accelerating voltage at beta=0.067, MV 0.10Power loss at maximum voltage, kW ≤3Frequency tuning range, kHz 100Maximum water supply pressure, Bar 20

CouplerCoupler Power Rating (full reflection), kW 4Coupling coefficient 1.0Coupler feeder – standard coaxial with impedance, Ohm 50

Purpose of Prototype Commissioning


From MEBT buncher cavity FRS

• Network Analyzer (NWA) connected to ACNET through GPIB.

• P:NWACF – Sweep center frequency

• P:NWAX1 – Marker x-position, Trace 1 (frequency)

• P:NWAY1M – Marker y-position, Trace 1 (S11 logmag)

• P:NWAY2M – Marker y-position, Trace 2 (S21 logmag)

• For low level measurements, NWA set to sweep frequency across cavity bandwidth.

• Marker set to track relative maximum (or minimum) of amplitude response.

• Reflection and field probe response measured simultaneously.

Low Level Measurement Setup


Smith chart plot of S11 measurement looking into coupler. Coupling factor represented by radius of circle, β = r/(1-r). Here β = 0.904.

Logmag plot showing cavity response through field probe. At resonant frequency, field probe coupling is -33dB and loaded Q is 5240.

Coupling and Q


• Plot at right shows cavity resonant frequency as a single tuner is varied over its range.

• Zero represents furthest incursion into cavity.

• Water temperature is 81°F and first tuner is at frequency mid-range.

• Response is non-linear; frequency mid-range and mechanical mid-range are different points.

• Total range is large (>300kHz).

Mechanical Tuners


162.15

162.2

162.25

162.3

162.35

162.4

162.45

162.5

0 5 10 15 20 25

Freq

uenc

y (M

Hz)

Tuner Position (mm)

Fixed Tuner Sensitivity (water cooling)

MechanicalMid-range

FrequencyMid-range

• Cavity body and stem cooling circuits are parallel water paths.

• Top plot shows cavity resonant frequency as a function of stem input water temperature (Body should be the same).

• Cavity frequency responds quickly relative to change in water temperature.

• Lower plot is parametric plot of water temperature and frequency. Linear scaling is -1.65kHz/°F.

Water Temperature (low power)


707274767880828486889010:48:00 12:00:00 13:12:00 14:24:00 15:36:00 16:48:00

162.305

162.31

162.315

162.32

162.325

162.33

162.335

Water Tem

perature (F)

Time (hours)

Resonant Frequ

ency (M

Hz)

Resonant Frequency and Water Temp Study

Resonant Frequency P:R2RT03 degF Hins

y = ‐1,653.04x + 162,452,728.21

162.3

162.305

162.31

162.315

162.32

162.325

162.33

162.335

70 75 80 85 90

Resonant Frequ

ency (M

Hz)

Temperature (F)

Resonant Frequency vs. Water Temperature

• Diagram above shows RF power conditioning instrumentation.• P:PMTR1 – forward power, P:PMTR2 – reflected power, P:PMTR3 – cavity power.• Signal generator frequency adjusted manually to track resonant frequency.

Conditioning Setup (performed by Ding Sun)


Plot above shows conditioning progress. Yellow and green are cavity power, red is reflected power, and blue is vacuum. Low power conditioning was noisy until power threshold was reached.

Plot above shows zoom of left plot. Apparent multipacting threshold at ~160W but clean beyond.

Low Power Conditioning


• Right plot shows over five continuous days of cavity operation at 1kW CW.

• 1kW is above the power level for the nominal cavity voltage.

• Spike in reflected power due to increased air temperature overnight at CMTF.

1kW Operation


Plot above shows conditioning progress up to 1750W, over 20% above 1400W needed for peak voltage specification.

Plot above shows long term, stable running at 1750-1800W.

High Power Conditioning


MEBT Buncher Voltage via X-ray Measurements

2-Stage Cooled CdTe detector• 3x3x1 mm3

• Low-noise and dark current• Good energy resolution

Broad energy acceptance - ~3 to 100 keV

Measure buncher cavity voltage by looking for end‐point of x‐ray energy spectrum

8/25/2015Vic Scarpine | Prototype MEBT Buncher Cavity Test13

MEBT Buncher Cavity X-ray Spectrum VS RF Power

Difficult to accurately estimate spectrum end‐points• Few keV of error?

X‐raydetector

Buncher cavity with detector

8/25/2015Vic Scarpine | Prototype MEBT Buncher Cavity Test14

• Right diagram shows instrument setup for high power response measurements.

• NWA set to CW mode 1 second sweep time. Actual duty factor about 95% due to 40ms blank between sweeps.

• P:NWACF – NWA drive frequency.

• P:NWAX1 – Marker x-position (time in sweep).

• P:NWAY1(M&P) – forward power logmag and phase.

• P:NWAY2(M&P) – cavity power logmag and phase.

• P:NWAY3(M&P) – reflected power logmag and phase.

Setup for High Power Response Measurements


• Right plot shows cavity resonant frequency and cavity power over time of study.

• Power would read low if sampled during NWA blanking and was cleaned up in the plot.

• Resonant frequency is calculated from NWA CW frequency setting and measured phase between forward power and cavity power.

• Power amplifier would trip off at 1500W due to NWA blanking process.

High Power Cavity Response


162.446

162.448

162.45

162.452

162.454

162.456

162.458

162.46

162.46213:26:24 13:55:12 14:24:00 14:52:48 15:21:36 15:50:24

0

200

400

600

800

1000

1200

1400

1600

Resonant Frequ

ency (M

Hz)

Time of Day

Cavity Pow

er (W

)

PXIE MEBT Buncher Cavity Resonant Frequency Variation

Cleaned Up Power Resonant Frequency

Parametric plot of resonant frequency vs power. Linear interpolation is:-9.5kHz/kW.

Parametric plot of differential water temperature vs. power. Bulk of power is dissipated in the stem.

High Power Response (Cont’d)


y = ‐9.53x + 162,461,056.01

162.446162.448162.45162.452162.454162.456162.458162.46162.462

0 500 1000 1500

Freq

uency (M

Hz)

Power (W)

PXIE MEBT Buncher Resonant Frequency vs Power

y = 1.209E‐03x + 1.868E‐02

‐1

‐0.5

0

0.5

1

1.5

2

0 500 1000 1500Tem

perature Delta (F)

Cavity Power (W)

Water Temperature Increase vs Power

Stem Water TempDelta

Body Water TempDelta

Linear (Stem WaterTemp Delta)

Test Results Summary

• Prototype cavity meets all operational specifications.• Mechanical tuning range is not well centered but still has

plenty of available tuning space to compensate for water temperature and power variations.

• Operational power input should be kept above 200W to avoid multipactor thresholds.

• X-ray spectroscopy looks promising as means of measuring gap potential.

• Cooling is sufficient to maintain stable operation at high power.


prototype mebt buncher cavity test - pxie · 2015. 8. 25. · 7 jim steimel | prototype mebt...

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