simulations of dark current dependencies on frequency ...€¦ · simulations began in cst using...

Simulations of Dark Current Dependencies onFrequency

using CST Particle Studio

T. G. Lucas, M. Volpi, T. Argyropoulos, C. Serpico and W. Wuensch

University of Melbourne and CERN

January 2018

T. G. Lucas, M. Volpi, T. Argyropoulos, C. Serpico and W. Wuensch (University of Melbourne and CERN)[email protected] CLIC Workshop 1 / 21

1 Radiation vs Structure Temperature

2 Frequency Issue?

3 CST Simulations

4 Simulation Results and Comparison

5 Conclusions and Future Work


Observation of Dark Current Dependencies on StructureTemperature

During normal operation, an event occurred where a gradual increasein the radiation was observed. Over an hour the radiation increasedsteadily to around 4 times the expected level. This was discoveredafter an interlock stopped Xbox 2.

Figure: T. Argyropoulos CLIC Workshop 2017T. G. Lucas, M. Volpi, T. Argyropoulos, C. Serpico and W. Wuensch (University of Melbourne and CERN)[email protected] CLIC Workshop 3 / 21


StructureTemp.

Radiation

Avg. Power

Peak Power

Pulse Width



Investigating structure parameters found temperature changes duringthis time. Systematic changes to the structure temperature confirmeda strong correlation between the structure temperature and radiation.

Change was unable to be accounted for in any field emission modelgiven such small changes in temperature.


A Frequency Issue?

Figure: TD26CC


A Frequency Issue?

Figure: SiC (Left) and T24 PSI (Right)


A Frequency Issue!

3 scans of frequency and temperature for the 3 structures.

Good agreement of radiation change for frequency and the equivalenttemperature for all three structures.

Gradient of change for three structures when normalised to the baseradiation similar for three structures.

I T24PSI: 2.52e-4 µSv h−1kHz−1

I TD24SIC: 2.49e-4 µSv h−1kHz−1

I TD26CC: 2.16-4 µSv h−1kHz−1


Capture Efficiency with FrequencyIt is hypothesised that the temperature change results in a change inthe phase velocity of the structure equivalent to if the driverfrequency was changed. Consequently the lower phase velocitycaptures lower energy dark currents better.

Figure: T. Argyropoulos CLICWorkshop 2017

An increase in temperatureexpands the structure→ dispersion curve shifts down→ vp < c→ Relative velocity of phaseand dark current lower→ Lower energy dark currentexperience better capture(acceleration).

Same effect if one increases thedriver frequency.


Simulations of Dark Current Capture

To test the hypothesis RF and particle simulations were performed.

Simulations began in CST using the eigenmode solver for fields andPIC for particle simulations.


Eigenmode Solutions

To find the field profile, eigenmode solutions were found for the cells.

Boundary conditions were varied to change the frequency.


Dispersion CurveGives a phase velocity of c and group velocity of 0.01c as expected.


Particle Source

Particle source used the field emission model from CST. Modified tohave a field enhancement factor of 20.


CST PIC

Simulation for 1 ns.

Rf fields imported for single cell, duplicated then shifted spatially andin phase to fit the structure.

Used the field emission model of CST with an adjustment to have asurface enhancement factor of 20.


Energy vs Position: Final BunchIt is known that the downstream bunches cause most of the radiation.

-2.4 MHz

-1.18 MHz

0 MHz

2.44 MHz

1.25 MHz


Histogram of Energy Spectrum

The energy spectrum of those bunches seen are.

Double peak in spectra result from the two peaks in emission duringan RF oscillation.

Figure: A histogram of the energies in the leading bunch.

The number of particles reaching the end of the structure alsoincreases.

Capture is approximately 1% though varies with frequency.


Energy and N particles


Bunch Total Energy vs Frequencies

Finding the total energy of the final bunch and plotting against thefrequency.

Similar trend to that measured using Cherenkov fibres which measureonly electrons in the CLIC structure radiation emission.


Conclusions and Future Work

A change in temperature led to a change in radiation and wasattributed to frequency changes due to thermal expansion.

A PIC simulation were developed using CST.

Simulations demonstrated that the dark current which was laggingbehind the ”in-phase” bunch experienced better capture andacceleration for higher frequencies. Attributed to a lower phasevelocity.

This should be measurable with a spectrometer!

Next steps using steps is testing all the dark current properties(capture percentage, energy spectrum, upstream/downstream ratio,etc.)

Using another code to verify: RF Track.


Thank YouThanks to the Xbox team, Thibaut Lefevre and Andrea Latina, andto the Australian Team!See you at IPAC19!


Current Emission Double Peak


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