Funded by the

European Union

CompactLight@elettra.eu XLS www.CompactLight.eu 1

INFN-LNF

Simulation status for the C-band injector

Michele Croia

Athens 21/1/2019

Funded by the

European Union

M. Croia 2

The layout of the C-band photoinjector has been optimized. It was made by a 1.6 cells

C-band gun with 240MV/m peak field (160MV/m for the high repetition rate case), a

properly scaled solenoid and by 3 C-band structures (2m).

To match the Compact Light beam parameters at the injector end a soft velocity

bunching was performed in the first section and a booster made by 7 X-band

structures (0.9m) was inculded in the injector layout.

To have a final symmetric current profile a Ka-band structure (0.1m) was inserted in

the injector line.

To increase the repetition rate toward the kHz, a study on the beam brightness at

different cathode peak field was done.

After several optimizations a final layout was fixed to match the COMPACTLIGHT

beam parameters both in High and Low repetition rate regime.

INTRODUCTION

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M. Croia 3

• The same layout is able to work in two different regimes, Low and High repetition rate, matching inboth cases all the COMPACTLIGHT bunch parameters at the injector end.

• To pre-compensate the charge accumulation on the bunch head, a short Ka-band section was inserted(l = 0.1m). It operates at 180°respect to the crest. The gradient was searched to have a finalsymmetric current profile and an accettable rms emittance value 𝜀𝑛,𝑟𝑚𝑠 < 0.2 𝜇𝑚.

• Using the GPT code the section positions and the integrated magnetic fields were optimized. The firstsection RF phase was setted to perform a soft RF compression (velocity bunching).

1.6 cells C-BAND GUN + SOLENOID

During the RF compression solenoids on thefirsts two sections are used to keep undercontrol the beam spot size and the emittance

7 X-BAND (𝑙 = 0.9𝑚) SECTIONS

C-band injector layout

3 C-BAND (𝑙 = 2𝑚) SECTIONS Ka-BAND (𝑙 = 0.1𝑚) STRUCTURE

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M. Croia 4

High repetition rate studies

• To explore beam final parameters for lower gradients we optimized different layoutsdecreasing the cathode peak field.

• The scan was performed re-optimizing the entire line (laser on cathode dimensions,magnetic fields, launch phases and structure positions) for each case.

• This study was useful in the case we will reduce the cathode peak field, and to exploregradients for the High repetition rate scenario toward the KHz.

Beams are on crest and solenoidsaround sections are switched OFF

𝑬𝒄𝒂𝒕𝒉 𝟐𝟒𝟎, 𝟐𝟐𝟎, 𝟏𝟔𝟎, 𝟏𝟑𝟎𝑴𝑽

𝒎Re-optimized magnetic field for each case

Laser on cathodedimensions andintrinsic emittanceare properly scaledfor each case

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M. Croia 5

Using 𝟏𝟔𝟎𝑴𝑽/𝒎 it is possible to improve the beam Brightness and increase the repetition rate toward kHz

Comparison for different cases

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M. Croia 6

• 𝑄 = 𝟕𝟓 𝒑𝑪

• GPT 250k particles

• Laser on cathode : ൞

𝑡𝑙𝑒𝑛𝑔𝑡ℎ = 𝑝𝑠 (𝑢𝑛𝑖𝑓𝑜𝑟𝑚)

𝑟𝑎𝑑𝑖𝑢𝑠 = 294 𝜇𝑚 𝑢𝑛𝑖𝑓𝑜𝑟𝑚𝐸 = 4.66 𝑒𝑉 (corresponding to 𝜆 = 266.7 𝑛𝑚)

• The field on the cathode is: 𝐸𝑧 = 𝐸0𝑠𝑖𝑛𝜙𝑙𝑎𝑢𝑛𝑐ℎ . In this case

𝐸0 = 240𝑀𝑉/𝑚 and 𝜙𝑙𝑎𝑢𝑛𝑐ℎ = 35° → 𝑬𝒛 ≈ 𝟏𝟑𝟕𝑴𝑽/𝒎

• The starting intrinsic emittance was analitically re-calculated for an ideal high gradient case and setted to: 𝜀𝑖𝑛𝑡 ≈ 120 𝑛𝑚.

3 C-BAND SECTIONS @ 40MV/mGUN @ 240 MV/mKa-band @ 12.7 MV/m

X-BAND SECTIONS @ 65MV/m

LOW REPETITION RATE

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M. Croia 7

Velocity bunching

C-BAND SECTIONS SOLENOIDS ONX-BAND SECTIONS

Soft RF compression

Ka-band

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M. Croia 8

Beam parameters trends

𝝐𝒏,𝒓𝒎𝒔,𝒇𝒊𝒏𝒂𝒍 ≈ 𝟎. 𝟏𝟒 𝝁𝒎

𝑬𝒇𝒊𝒏𝒂𝒍 ≈ 𝟔𝟎𝟎𝑴𝒆𝑽

𝑬𝒔𝒑𝒓𝒆𝒂𝒅 ≈ 𝟎. 𝟐𝟒 %

𝐸𝑔𝑢𝑛,𝑒𝑥𝑖𝑡 ≈ 6𝑀𝑒𝑉

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𝝈𝒕,𝒇𝒊𝒏𝒂𝒍 ≈ 𝟑𝟓𝟎 𝒇𝒔

𝑰𝒔𝒍𝒊𝒄𝒆,𝒑𝒆𝒂𝒌 ≈ 𝟔𝟓 𝑨

Rolling slice analysis𝑳𝒔𝒍𝒊𝒄𝒆 = 𝟏𝟎 𝝁m

Compression factor ≈ 3

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M. Croia 10

• 𝑄 = 𝟕𝟓 𝒑𝑪

• GPT 250k particles

• Laser on cathode : ൞

𝑡𝑙𝑒𝑛𝑔𝑡ℎ = 𝑝𝑠 (𝑢𝑛𝑖𝑓𝑜𝑟𝑚)

𝑟𝑎𝑑𝑖𝑢𝑠 = 386 𝜇𝑚 𝑢𝑛𝑖𝑓𝑜𝑟𝑚𝐸 = 4.66 𝑒𝑉 (corresponding to 𝜆 = 266.7 𝑛𝑚)

• The field on the cathode is: 𝐸𝑧 = 𝐸0𝑠𝑖𝑛𝜙𝑙𝑎𝑢𝑛𝑐ℎ . In this case

𝐸0 = 160𝑀𝑉/𝑚 and 𝜙𝑙𝑎𝑢𝑛𝑐ℎ = 29° → 𝑬𝒛 ≈ 𝟕𝟖𝑴𝑽/𝒎

• The starting intrinsic emittance was analitically re-calculated for an ideal 160MV/m case and setted to: 𝜀𝑖𝑛𝑡 ≈ 155 𝑛𝑚.

3 C-BAND SECTIONS @ 20MV/mGUN @ 160 MV/mX-BAND SECTIONS @ 32MV/m

HIGH REPETITION RATE

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M. Croia 11

Velocity bunching

C-BAND SECTIONS SOLENOIDS ONX-BAND SECTIONS

Soft RF compression

Ka-band

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M. Croia 12

Beam parameters trends

𝝐𝒏,𝒓𝒎𝒔,𝒇𝒊𝒏𝒂𝒍 ≈ 𝟎. 𝟏𝟗 𝝁𝒎

𝑬𝒇𝒊𝒏𝒂𝒍 ≈ 𝟑𝟎𝟕𝑴𝒆𝑽

𝑬𝒔𝒑𝒓𝒆𝒂𝒅 ≈ 𝟎. 𝟐𝟓 %

𝐸𝑔𝑢𝑛,𝑒𝑥𝑖𝑡 ≈ 4.2 𝑀𝑒𝑉

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𝝈𝒕,𝒇𝒊𝒏𝒂𝒍 ≈ 𝟑𝟑𝟎 𝒇𝒔

𝑰𝒔𝒍𝒊𝒄𝒆,𝒑𝒆𝒂𝒌 ≈ 𝟖𝟎 𝑨

Rolling slice analysis𝑳𝒔𝒍𝒊𝒄𝒆 = 𝟏𝟎 𝝁m

Compression factor ≈ 3.5

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Axial symmetric 2D simulations

have been performed with

Poisson Superfish.

Integrated field the same of

the SPARC_LAB S-band gun

solenoid.

Bucking coil for the

cancellation of the field on

cathode (from almost 360 G to

≈1 G).

Cathode position

(110.5mm from the

solenoid center)

Bucking

Coil

Courtesy of

A.Vannozzi

C-band gun solenoid

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M. Croia

XLS

REQUESTS

High Repetition

Rate

Low Repetition

RateCharge (Q) pC 75 75 75

Beam energy MeV 300 307 600

rms bunch

length (σt)

fs 350 330 350

Peak current

(Q/sqrt12σt)

A 60 66 (80 slice) 61 (65 slice)

rms Energy

Spread

% 0.5 0.25 0.24

Projected rms

norm.

emittance

mm 0.2 0.19 (0.19 slice) 0.14 (0.13 slice)

Repetition rate KHz 1 / 0.1 1 0.1

Total Length m 15.5 15.5

TRL > 3 2 2

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Thank you!

CompactLight is funded by the European Union’s Horizon2020 research and innovation programme under Grant Agreement No. 777431.

CompactLight@elettra.eu www.CompactLight.eu