DDS limits and perspectives - PowerPoint Presentation

Slide1

DDS limits and perspectives

Alessandro D’Elia on behalf of UMAN Collaboration

1Slide2

Damped and detuned design

Detuning: A smooth variation in the iris radii spreads the dipole frequencies. This spread does not allow wake to add in phase

Error function distribution to the iris radii variation results in a rapid decay of

wakefield

.Due to limited number of cells in a structure wakefield recoheres.Damping: The recoherence of the wakefield is suppressed by means of a damping waveguide like structure (manifold).Interleaving neighbouring structure frequencies help enhance the wake suppression

2Slide3

3

VDLSlide4

Why a Detuned Damped Structure (DDS) for CLIC

4

Huge reduction of the absorbing loads: just 4x2 loads per structure

Inbuilt Wakefield Monitors, Beam Position Monitors that can be used as

remote measurements of cell alignmentsHuge reduction of the outer diameter of the machined disksSlide5

CLIC_DDS_A: regular cell optimization

The choice of the cell geometry is crucial to meet at the same time:

Wakefield suppression

Surface fields in the specs

Consequences on wake functionCell shape optimization for fields

DDS1_C

DDS2_E

5Slide6

RF Properties of CLIC_DDS_A in comparison with CLIC_G

6

Parameters

Units

CLIC_DDS_A8 x DDS_A8 x DDS (Circular cells)CLIC_GFc (Amplitude)

-

1.29 x 10

24

*

3.4 x 10

5

*

6573

*

1.06 **

Frms

(Amplitude)

-

1.25 x 10

27

*

2.8 x 10

7

*

5 x 106 *5.9 **Fworst (Amplitude)-1.32 x 1028 *7.5 x 108 *1.55 x 108 *25.3 **Pulse lengthns276.5--240.8Peak input power (Pin)MW70.8--63.8No. of bunches-312--312Bunch population1094.2--3.72Max EsurfMV/m220--245TK51--53, 47SCW/m26.75--5.4bXm-21.36 x 1034--1.22 x 1034RF-to-beam efficiency%23.5--27.7RF cycles-8--6Cost--

* 312 bunches, only first dipole band

** 120 bunches, quarter structure

GdfidL

wake Slide7

A new approach: a Hybrid Structure for CLIC_DDS_B

7

WGD_Structure

+

DDS_Structure=

Hybrid StructureSlide8

Study of the wake function

The problem

F



571MHz; F=2GHZ

Question: How big must be

F

in order to have acceptable wake damping starting from 0.5ns?

8Slide9

Study of the wake function

W

t1

6-7V/[

pC mm m], considering that W(0)170-180V/[pC mm m], the maximum acceptable bump must be 4% F2.9GHz and 0.830GHz F=2GHZ

F=2.5GHZ

F=2.9GHZ

9Slide10

What about a “

Sinc

” wake?

Wake uncoupled

Wake coupledThis is the wakefield considering only the first dipole band2Kdn/df

Real(

Zx

)

10Slide11

GdfidL

“Full Wake”

1

st

Dipole wake from GdfidLThe presence of the higher order bands makes the scenario even less comfortable

Conclusion: It is not possible to control the position of the zeros along the wake, a smooth function of the impedance is needed

What about a “

Sinc

” wake?

11Slide12

Can other types of distributions improve the wake decay?

906MHz F=2.9GHZ

830MHz

12Slide13

Can other types of distributions improve the wake decay?

967MHz F=2.9GHZ

1.036GHz

13Slide14

Can other types of distributions improve the wake decay?

=1GHz

926MHz

F=2.5GHZ

14Slide15

What about 0.67ns?

F=2GHZ

15Slide16

How big is the bandwidth we may achieve?

Assuming

SlotW

constant throughout the full structure

We must consider that 400-500<Av. Cross.<800-900 in order to get Qs in the order of 500-600 which will preserve the fsyn distribution

NB:

The BW has been evaluated considering the difference between 1

st

Reg. Cell and Last Reg. Cell, i.e. Cell#27, but the total number of the cells is 26 (26 cells

 27 irises

); then the real BW will slightly decrease in the real structure

Geometric Parameters

a (mm)

4.04-1.94

L (mm)

8.3316

t (mm)

4-0.7

eps

2

WGH (mm)

5

WGW (mm)

6

16Slide17

Bandwidth coupled and uncoupled

- Uncoupled 27 cells: F= 2.685GHz

Uncoupled 26 cells (not shown): F= 2.47GHz

Coupled (

GdfidL): F= 2.363GHz From theoretical distribution to real structure one must take into account a reduction of ~200MHz in the BW

Av. Cross~600MHz

17Slide18

What is the bandwidth of the real coupled structure?

GdfidL

Reconstructed wake (only 1

st

Dipole band)

Uncoupled wake with 25 peaks (



F=2.314GHz

)

The uncoupled wake with 25 frequencies (black dashed curve,

F=2.314GHz

) falls faster than the 1

st

dipole band reconstructed wake from

GdfidL

(red dashed curve): is there any strange effect from uncoupled to coupled that further reduce the bandwidth?

18Slide19

Non Linear Fit to improve wake reconstruction

The procedure:I take

GdfidL

wake as “objective” function of my non linear regression

I use reconstruction formula as my fitting function Fsyn are considered as given from Lorentzian fit of the impedance peaks while Qdip and Kicks are the parameters to be optimizedInitial guess for Qdip and kicks are from Lorentzian fit19Slide20

Results (1)

The agreement with GdfidL is quite good and, as expected, the new procedure produces a major correction at the beginning of the curve while for the rest there are no appreciable variation with the wake reconstructed using the data from

Lorentzian

fit.

<Qdip>=312<Qdip>=512

=94

=

67

It is clear that the wake is reconstructed from unphysical values of kicks and

Qdip

. Constraints on the parameters are needed.

20Slide21

Results (2)

<

Qdip

>=312

<Qdip>=337=94=67

With same constraints and an appropriate length of the wake, kicks and

Qdip

starts to converge.

21Slide22

First results for sech

1.5

2Kdn/

df

Very sharp deep, before 0.15mNeed to finalize the simulation to finalize the analysis

22

Very preliminarySlide23

Conclusions

With conventional DDS (DDS_A) it seems very difficult to meet beam dynamics criteria

With hybrid DDS, using Gaussian distribution, it seems non realistic to get damping within 6 RF cycles

With different distribution (in particular sech

1.5) it is possible to relax the constraint on the BW and this could allow to stay in the 0.5ns bunch spacingPlay with Kdn/df would be interesting to see what happen and especially whether it is possible to increase the bandwidth by distributing differently the frequenciesHowever the requirement of 0.5ns is quite tricky and we have not yet considered surface fields…I would not close totally the door to 8 RF cycles23Slide24

24

THANKS

Igor

Slide25

Additional slides

25Slide26

Physical interpretation of the result

Constraints:

First and last three peaks in the impedance are well separated then their

Qdip

and kicks are considered fixed The rest of the kicks must be positive and spanning in a range from zero to roughly 10 The rest of the Qdip can span from zero to a maximum of 1500<Qdip>=312<Qdip>=576=94=67

Wake is still well approximated but kicks and especially

Qdip

do not seem correct. The constraints I gave are still not enough.

26Slide27

Extrapolation for longer wake

If I extrapolate for a longer wake it is clear that

Qdip

and kicks evaluated from Non Linear Fit are not correct.

I need more wake to improve Qdip calculation27Slide28

Increasing the length of the wake: 10m

<

Qdip

>=315

<Qdip>=312=67=67This makes me much more confident on the wake reconstruction

28Slide29

Going back to the beginning

Question was: can I evaluate the bandwidth reduction from uncoupled?

From

GdfidL

Uncoupled 25 Cells

Uncoupled 27 Cells

Uncoupled 25 Cells

Uncoupled 26 Cells

2Kdn/

df

Answer: It seems Yes, with some minor approximation.

In particular in this case it is clear that the major reduction comes from one peak which is missed. Then I estimate a reduction of ~230MHz and not of 322MHz

 if I choose ~2.75GHz, I should stay around 2.5GHz which is the minimum required for sech

1.5

distribution.

29