How to reduce the heating of the ALFA Roman Pots

ALFA = Absolute Luminosity For ATLAS

O.Berrig, F.Caspers, P.Fassnacht, S.Jakobsen, J. Kuczerowski, B.Salvant

The problem with the ALFA is:

Even though the POTs are in parking position

i.e. removed as much from the beam as possible (40mm)

They heat up!This comes from resonances

Primer on beam impedance1. When a particle passes a structure, it will generate

electro-magnetic fields.

2. These electro-magnetic fields will react back on the particle itself and act as well on other particles [in the bunch itself or on following bunch(es) - if the fields have not died out before the next bunch(es) arrives]

3. These electro-magnetic fields have two effects: A. They will give the bunch a kick Which can lead to beam instability B. They will heat up the structure Which might destroy the structure

4. The integral of the electrical fields over the distance of the structure is voltage. The voltage divided by beam current is the beam impedance

http://emetral.web.cern.ch/emetral/LRFF/LRFF.htm

Example of destructive heating:Melting of spring that holds RF fingers

Primer on heating (by resonances)

Fig. A: The energy deposited in the resonator: I*V*dt (where dt is the length of the bunch)

https://cds.cern.ch/record/1058086/files/p139.pdf

It is even more a thought experiment, because the bunch induces a voltage before it reaches the resonant circuit

Fig. B: The energy deposited in the resonator for the different bunches:bunch 1: E= I*V*dt Accumulated energy: I*V*dtbunch 2: E= 2*I*V*dt Accumulated energy: 3*I*V*dtbunch 3: E= 3*I*V*dt Accumulated energy: 6*I*V*dtbunch 4: E= 4*I*V*dt Accumulated energy: 10*I*V*dt

This is a thought experiment, where the resonator consist of basically only a capacitor. The R and L are very, very low:

The energy which is deposited in the structure grows like a parabola. That is why resonances are so dangerous.

This leads to the paradoxical statement, that in order to reduce the heat deposition, we need to add more lossy material! (when the Q of the resonator is low, the voltage has died out before the next bunch arrives)

http://indico.cern.ch/getFile.py/access?contribId=43&resId=1&materialId=slides&confId=14017

= The elastic scattering rate

= Luminosity

= Cross section of protons

For an ideal detector, the following formula applies:

Done during specific runs, high b* values and low luminosity (few bunches)

http://www.vakuum.cz/pdf_totem/M_Oriunno.pdf\\cern.ch\dfs\Users\s\SJAKOBSE\Public

Measurement setup

DUT = Device under test

Measurements were done on three configurations:

ALFA - Original ALFA – Originalwith Station filler

ALFA – Originalwith Roman POT filler(Designed with the help of many CST simulations by B.Salvant)

NB! In addition, all three configurations were tested with different ferrite options e.g.

0.00E+00 5.00E+08 1.00E+09 1.50E+09 2.00E+09 2.50E+09 3.00E+09

-40

-35

-30

-25

-20

-15

-10

-5

ALFA_24_5_13_NO_FILLER_NO_FERRITES

ALFA_8_8_13_REFERENCE_CHAMBER

ALFA_28_5_13_WITH_STATION_FILLER_AND_FERRITE

ALFA_7_8_2013_ROMAN_POT_FILLER_WITH_ONLY_RING_FERRITES

Nothing should propagate with less losses than the reference chamber => Unphysical(Still to be investigated further)

Wire measurements in garage position

[dB]

[Hz]

In operation in 2012

-16.6 dB comes from matching resistors

Candidate to be installed in LHC in the future

In the following days (week) we might need to calculate the energy deposited in the ALFA station for the configuration with the CERN filler and with/without ring ferrites, in order to see if we can avoid the ring ferrites.

0.00E+00 5.00E+08 1.00E+09 1.50E+09 2.00E+09 2.50E+09 3.00E+09

-19

-17

-15

-13

-11

-9

-7

-5

ALFA_8_8_2013_REFERENCE_CHAMBERALFA_7_8_2013_ROMAN_POT_FILLER_WITH_ONLY_RING_FERRITESALFA_5_8_2013_ROMAN_POT_FILLER_NO_FERRITES

The purpose of the ring ferrites, is to remove this peakSince the ferrites are expensive; are they really necessary ?NB! 1 dB means we have lost 20% of the power at this frequency 3 dB means we have lost 50% of the power at this frequency 10 dB means we have lost 90% of the power at this frequency

[dB]

[Hz]

The position of the ring ferritesThe ring ferrites absorb the fields that propagate along the filler to the bottom of the POT.

Measurements vs. SimulationsALFA - no roman pot filler – no ferrites

2.00E+08 4.00E+08 6.00E+08 8.00E+08 1.00E+09 1.20E+09 1.40E+09 1.60E+09

ALFA_24_5_2013_NO_FILLER_NO_FERRITESALFA_8_8_13_REFERENCE_CHAMBER

Measurement for garage position(40 mm)

The simulations are done with wire inserted, but without matching resistors!

Measurements vs. SimulationsALFA with roman pot filler – no ferrites

2.0E+08 4.0E+08 6.0E+08 8.0E+08 1.0E+09 1.2E+09 1.4E+09 1.6E+09

ALFA_5_8_2013_ROMAN_POT_FILLER_NO_FERRITESALFA_8_8_13_REFERENCE_CHAMBER

Measurement for garage position(40 mm)

Do we need to simulate higher frequencies?

The simulations are done with wire inserted, but without matching resistors!

ConclusionThe current Roman pots should not be left as they were before LS1. In order to reduce the heating of the ALFA station, two modifications were tested: “STATION FILLER” and “ROMAN POT FILLER”. The test measurements showed that the two modifications were similar in their ability to reduce the heating. The “ROMAN POT FILLER” is preferred as it is more mechanically robust.The “ROMAN POT FILLER” upgrade proposal clearly goes in the right direction if we believe simulations and bench measurements (as reviewed at LTEX of July 11th 2013 and impedance meeting of August 2nd 2013 in particular). With the information and results at our disposal, the impedance team would recommend to install the Roman pot filler and the ring ferrites. https://edms.cern.ch/comment/1288051/0.2

FUTURE DEVELOPMENTS

0.00E+00 5.00E+08 1.00E+09 1.50E+09 2.00E+09 2.50E+09 3.00E+09

-18

-16

-14

-12

-10

-8

-6

50mm_NO_ATTENUATOR35mm_NO_ATTENUATOR

[Hz]

[dB]

0.00E+00 5.00E+08 1.00E+09 1.50E+09 2.00E+09 2.50E+09 3.00E+09

-39

-37

-35

-33

-31

-29

-27

50mm_WITH_ATTENUATOR35mm_WITH_ATTENUATOR

[Hz]

[dB]

Frequency interval corresponds to twice the length of the REFERENCE CHAMBER + length of the two SUCOBOXES*.Δf = c/((LREFERENCE CHAMBER + 2*LSUCOBOX)*2)Δf = c/((0.420+ 2*0.035)*2) Δf = 0.306 GHz

* It is probaly more the length between the end of the REFERENCE CHAMBER and where the reflection happens in the resistor.

Measurements will be done with short SUCOBOXes and attenuators in the future.(For compatibility reasons, all the ALFA measurements were done with 50mm SUCOBOXes and no attenuators)

The above measurements were done on the ALFA reference chamber

FUTURE DEVELOPMENTS

Replace the matching resistor with a small printed circuit and SMD type resistor.This should improve matching over a larger frequency range. The standing wave along the length of the DUT, would then disappear.

Also some way should be found to replace the soldering!

FUTURE DEVELOPMENTSReplacing ferrites with loops. Proof of principle

One loop

The measurement was done at the position, which is closest to the beam that we can measure i.e. 3mm. We managed to reduce the resonance at 1.08 GHz, however we created a new resonance at 0.948 GHz. The resonance at 1.97 GHz was suppressed, but a new resonance at 1.87 GHz was created. The huge resonance at 2.31 GHz was reduced by 10 dB. However, the remaining resonance is wider than the original.

Johannes Herbst