Outline of the presentation

First ideas on implementation of crystals in LHC collimation system
Valentina Previtali, AB/ABP and EPFL Thanks to R. Assmann (supervisor), S. Redaelli, W. Scandale PhD on LHC collimation upgrade (including crystal studies) in the AB/ABP group. Name Phd student in Phd focused Title

Outline of the presentation
The present LHC collimation system and its main features Using crystals for the LHC collimation system: Qualitative analysis of problems to face Preliminary optics studies for a crystal-based collimation system Conclusions Intro evaluation

Why collimation system is so important?
We have to deal with a very energetic beam in a superconducting enviroment! Stored beam energy : 360 MJ Quench limit for LHC magnets: 10 mJ over 1 cm3 Robustness Equivalent to 90 Kg of TNT Courtesy of R. Assmann 200 times more than Tevatron/HERA That means: @7 TeV it is enough to loose 1 proton over to make a SC magnet quench! Crytical issue in lhc Because there is high en And this energy must be managed in a sc enviroment Up to 2MJ in 6 ms in case of inj error! 1MJ in o.2 microsec in asyncronous beam dump Cleaning Efficiency

Requirements for collimators
Good robustness High efficiency SOLUTION? Low impedance Reasonable cost Fast schedule Low activation Reasonable tolerances No simple solution! => phased approach PHASE 1: Priority to robustness and flexibility (CFC). Luminosity might be limited by high impedance and efficiency restrictions PHASE 2 will allow to reach the nominal luminosity. To overcome the impedance limitations: innovative metallic collimators to be used during stable physics running. Room also for other innovative solutions (e.g. crystals). Fase 1 che lum e’ presvista?

Present layout of collimation system: multi-stage cleaning
15 10s 5 Beam ` center -5 Primary halo produce -10 Secondary halo -15 Tertiary halo Primary collimators CFC - 60cm Secondary collimators CFC - 1m Absorbers W - 1m Sensitive equipment LHC arc or IR triplet

Phase 1 collimation system
Overall ~150 collimator locations in LHC and transfer lines Two warm insertions dedicated to collimation: IR3 momentum cleaning IR7 betatron cleaning Layout has been optimized for phase 1 Courtesy of C. Bracco

Could Crystals help in upgrade?
The basic idea: Use the crystals instead of primary collimators, and deviate all the particles on secondary collimators/absorbers. Diffusion rate of halo particles: 2nm/turn center Impact parameter ≤ 1 m -5 Scattered particles (crystal as amorphous material) Primary halo How the cystal could be implemnted in this scheme? -10 Tertiary halo -15 Primary collimators crystal+CFC Slot avaiable Sensitive equipment LHC arc Extracted Particles (crystal for de- and/or reflection) ?

What do we have to understand
In order to study the feasibility of crystal collimation, we have first of all to understand: 1- which angle/position have the particles that hit the crystal? 2- which angle/position have the particles that will leave the crystals? 3- What will be the impact on the machine? (luminosity, impedance, efficiency...) How does the crystal work? What will be the best layout? Simulations!

Position of incoming particles
Particles of the halo naturally drift slowly outwards (~ 2 nm per turn). drift beam Is the crystal robust enough (robust as CFC!) to stand all the impacts in a small region, when acting as amorphous? What could be the increase in T after hours at nominal luminosity? And the effect? Deformation? Maybe damage or breakage? What is the impact parameter that the particle will have on the crystal? Since the tune is not integer, the particles will hit the crystal every ~10-20 turns. The FIRST impact parameter of the particles will be in the range of ~100 nm Grazing impact on crystal surface!!!

Angular distribution of incoming particles
If crystal is positioned where =0 : incoming angle does not depend on . Otherwise (≠0): divergence depends on  it changes during the ramping-up in energy (adiabatic emittance damping) Divergence of the beam! LHC Energy range crystal Particular case: crystal positioned instead of present primary collimators: Due to slow diffusion velocity, angular spread for fixed energy (contributions of impact parameter, drift of the closed orbit…) should be negligible. Angle variation rad from injection to top energy Reliability of the goniometer

How does the crystal work?
The effect of the crystal on an incoming particle depends on the impact parameter and on the incoming angle Perfect crystalline structure Impact parameter Amorphous layer Incoming angle We can consider two different zone: Amorphous layer: Multiple Coulomb Scattering Perfect crystalline structure: different processes, depending on the incoming angle and on the energy of the particle (see next slide)

High impact parameter particles: Dependence on incoming angle
(4) Volume capture (3) Channeling (2) Dechanneling (5) Volume reflection High impact parameter particles: Dependence on incoming angle For a given particle energy, probabilities depends on the angle x' - No deflection/Coulomb scattering (1) - Dechanneling (2) - Channeling (3) - Volume capture (4) - Volume Reflection (5) Precise evaluation of probability (also in function of energy, different geometries)? We take care of a fraction of 10-3 of protons colliding with the primary collimators! (1) (4) (5) (3) (2) (1) Courtesy of W.Scandale, see “Main outcomes of H8 runs in 2006”, this workshop

Low impact parameter particles
First impact parameter is around 100 nm=> thickness of the amorphous level is critical First impact: crystal will behave like an amorphous (small scattering angle) Multi-turn accumulation! channeling Estimated roughness: 100 nm [A.Vomiero] Is it a sufficient measure of amorphous level? What about dislocations, cracks? After some (?) iterations: Particles will have a larger impact parameter (~1mm is enough? need an estimation): Channeling or VR => depends also on the incident angle Volume Reflection

Angular kick given by the crystal: Volume Reflection
For particles with impact parameter > amorphous layer Crystal x x' Volume Reflection Volume capture (partial channeling) What’s the probability of de-channeling after Volume Capture? Using Volume Reflection: two processes in competition 1- Volume Reflection (announced to be up to 97%) 2- Volume capture (+ dechanneling?) 3- No deflection Angular kicks are given in opposite directions Secondary collimators must cover two different phase-space locations (~3%) Not low compared to 1e-5 cleaning requirement

Angular kick given by the crystal: Channeling
For particles with impact parameter > amorphous layer Three processes in competition 1- Channelling (~ 50% ) 2- Dechanneling 3- No deflection Angular kick given along a spread of angles Crystal x x' Dechanneling Channeling (~50%) Is the goniometer reliable? If there was an error, we would not reflect/channel anymore. “Best” case: particles will be undeflected. Risk of sending all the particles to a wrong direction (maybe the opposite one) if e.g. we jump from reflection to channeling regime (MP): where does it go?

In order to study the feasibility of crystal collimation, we have first of all to understand: 1- which angle/position have the particles that hit the crystal? 2- which angle/position have the particles that will leave the crystals? 3- What will be the impact on the machine? (luminosity, impedance, efficiency...) How does the crystal work? What will be the best layout? Simulations! For cleaning the beam, it is necessary to intercept and absorb the extracted particles => absorbers on the new trajectory

Which absorbers? For cleaning the beam, it is necessary to intercept and absorb the extracted particles => absorbers on the new trajectory! Energy will be extracted to an absorber with potentially small spot size. Concerns about robustness, efficiency, cooling and positioning of these absorbers in/out-side LHC vacuum. Where? Preliminary optics studies have been performed by R. Assmann and S. Redaelli for layouts of the absorbers in a possible crystal-based collimation system for LHC, in case of positioning the crystal at the present scraper location. Two options: 1 - keep the present layout and optimize the crystal angle 2 - study a new absorber layout optimized for a crystal-based collimation system  ≠ 0 only the option of inserting the crystal at the scraper location has been investigated up to now Alpha not 0!

Keeping the present layout
It is not easy to find an angle good for both injection and nominal energy, especially for the horizontal case.  ≠ 0 only the option of inserting the crystal at the scraper location has been investigated Available collimators Only the possibility of using channeling has been explored up to now (positive angles) An angle of 32 rad could ensure impacts in the whole energy range, but at the injections the losses would be concentrated far downstream of IR7, close to cold magnets Courtesy of R. Assmann and S.Redaelli

~ 400 m of warm LSS available in IR7!
Bends Q Q Q Q Bends SC magnets Crystal 6s 7s 7s 7s Absorber? 7s 7s ~ 400 m of warm LSS available in IR7! Courtesy of R. Assmann and S.Redaelli

New layout (optimized for crystals)
Optimizing means: (1) minimize crystal kick angles (2) minimize the difference between required angles at injection and top energies (3) move the absorber as much as possible upstream New absorbers would be positioned in space reserved for collimation upgrade and the first impact of channeled particles would occur at the beginning of the warm insertion. Courtesy of R. Assmann and S.Redaelli

In order to study the feasibility of crystal collimation, we have first of all to understand: 1- which angle/position have the particle that hit the crystal? 2- which angle/position have the particles that will leave the crystals? 3- What will be the impact on the machine? (luminosity, impedance, efficiency...) How does the crystal work? What will be the best layout? Simulations! The implementations of crystal could increase efficiency and/or reduce the impedance of the collimation system. This will be one of the main subjects of my future PHD thesis

And in case of failure? What if a machine operation error? Would it deviate the entire beam on a collimator or into the machine cold aperture? What if the crystal fails? (e.g. misalignment). Then the crystal would act like a very thin amorphous matherial.What would be the impact on the machine? Would we need a “backup” system? An object able to extract the full beam is dangerous! All the possible scenarios must be well studied and the impact on the machine evaluated The standard collimation system downstream of primary collimators (secondary collimators and absorbers) must be kept at or close to nominal settings to provide efficient multi-stage cleaning in case channeling/VR is lost.

Conclusions LHC collimation is a difficult task: phased approach
Phase 1 will guarantee robustness, but might limit the intensity. Need for new ideas! Could the crystals help? There is some hope but there are also still a number of open questions that must be addressed: Heating and radiation hardness What would be the increase in temperature after hours at nominal luminosity? And the effect? Deformation? What’s the risk of damage or breaking? Is the crystal robust enough to stand all the impacts in a small region, when acting as amorphous material? Long-term damage from radiation? Precise evaluation of probabilities Required cleaning efficiency in the LHC is 2×10-5 per meter! Even if the crystal cleaning process has an efficiency of 97%: what happens to the residual 3%? Since the critical angle depends on the energy, how does the angular kick/acceptance change with the energy on the various processes? Need of estimation in the LHC energy range. And the dependence on the geometry?

- Reliability of the goniometer
If there was an error, we would not reflect/channel anymore. “Best” case: particles will be undeflected. Worst case: Jump from reflection to channeling: all the particles sent to a wrong direction (maybe the opposite one): where? What is the operational stability of the goniomenter in vacuum when exposed to strong radiation? What will be its average lifetime? - Absorber Need for investigations about a possible material for the absorber and robustness, efficiency, cooling and positioning of these absorbers in- or outside LHC vacuum (design peak power load up to 1 MW). - Layout of crystal-enhanced collimation system Crystals must be placed in warm insertions, namely the cleaning insertions. With the present layout the implementation of a crystal is challenging. Nevertheless, the standard collimation system downstream of primary collimators (secondary collimators and absorbers) must be kept at or close to nominal settings to provide efficient multi-stage cleaning in case channeling/VR is lost. Possible scenarios in case of failure An object capable to extract the full beam is dangerous! All the possible scenarios must be well studied and the impact on the machine evaluated.

Outlook My PhD will focus (among a few other things) on a possible implementation of crystals into the LHC collimation system. Focus is accelerator physics! My future goals on crystals over the next 3 years: Participate in experiments and evaluate existing data. Implement crystals into numerical collimation simulation programs. Work out possible optics layouts Evaluate predicted cleaning performance and expected energy deposition (with FLUKA team). Address MP issues. Initiate a first look at an engineering solution, including design of required absorbers. Conclusion should be ready at end of 2009 for decision on collimation upgrade. For my work I need the support of the crystal channeling community. Thanks in advance for your support!

First ideas on implementation of crystals in LHC collimation system
Valentina Previtali, AB/ABP and EPFL Thanks to R. Assmann (supervisor), S. Redaelli, W. Scandale PhD on LHC collimation upgrade (including crystal studies) in the AB/ABP group.

Questions related to impact parameter
Proton (0.2 mm) Roughness (several mm?) At what depth into the crystal will the protons be de- or reflected?

“Phased” approach for collimation system
No ideal solution for LHC collimation system PHASED APPROACH divide goals and difficulties of LHC in time. LHC collimation as a mountain to climb: any good climber would divide the journey in stages! PHASE 1: Priority to robustness and flexibility (CFC). Luminosity might be limited by high impedance and efficiency restrictions. PHASE 2 will allow to reach the nominal luminosity. In order to overcome the impedance limitations: innovative metallic collimators to be used during stable physics running. Room also for other innovative solutions (e.g. crystals). To tackle such a difficult problem, in a limited amount of time, a staged approach has been devised .... Phase I, Phase II …

Volume capture Channeling Dechanneling Volume reflection

Requirements for collimators: robustness
For normal proton losses, the collimation system must be able to deal with a minimum lifetime of 0.1 injection and 0.2 collision energy for up to 10 seconds. 1% OF BEAM LOST OVER 10 s This can mean energies up to 500 KW in the collimation insertion (250 m) during 10 seconds (5 times less continuously). In case of failures (MP functionality) losses are much higher (up to ??). => we need robust materials! 1

Requirements for collimators: high cleaning efficiency
Max total intensity of protons Quench level cleaning inefficiency Number of escaping p (>10) Number of impacting p (6) Beam lifetime (0.2h) Diluition lenght Collimation performance can limit the intensity and therefore LHC luminosity. Courtesy of R. Assmann Quench 7 TeV: p/m/sec. For reaching nominal luminosity: we need high efficiency! (must care for 10-5 of the p impacting on collimators) 2

Requirements for collimators: Small gaps and low impedance
Collimators are the closest elements to the beam. The collimator gaps are a function of the lowest b* in the ring. Materials close to the beam: potentially high impedance induced on the ring (Impedance scales inversely proportional to the third power of gap size) We would like to have a low-resistivity material 3 Material chosen: Fiber-Rinforced Graphite with higher resistivity than e.g. copper (priority given to robustness, accepting possible limitations due to impedance).

Angular distribution of incoming particles: fixed energy
Let's imagine that the crystal is 6 x' < 0.5 rad a. Spread (both 450 GeV & 7 TeV) of the impact angle for 6s halo particles: the angular spread is determined by the impact parameter on the crystal (here considered of 1 m) x x' b. Moving orbit: the closed orbit might drift. For a 1 s drift in orbit around the machine the angle can change by less than 3-4 mrad at 7 TeV (4 times bigger at 450 GeV). Orbit tolerance: ~0.4 s. x 450 GeV: ~6-7 rad total 7 TeV: ~ 2 rad total

Low impact parameter particles
Very low impact parameter = crystal behave like an amorphous ( How do we deal with this problem with standard collimators? First time: small impact parameter  small amount of material  small scattering angle The next time: larger impact parameter  larger amount of material  larger scattering angle => iterative, cumulative process What matters is the integrated path (must consider relative nuclear cross-sections; absorption vs. radiation length for material)

Variation of acceptance with energy?
Could became something like this???? (energy increase = lower critical angle = lower angular acceptance)

Why do we need collimators?
In a real machine several mechanisms lead to beam losses - beam physics and operational instabilities - intra beam scattering - scattering with residual gas - synchrotron radiation - collective instabilities … Core of the beam Single and collective instabilities Single and collective instabilities HALO HALO These particles constitute the HALO of the beam

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