# Presentation: 356195

## PIC & REMEX(Parametric-Resonance Ionization Cooling and Reverse Emittance Exchange for Muon Colliders)

- Richard Sah
- Muons, Inc.
- Muon 2007
- January 27, 2007

## List of Contributors

- Muons, Inc.
- Richard Sah, Rolland Johnson,
- David Newsham, Tom Roberts, and
- Katsuya Yonehara
- Jefferson Lab
- Yaroslav Derbenev, Alex Bogacz, and
- Yu-Chiu Chao

- 27 October 2006

## PIC & REMEX

- Introduction to PIC and REMEX
- Motivation
- Technical Issues
- Solutions and Benefits
- Cooling Channel Studies
- Cell Designs
- Simulations
- Higher-Order Effects
- Summary

- 27 October 2006

## PIC & REMEX: Motivation

- It is important for a muon collider to have a high luminosity L, on the order of 1035
- L is proportional to (N+N-)/N, so a lower normalized emittance requires fewer muons to achieve a given L
- As muon-collider designs evolve, there is an increasing focus on _high-luminosity designs_ that feature _lower beam emittances and fewer muons_, on the order of 1011 per bunch

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## PIC: Technical Issues

- New, advanced designs for muon colliders feature ionization cooling, high-pressure RF cavities, and helical cooling channels to provide very fast muon beam cooling
- Unfortunately, ionization cooling as it is currently envisioned is limited at an equilibrium value and will not cool  transverse beam sizes sufficiently well to provide high luminosity without high muon intensities
- PIC is a new method to provide much smaller transverse beam emittances

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## IC Reaches an Equilibrium

- The equation describing the rate of cooling is a balance between cooling (first term) and heating (second term):
- Here n* *is the normalized emittance, Eμ* *is the* *muon energy in GeV, dEμ/ds and X0 are the energy loss and radiation length of the absorber medium,  is the transverse beta-function of the magnetic channel, and  is the particle velocity.

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## PIC is a Solution

- Excite 1⁄2 integer parametric resonance (in Linac or ring)
-   Similar to 1⁄2-integer slow beam extraction
-   Weak lenses every half period drive a parametric   	resonance, and the elliptical phase space motion 	becomes hyperbolic
-   Use xx’=const to reduce x, increase x’
-   Use IC to reduce x’
- Detuning issues being addressed (chromatic and spherical aberrations, space-charge tune spread).  Simulations underway.

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## Snake PIC

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## REMEX: Motivation

- The muon beam is cooled at a few hundred MeV/c, and it is then accelerated to the TeV/c range for an energy frontier collider.
- Consequently, the relative momentum spread p/p is greatly reduced during this acceleration process, and in fact _it becomes smaller than is required for a collider_
- Reverse Emittance Exchange (REMEX) is a concept to take advantage of this situation, in order to reduce further the transverse beam emittance

- 27 October 2006

## REMEX is a Solution

- Thin wedge absorbers are used to exchange the transverse and longitudinal emittances, in order to provide an additional reduction in the transverse emittance.
- The longitudinal emittance grows, but it remains low enough for a muon collider

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## Benefits of PIC and REMEX

- After IC is used to cool the beam in a muon collider, PIC can further reduce the transverse emittance by about an order of magnitude
- PIC reduces x, at the expense of increasing x’
- IC is then used to reduce x’
- REMEX can reduce the transverse emittance by one more order of magnitude – the longitudinal emittance is increased, but that does not matter
- Together, PIC and REMEX can reduce the transverse emittance by about _two orders of magnitude_ during the very last stages of beam cooling.  This emittance reduction is crucial to the performance of an energy frontier collider.

- 27 October 2006

## Further Developments of REMEX

- Optimization of REMEX/coalescing: three stages to increase transverse brightness, at the expense of increasing longitudinal emittance
- 1st Stage REMEX: after 6-D cooling using helical and PIC techniques, wedge absorbers provide REMEX up to the limit of RF acceptance
- 2nd Stage REMEX: the beam is accelerated to an intermediate energy, now with reduced p/p, for another stage of REMEX (which can take place during the acceleration process).  This process can continue until about 2.5 GeV, at which time energy straggling in the absorber becomes too large.

- 27 October 2006

## Further Development of REMEX

- Optimization of REMEX (continued)
- BUNCH COALESCING follows two stages of REMEX: the beam is accelerated to about 20 GeV, to achieve smaller p/p and longer muon lifetime.  6 to 15 bunches in a batch are then kicked into a special ring for coalescing into one intense bunch of short time but large momentum spread.  This newly-formed bunch is then extracted for further acceleration.
- Optimization issues will be hot topics at the upcoming LEMC Workshop on February 12-16, 2007. e.g.
- use of different RF frequencies to let the bunches get longer either before or during the acceleration stages of REMEX
- Forming and cooling bunches suitable for coalescing

- 27 October 2006

## Cooling Channel Studies

- Beamline Designs (ppt from Alex Bogacz)
- Particle-Tracking Simulations
- 	(ppt from David Newsham)
- Higher-Order Effects (ppt from Alex Bogacz)

- 27 October 2006

## Prototype Lattice

- Momentum = 300 MeV/c
- Dispersion at the absorber = ±22 cm
- Minimum betas at the absorber (h/v) =1/1 cm
- Maximum dispersion = 80 cm
- Maximum betas (h/v) = 5/5 m
- Periodic cell length = 2 m
- Betatron phase adv/cell (h/v) = /2

- Lattice implementation:
- Solenoid focusing around the absorber – axially symmetric * region
- Two pairs of dipoles with opposing bends – ‘dispersion flip’
- Quadrupole compensation for the dipole edge focusing – balanced  betas

- Lattice requirements:
- Integer/Half-integer betatron phase advance per cell
- One dispersion period per cell

## Lattice Layout

- Solenoid

- Quad

- Sector Dipole

- Absorber Plane

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## Periodic Cell – Magnets

- ‘inward half-cell’

**solenoids:**
- L[cm] 		B[kG]
- 22		105
- 22		105

- ‘outward half-cell’

**dipoles:**
- $L=20;  =>          20 cm
- $B=          25;  =>          25 kGauss
- $Ang=$L*$B/$Hr;  => 0.4996 rad
- $ang=$Ang*180/$PI;  =>   28.628 deg

**quadrupoles:**
- L[cm] 		G[kG/cm]
- 8		1.79754
- 8		-0.3325
- 8		1.79754

- betatron phase adv/cell (h/v) = /2

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## Periodic Cell  Optics

- Periodic Cell  Optics

- ‘inward half-cell’

- ‘outward half-cell’

- betatron phase adv/cell (h/v) = /2

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## Entr/Exit into the Snake channel – Dispersion suppression 

-  periodic cells

-  entrance cell

-  exit cell

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##  PIC/REMEX Channel   Optics

-  n periodic PIC/REMEX cells (n=2)

-  beam extension

- beam extension
- RF cavity
- skew quad

-  disp. anti-suppr.

-  disp. suppr.

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## Detuned Cell for Initial Simulations 

- Resonant cell

- Detuned cell

- tune per cell (Qx/Qy) = 0.700/0.720

- tune per cell (Qx/Qy) = 0.964/0.545

- 27 October 2006

## Detuned Single-Cell Lattice: Particle-Tracking Simulations with G4BL

- 15 mm aperture

- 50 mm aperture

- 100 mm aperture

- 200 mm aperture

- Solenoid

- Quad

- Sector Dipole

- Absorber Plane

- Must Use Small Solenoid Apertures to Avoid End Effects

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## Average Motion in “Center Line” 

- Understand the Corners

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## Detuned Single-Cell Beta

- 15 mm aperture

- 200 mm aperture

- Detuned
- OptiM
- Result

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## Initial Beam Profile

- 15 mm aperture

- 200 mm aperture

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## Initial Phase Space

- 15 mm aperture

- 200 mm aperture

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## Detuned 10-Cell Lattice

- 15 mm

- 200 mm

- 150 mm

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## Detuned 10-Cell Beta

- 15 mm aperture

- 150 mm aperture

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## Results of Simulations of Detuned Lattice

- Care must be taken in using G4Beamline to simulate channels designed with OptiM:
- Lumped linear elements (transfer matrices) vs. “real” objects
- Accurate injection is required to avoid beam loss
- Linear optics must be understood before adding absorbers
- The effects of fringe fields and end fields must be understood.  The Detuned Lattice was too compact, and only small-aperture solenoids could be simulated successfully, because of end-field issues.
- Next: try simulations of more “spread out” channel, where elements are spaced further apart and interact less.  Then more realistic solenoid apertures may be used.

- 27 October 2006

## Simulations of a Spread-Out Cell Have Begun

- betatron phase adv/cell (h/v) = /2

- 27 October 2006

## Higher-Order Effects

- The purpose of the PIC and REMEX concepts is to reduce greatly the transverse emittance of the muon beam, at the very end of the beam cooling process in a collider
- Therefore, it is imperative that the final beam spot is not spoiled by any higher-order effects
- A number of effects must be considered:
- Chromatic and geometric aberrations
- Space charge effects
- Effects of beamline elements – fringe fields, straggling in absorbers, etc.

- 27 October 2006

## Optical Aberrations

- Notation: second-order aberrations may be described in terms of a tensor T such that
- vif = Tijk vji vki   where v = (x, x’, y, y’,t,)
- From beamline and spectrometer design, it is known that two of the most important tensor elements (out of a total of 216 elements) are T126 and T122

- 27 October 2006

## Chromatic Aberrations

- T126 is a chromatic aberration, which depends on the initial value of v6 = 
- In a ring, this tensor element is the _chromaticity_, and it is usually corrected using carefully placed sextupole magnets

- 27 October 2006

## Geometric Aberrations

- T122 is a spherical aberration, which is one type of geometric aberration
- The magnitude of this aberration, at the end of a section of beamline, depends on the square of the initial value of x’
- This aberration arises unavoidably from the use of dipole magnets, but beamlines with the proper reflection symmetry exhibit a exact cancellation of this aberration
- Simulations indicate that this cancellation can indeed be effectively employed in a PIC/REMEX cooling channel

- 27 October 2006

## ‘Snake’ vs. ‘Chicane’  Layout

**L**

**L**

**R**

**R**

**L**

**R**

**L**

**R**

**‘****snake’**

**‘****chicane’**

- 27 October 2006

## Initial Transverse Phase-space

**Initial phase-space**
- BunchLength[cm]=1.0   dP/P=0.000001
- 	Beta_X_1[cm]=1.36483 	Beta_X_2[cm]=0.
- 	Alfa_X_1=0.0      	Alfa_X_2=0.
- 	Beta_Y_1[cm]=0.  	Beta_Y_2[cm]=1.36448
- 	Emit1[cm]=1.e-2 	Emit2[cm]=1.e-2
- 	DispX[cm]=0.     	DispY[cm]=0
- 	DispXpr[cm]=-59.8339   	DispYpr[cm]=0

- x,y = 100 mm mrad geometric emittance at p =100 MeV/c
- Very small momentum spread, to remove chromatic aberrations

- x - y

- x – x’

- y – y’

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## ‘Snake’ vs. ‘Chicane’  Tracking

- ‘Snake’ vs. ‘Chicane’  Tracking

**chicane**

- x - y

- x – x’

- y – y’

- 27 October 2006

## Cancellation of Geometric Aberration

- The speculation that T122 is the most important geometric aberration in this cooling channel is supported by the simulation results
- That is, the reflection symmetry of the “chicane” cooling channel cancels this specific geometric aberration and greatly reduces the total geometric aberrations observed

- 27 October 2006

## Summary

- The PIC and REMEX concepts have been invented and developed to provide the crucial final beam cooling in a muon collider.  Their use can greatly reduce the final beam emittance and can permit the construction of a high-luminosity collider that requires fewer muons.
- Significant progress has been made in the design of these cooling channels and in the corresponding particle-tracking simulations
- Reflection symmetry in the cooling channel design can be used effectively to cancel the largest geometric aberration in the beamline optics

- 27 October 2006