NICA start-up scenario + questions of instabilities

A.SidorinFor NiCA team

NICA Machine Advisory Committee at JINR (Dubna)October 19-20, 2015

• Total and start-up configuration of the NICA collider equipment

• NICA start-up scenario • Luminosity:- gold-gold collisions- Light ionsQuestions of instabilities

2

Contents

Staging of the NICA project

1. Fixed target experiment (BM@N), collider and MPD commissioning at start-up configuration

2. Upgrade to total version, energy scan and scan over the ion masses at collider.Both collider rings are operated at the same rigidity (symmetry collisions)

3. Upgrade of the interaction region optics, gold-light ion and Au-p collisionsThe rings are operated at different rigidity (asymmetry collisions)

4. Upgrade of the collider injection kickers, installation of Siberian snakes, spincontrol and spin diagnostic devices, collisions of polarized beams

Goal of this reportComparison of start-up and total version

Total: Peak luminosity maintains

1. Maximum rms bunch length is chosen to be 0.6 m:to concentrate the luminosity inside the MPD inner tracker

2. Maximum peak luminosity (at tune shift limit) corresponds to maximum achievable emittance:rms emittance is 1.1 mmmrad (beam rms radius = 1/6 aperture)

3. Ratio between horizontal, vertical emittance and momentum spread is chosenfrom equality of the IBS heating rates (thermal equilibrium – minimum heating rate)

4. Particle number in bunch is limited by tune shift 0.05

5. Number of bunches = 22: absence of parasitic collisions

6. RF amplitude (up to 1 MV) and harmonics (66) are chosen to provide required momentum spread and bunch length (the bunch phase area is about 1/25 of the bucket)

4

Circumference of the ring, m 503.04Number of bunches 22

R.m.s. bunch length, m 0.6β-function in IP, m 0.35

Betatron frequinces, Qx/Qy 9.44/9.44Chromaticities, Q’x/Q’y -33/-28

Acceptance of the ring, π mm·mrad 40Momentum acceptance, Δp/p ±0.010

Critical energy factor , γtr 7.088Energy of 79Au, GeV/u 1.0 3.0 4.5

Number of ions per bunch 2.0·108 2.4·109 2.3·109

R.m.s. momentum spread, Δp/p 0.55·10-3 1.15·10-3 1.5·10-3

R.m.s. emittance, π mm·mrad 1.1/0.95 1.1/0.85 1.1/0.75Luminosity, cm-2 s-1 0.6·1025 1.0·1027 1.0·1027

IBS growth time, s 160 460 1800

NICA passport parameters for Au-Au

5

6

Total: Mean luminosity maintains

1. Effective scheme of the beam storage and bunching (the storage time is less than 10 minutes)

2. Vacuum life-time is about ~ 10 hours

3. Suppression of instabilities by feed-back systems

4. Suppression of IBS by beam cooling:

1 – 3 (4.5) GeV/u – electron cooling

3 – 4.5 GeV/u – 3D stochastic cooling, Palmer method for longitudinal cooling

due to large momentum spread

7

Total: Beam storage and bunch formation Structure of the RF system

1. Storage of a coasting beam occupied half of the ring circumferenceusing barrier-bucket system (RF1, 5 кV, 1 resonator per ring)with cooling (longitudinal):Simulated efficiency up to ~ 95%About 120 injections (55 into each ring) repetition period is 5 secondsStorage time is 10 minutesMinimum momentum spread of the stored beam is limited by microwave longitudinal instability

2. Formation of 22 bunches at the length of 1.2 m (RF2, 100 kV, 4 resonators per ring)with longitudinal cooling

3. Increase of harmonics to 66, bunch compression down to 0.6 m (RF3, 1 МV, 8 resonators per ring) with cooling

8

NICA start-up configuration

At the MAC meeting in October 17-18, 2013 the staging of the NICA collider commissioning was discussed.15 January 2014 NICA coordination committee approved the staging of the NICA collider and MPD commissioning.

The goals of the first stage (2019-202x):

-Investigation and optimization of the beam dynamics: injection, storage scheme, bunch formation optimum working point

- MPD commissioning at peak luminosity of 51025 cm-2s-1

MPD inner tracker is excluded from the start-up configuration,the maximum acceptable bunch length is 1.2 m instead of 0.6 m

9

Energy range 3 – 4.5 GeV/u

NICA start-up configuration

1. Most interesting for physics

2. Does not covered by CBM at SIS-100

3. Achievable at Nuclotron

4. Better for luminosity

One can use stochastic cooling only (tested at Nuclotron)

10

Electron cooling system can be postponed

The bunch intensity is less – one can work without feed-back systems

The bunch length of 1.2 m has to be achieved with RF2,RF3 is not necessary at this stageRF2 Voltage can be reduced (50 kV) because of smaller beam intensity(it is possible to cool down to less momentum spread)

In comparison with total configuration:

Total set Start-up

Barrier bucket (RF1) 2 2

RF2 8 4

RF3 16 0

RF system configuration

11

Start-up: Beam storage and bunch formationThe scheme is the same:

2. The bunching using RF2 leasd to formation of 22 bunching at the rms length 1.2 мThe final momentum spread ~ 510-4 (three times less than in the total version)

1. The longitudinal cooling only id necessary for the storage – Expected transverse emittance from Nuclotron is 0.10.3 mmmrad Filter method can be used (requirements are simple, tested at the Nuclotron)

The rms momentum spread corresponding to 1.2 m of the bunch length at RF voltage of 50 kV, harmonics number = 22 as function of the beam energy.

12

Start-up: Cooling strategy during collisions

«Temperature» of longitudinal degree of freedom is much less than transverse

IBS leads to two effects:- Energy transfer from transverse degree of freedom to longitudinal one(relaxation)- Slow increase of 6D phase volume.

Longitudinal heating rate is sufficiently larger than transverseAt the equal emittances: – the horizontal increases,- the vertical decreases.

At working point Qh Qv

One can use the coupling between planes

For given momentum spread one can find the emittance at whichthe horizontal heating is compensated by vertical cooling (sympathetic)

13

Start-up: Stabilization of transverse emittance“Equilibrium” emittace as function of energy.

At energy larger than 4 GeV/u the “equilibrium” emittace is larger than acceptancelimit. However at maximum acceptable emittance of 1.1 mmmrad the transverse heating time is about 15 hours

14

Start-up: Requirements for longitudinal cooling

Particle number per bunch atLuminosity of51025 cm-2s-1

Heating time for longitudinal degreeof freedom (the transverse emittanceis stable):The cooling time has to be shorter

15

Comparison with total version

Energy, GeV/u

Req

uire

d co

olin

g tim

e, s

ec

Stochastic cooling time is scaled as peak current, decrease of the particle numberand increase of the bunch length simplify requirements for stochastic cooling

SUs

s

SUSUeqSU N

N

,_

To compare one can recalculate

It is possible to reach the Luminosity of 51025 cm-2s-1

in start-up configurationusing the same longitudinal cooling,however optimum gain is larger

16

Start-up versus total

Total Start-up

Energy range, GeV/u 1 – 4.5 3 – 4.5

Number of bunches 22 22

Rms bunch length, m 0.6 1.2

Maximum ion number per bunch 2.4·109 7·108

Beam storage time, min 10 3.4

R.m.s. emittance, π mm·mrad 1.1 0.5 – 1.1

IBS growth time, s 160 – 1800 37 – 470

Luminosity, cm-2 s-1 1.0·1027 (over 3 GeV/u)

5.0·1025

Bunch parameters for Au-Au collisions

17

Start-up: technical reserve

1

WNeq

(1 1/M pk2 )2

Mkp

Well known formula:

Neq NC

2 s

M pk 1

2( fmax fmin )pkTpkpp

Mkp 1

2( fmax fmin )kpTkppp

At given phase volume of the bunch the maximum luminosityis determined by the equality

coolIBS 11

Analytical estimation

18

Particle number correspondingto the equilibrium (analytical formula for the cooling)

Equilibrium between heating and cooling

Luminosity in 1026 cm-2s-1 (from four times to one order of magnitude larger,than necessary for start-up)

Characteristic cooling/heating times

20 – 140 s

More accurate estimations for the stochastic cooling - N.Shurkhno

19

Start-up: Luminosity for different ion speciesThe IBS heating rate is scaled as Z2/A

The momentum spread (at fixed bunch length and RF Voltage) ~ sqrt(A/Z)

«Equilibrium» emittance approximately proportional to the momentum spread

At the energy of 3.7 GeV/u (optimum for the stochastic cooling)

p, 10-4 , mmmrad

Nb L, cm-2s-1

197Au79+ 4.14 0.805 1.49109 3.051026

124Xe42+ 3.8 0.678 2.53109 8.91026

84Kr36+ 4.28 0.86 3.31109 1.521027

40Ar18+ 4.39 0.92 6.75109 5.531027

AZN

IBS

2

~1 Nsc

1~1 2

2 ~~ZANL

20

Start-up: General problems

1. Smaller momentum spread – less Landau dumping:microwave longitudinal instability

2. Absence of a feed-back system:- Weak head-tail instability-Transverse and longitudinal Multi-bunch instabilities

Influence of the fringe fields of quadrupole lenses:- restrictions of the working point choice(this problem exists for the total version also)

Collective effects for the total version were analyzed by P.Zenkevich, A. Bolshakov(MAC October 2013)

At start-up configuration:

21

Threshold particle number in accordance with Keil-Shnell criterion for microwave longitudinal instability as function of the beam energy.

Start-up: Microwave longitudinal instability

Particle number corresponding to51025 cm-2s-1

Reserve is about 6 times

22

-To avoid weak head-tail instability the collider will be operated at small

negative chromaticity, the dipole mode will be stable

the high order modes will be suppressed by use of octupole families.

Transverse and longitudinal Multi-bunch instabilities:

Transverse is driven by resistive wall impedance, the peak current

for start-up configuration is about 10 times less, the bunch is longer

Longitudinal is driven by resonant elements: BB cavities are shortened

during collisions, technical requirements for RF2 include

impedance of high frequency HOM

are to be investigated experimentally to concretize requirements for the

feed-back systems

Start-up: Other instabilities

23

Fringe fields of quadrupole lenses

* in cm

Preliminary analysis (P.Zenkevich, A.Bolshakov) showed decrease of DAin chosen working point

- Appropriate design of the final focus lenses

- increase of the beta function in collision point

2

*

2

**

1

)exp(1~)(

su

duuL

(Half of the effect – final focus lenses)

Increase of the * by 2 timesleads to the luminosity decreaseby 20% only

1

0.6

L(*

)/L(3

5)

24

coolIBS 11

IBS

IBS

NC

1

IBS

cool

CCNL ~~ 2

NCsc

sc

1

At equilibrium

Luminosity increase at total set of equipment

1. The energy range will be increased due to electron cooling application

2. The luminosity at energy range from 3 to 4.5 GeV/u

upstartIBS

IBS

upstart CC

LL

_

Increase from 58 (3 GeV/u) to 13 (4.5 GeV/u) times

25

Conclusions

- The luminosity required for the first stage of the collider operation can be achieved with sufficient technical reserve

- Requirements for stochastic cooling are simple than in total version

- For ions at intermediate mass (like Ar) one can expect the luminosity up to 1027 сm-2s-1

-The maximum achievable luminosity can be limited by MCW longitudinalor multi bunch instabilities

-To avoid decrease of DA optimization of the final focus lenses and * is necessary

26

Thank your for attention