NICA start-up scenario + questions of instabilities A.Sidorin For 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 ions Questions of instabilities 2 Contents

Staging of the NICA project 1.Fixed target experiment 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 collisions The rings are operated at different rigidity (asymmetry collisions) 4. Upgrade of the collider injection kickers, installation of Siberian snakes, spin control and spin diagnostic devices, collisions of polarized beams Goal of this report Comparison 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  mm  mrad (beam rms radius = 1/6 aperture) 3. Ratio between horizontal, vertical emittance and momentum spread is chosen from equality of the IBS heating rates (thermal equilibrium – minimum heating rate) 4. Particle number in bunch is limited by tune shift  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 Number of bunches22 R.m.s. bunch length, m0.6 β-function in IP, m0.35 Betatron frequinces, Q x /Q y 9.44/9.44 Chromaticities, Q’ x /Q’ y -33/-28 Acceptance of the ring, π mm·mrad 40 Momentum acceptance, Δp/p±0.010 Critical energy factor, γtr7.088 Energy of 79 Au, GeV/u Number of ions per bunch2.0· · ·10 9 R.m.s. momentum spread, Δp/p0.55· · ·10 -3 R.m.s. emittance, π mm·mrad1.1/ / /0.75 Luminosity, cm -2 s · ·10 27 IBS growth time, s 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 circumference using 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 seconds Storage time is 10 minutes Minimum 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 ( x): -Investigation and optimization of the beam dynamics: injection, storage scheme, bunch formation optimum working point - MPD commissioning at peak luminosity of 5  cm -2 s -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 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 stage RF2 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 setStart-up Barrier bucket (RF1)22 RF284 RF3160 RF system configuration

11 Start-up: Beam storage and bunch formation The scheme is the same: 2. The bunching using RF2 leasd to formation of 22 bunching at the rms length  1.2 м The final momentum spread ~ 5  (three times less than in the total version) 1. The longitudinal cooling only id necessary for the storage – Expected transverse emittance from Nuclotron is 0.1  0.3  mm  mrad 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 transverse At the equal emittances: – the horizontal increases, - the vertical decreases. At working point Q h  Q v One can use the coupling between planes For given momentum spread one can find the emittance at which the 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 acceptance limit. However at maximum acceptable emittance of 1.1  mm  mrad the transverse heating time is about 15 hours

14 Start-up: Requirements for longitudinal cooling Particle number per bunch at Luminosity of 5  cm -2 s -1 Heating time for longitudinal degree of freedom (the transverse emittance is stable): The cooling time has to be shorter

15 Comparison with total version Energy, GeV/u Required cooling time, sec Stochastic cooling time is scaled as peak current, decrease of the particle number and increase of the bunch length simplify requirements for stochastic cooling To compare one can recalculate It is possible to reach the Luminosity of 5  cm -2 s -1 in start-up configuration using the same longitudinal cooling, however optimum gain is larger

16 Start-up versus total TotalStart-up Energy range, GeV/u1 – 4.53 – 4.5 Number of bunches22 Rms bunch length, m Maximum ion number per bunch2.4·10 9 7·10 8 Beam storage time, min103.4 R.m.s. emittance, π mm·mrad – 1.1 IBS growth time, s160 – – 470 Luminosity, cm -2 s ·10 27 (over 3 GeV/u) 5.0·10 25 Bunch parameters for Au-Au collisions

17 Start-up: technical reserve Well known formula: At given phase volume of the bunch the maximum luminosity is determined by the equality Analytical estimation

18 Particle number corresponding to the equilibrium (analytical formula for the cooling) Equilibrium between heating and cooling Luminosity in cm -2 s -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 species The IBS heating rate is scaled as Z 2 /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, ,  mm  mrad NbNb L, cm -2 s Au   Xe   Kr   Ar   10 27

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 to 5  cm -2 s -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 DA in chosen working point - Appropriate design of the final focus lenses - increase of the beta function in collision point (Half of the effect – final focus lenses) Increase of the  * by 2 times leads to the luminosity decrease by 20% only L(  *)/L(35)

24 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 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 сm -2 s -1 -The maximum achievable luminosity can be limited by MCW longitudinal or multi bunch instabilities -To avoid decrease of DA optimization of the final focus lenses and  * is necessary

26 Thank your for attention