EU contract number RII3-CT-2003-506395 CARE-Conf-2008-034-HHH

HIGH ENERGYHIGH INTENSITYHADRON BEAMS

Summary of the CARE-HHH-APD WORKING MEETING on

« LHC BEAM-BEAM EFFECTS AND BEAM-BEAM COMPENSATION»

August 28, 2008 – CERN

J.-P. Koutchouk, G. Sterbini, F. Zimmermann

A one-day CARE-HHH-APD working meeting on LHC Beam-Beam Compensation & Beam-Beam Effects for the LHC Upgrade" was held at CERN on 28 August. The goals of this meeting were to develop an implementation plan for wire compensation at the LHC, and to study beam-beam effects in the LHC luminosity upgrade perspective, including the large Piwinski angle regime, the impact of a few encounters at reduced distance, and options for a crab-waist scheme in the LHC. The meeting had about 20 participants, about half of which came from CERN, 1 from KEK, and 8 from US-LARP (BNL, FNAL and SLAC), including 5 or 6 connecting and presenting via WebEx. All presentations can be found in INDICO at http://indico.cern.ch/conferenceDisplay.py?confId=38353. The meeting was structured in 3 sessions, on wire compensation, simulations & concepts for upgrade scenarios, and experiments on beam-beam effects, respectively, each terminating in 15 minutes of discussions. This report contains the session summaries and conclusions from the three session chairs.

We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 "Structuring the European Research Area" programme (CARE, contract number RII3-CT-

2003-506395)

Geneva 2008

SUMMARY OF SESSION 1LHC WIRE COMPENSATION

Frank Zimmermann, CERN, Geneva Switzerland

TOPICS AND CONTRIBUTIONSThe opening session was devoted to wire compensation. Jean-Pierre Koutchouk reviewed the

LHC wire compensation scheme, Amalia Ballarino presented first ideas on an LHC wire implementation using HTS, and Ulrich Dorda discussed a possible RF wire system.

MOTIVATION, PRINCIPLE, EFFICIENCY (JEAN-PIERRE KOUTCHOUK)In his opening talk, Jean-Pierre covered the motivation for wire compensation in the nominal

LHC, the motivation for wire compensation at the LHC upgrade(s), the basic principle of LHC wire compensation, the compensation efficiency in simulations, experiments, and operations, and conclusions.

He recalled that the nominal LHC machine performance is limited by the long-range beam-beam effect, with constraints on the crossing planes, that the crossing angle already once had to be increased from 200 to 300 μrad (due to a revised relation between tune footprint and dynamic aperture) and that its margin seems small, as simulations show chaotic particle trajectories at 4-6σ due to the long-range collisions with possible implications for beam lifetime, detector background and collimation. For the nominal LHC, the wire compensator promises a (modest) recovery of some of the ~15% geometric luminosity loss from the crossing angle, plus it offers the possibility of investigating the potential severity of the long-range beam-beam effect ahead of time, well before the nominal beam current is reached. For the LHC upgrades, the crossing angle has to be significantly increased with an associated large loss of geometric luminosity, due to the reduction of β*, due to the increased bunch current and due to the larger number of long-range collisions for a possibly increased interaction length (longer quadrupoles). The wire compensation has the potential to minimize the required increase in crossing angle (and required quadrupole aperture) and the corresponding rapid loss in luminosity, and to compensate all long-range collisions at large enough separation, leaving more freedom to keep a few at reduced separation (i.e. in the “D0” scheme which was discussed in a later presentation by Guido Sterbini).

The wire compensation was first proposed in 2000. The idea was to install a wire on either side of IP1 and IP5. In the case of H crossings the wires would be located between the two beams – a probable complication, while for V crossings the wires would be mounted above or below the beams. A wire location has been reserved 104.931 m from the head-on collision point on either side of IP1 and 5, in EDMS503722 (2004).

Concerning compensation efficiency, several generations of mostly weak-strong, but also some strong-strong simulations were performed since 2000, by Jean-Pierre Koutchouk, Frank Zimmermann, Jicong Shi, Lihui Jin, Werner Herr (strong-strong), and Ulrich Dorda, yielding consistent conclusions: The compensation is very “efficient” (footprint, dynamic aperture); it is robust (need not be exact); the noise level shall be under control (FZ: <0.1%; J. Shi: <0.5%; the Tevatron electron lens achieves < 1% in practice, with much less current but also smaller distance). An example footprint with compensation was presented. Another simulation from Ulrich Dorda showed that even though a pulsed wire compensation should be the final goal, a first

generation of simple dc wire compensation is worth considering. Beam experiments with wires have been performed in the SPS for several years. Compensation was studied with two wire units about 2.6 degrees apart in betatron phase advance. The lifetime that is greatly reduced by a single wire was recovered by a second wire except for vertical tunes below 0.285. The reason for the degraded compensation efficiency in certain tune regions is not entirely clear.

Other wire experiments were conducted at the SPS and also at RHIC. Jean-Pierre stressed that RHIC has a high potential to advance the understanding of (simulated) long-range beam-beam encounters, given its long beam lifetime, observation time and precision instrumentation, so that more RHIC MD time would be extremely useful.

Concerning the use of wire in actual operation, the example of DAFNE was quoted, where a wire was installed to compensate the long-range beam-beam interaction.(C. Milardi et al, 2008; also see the proceedings of the HHH-IR’07 workshop): “The long-range beam-beam interactions (parasitic crossings) were one of the main luminosity performance limitations for the lepton Φ-factory DAΦNE in its original configuration. In particular, the parasitic crossings led to a substantial lifetime reduction of both beams in collision. The wires installed in the DAΦNE IRs proved to be effective in reducing the impact of BBLR interactions and improving the lifetime of the positron beam especially during the KLOE run.”

Jean-Pierre drew the following conclusions: (1) By beam-beam standards, the efficiency of the compensation of the long-range beam-beam effect appears well established. (2) A wire has given a concrete improvement in DAFNE. (3) The compensation has a potential merit both for the nominal and for the upgraded LHC. In addition, it would allow early and efficient studies of one of the most difficult and limiting phenomenon in the LHC. (4) The implementation of a first dc solution should be relatively simple and of limited cost. (5) It appears timely to consider an implementation plan. Jean-Pierre pointed out that the BI group was ready to support the LHC wire implementation. The approximate time line for the implementation is 2 years. Implementation options needed to be decided. A movable tank could interfere with collimation, as the wire must remain in the shadow of the local protection of the triplet. If the wire motion is restricted, the control of the beam-wire distance is an issue. Local orbit bumps are an option. It remained to be verified that these do not induce intolerable coupling or higher-order effects.

In conclusion, long-range collisions are expected to severely limit the performance of the nominal LHC; their effect gets worse in all upgrade scenarios; wire compensation can relax the constraints and open up new parameter regions; simulation studies, beam experiments, and the successful use of wires at DAFNE all support the concept; and an implementation plan for an LHC dc wire system should be developed with a time line of 2 years.

HTS WIRE (AMALIA BALLARINO)Amalia discussed a possible wire implementation using high-temperature superconductor

(HTS). She first summarized the requirements: the HTS wire shall be able to transport ∼80 A m*, in dc mode, at a “convenient” temperature; the HTS wire will be integrated between D2 and D1, on both sides of IP1 and IP5, where at the moment warm beam pipes are used. The system shall provide the cryogenic conditions required for the operation of the wire in a compact and cost-effective set-up; and, finally, the design of the HTS wire and the associated electrical devices (current leads) shall ensure optimized cryogenic and electrical performances and reliable operation.

After comparing properties of BSCCO2223 tape, YBCO123 tape, BSCI2212 wire and MgB2

wire, the latter - MgB2 wire – was identified as preferred conductor thanks to its superior mechanical properties. At CERN a critical current of 800 A was measured at 1.5 T and 24 K for

an MgB2 wire with 1.1 mm diameter, and with a bending radius equal to 100 times the radius of the wire. This is more than sufficient for the LHC application.

A cryostat around a cold beam pipe is required to house the HTS wire. The total length of the apparatus is about 4 m (Fig. 1). A two-stage compact cryocooler from Cryomech (with radii of 150/ and 300 mm for the two stages) is available on the market, which could do the job. The wire would be at a temperature of 10-20 K. Amalia presented a schematic stainless-steel support structure for the wire proper, including connections. The outer wire diameter is about 2 mm. The following details were discussed: The current leads and the HTS wire are in the technical vacuum insulation of the cryostat. The HTS wire is electrically insulated and fitted inside the pre-shaped stainless steel pipe. The unit is positioned inside the beam pipe, welded and electrically connected to the cold end of the leads. The separation between the technical vacuum of the cryostat and the beam pipe is assured by welds. The heat sinks for the current leads are respectively on the thermal screen (T∼ 80 K, 3.6 W/lead at 80 A), and on the outer wall of the beam pipe (T∼ 20 K, 1 W/lead at 80 A). The current leads have a design similar to that of the leads powering the LHC dipole corrector circuits. A lot of experience is available at CERN with the required current leads: The LHC 60-A and 120A dipole correctors make use of about 2000 conduction-cooled current leads.

Figure 1: Possible cryostat layout for an HTS wire compensator in LHC from A. Ballarino.

An interesting alternative option worth exploring is to use a cold copper wire: the resistivity of copper (RRR=120) at 20 K is ∼ 1.3x10-10 Ω m – about 100 times lower than at room temperature. A copper wire (RRR=120) of about 0.9 mm2 cross section (Φ∼1 mm) would dissipate about 1 W when transporting 80 A in dc mode at 20 K (the cryostat, the integration and assembly procedures, and the size of the stainless steel tube would not change compared with the HTS wire).

Two open points are heat losses from the beam, and radiation hardness: The proposed MgB2

conductor would operate with a temperature margin of about 10 K. The radiation resistance properties of MgB2 seem to be good – similar to Nb3Sn. Neutron done irradiation tests were performed at very high fluence (up to 3.9·1019 n·cm-2, at INFN and University of Genova). Up to a fluence of 1·1018 n·cm-2 no degradation of Tc was observed. Amalia concluded that the use and implementation of an HTS wire for the correction of the long range beam-beam effect in LHC is feasible, that the preferred HTS superconductor is MgB2 in the form of wire, but that a cold Cu wire can also still be considered. The electrical and mechanical properties of MgB2 largely satisfy the requirements for this specific application. A unit length of 100 meters of this wire will be delivered to CERN in October 2008 for a different application. Detailed drawings and some additional calculations should be done to finalize the present proposal.

In conclusion, both MgB2 HTS wire and cold Cu wire are viable candidates for the LHC compensation system; a possible cryostat layout was proposed.

RF WIRE (ULRICH DORDA)Ulrich Dorda discussed a possible rf-wire implementation. He first recalled the best performance

that can be obtained with a dc wire (“BBLR”). PACMAN bunches experience less long-range encounters than the nominal bunches. By choosing an intermediate dc current of a wire, it is possible to improve the dynamic aperture of the nominal bunches by more than 1, to a value comparable to that for the PACMAN bunches, while not degrading the dynamic aperture of the latter. To perfectly compensate both the nominal and PACMAN bunches, one must vary the wire current from bunch to bunch in a pre-defined way that follows from the bunch pattern. The field should increase from 0 for the first bunch to a maximum value for the first bunch after a gap to the maximum value over 14 bunches or 350 ns. The rise is followed by a microsecond-long period of constant excitation, and afterwards by a downward ramp. The maximum wire current is of order 100 A. The challenge is to repeat the required pattern very reproducibly, train after train (with some variation between trains due to the non-perfectly symmetric filling pattern) and especially turn after turn.

Originally it had been thought to implement a linear ramp using fast switches. Such scheme however had a number of disadvantages:

Extreme timing precision requirements; Need for huge relative bandwidth over several decades; Need for long cables to avoid placing electronics closer to the beam;

o Skin effect in the long cable;o Cable dispersion between DC and several kHz (characteristic impedance is

complex and strongly frequency-dependent); Generator impedance is also complex and strongly frequency dependent (and possibly

time dependent due to switching elements in the generator); this leads to multiple reflections which are difficult to control over a wide frequency range between the pulsed “quasi-DC”-BBLR and the generator;

Non-existence of passive circulators in the DC to MHz frequency range which could take the high power load;

Field shielding issue; Dependence of power requirements on the termination.

Figure 2: Ideal field evolution in an RF-BBLR (blue curve) and possible bunch positions (red circles) [left] and schematic RF-BBLR layout [right] after U. Dorda.

An RF BBLR (see Fig. 2) avoids all these problems. In the resonating RF-BBLR the passing bunch experiences a counter-propagating electromagnetic wave. With a strip-line structure of optimum length /4, the bunch integrates over /2 of the pulse. Advantages of the RF-BBLR are:

It is a resonator:o Well-known technology;o Low noise due to timing stability;o Resonator gain depends on coupling (partially cancelled by transit-time effect);

Both the electric and magnetic field of the wave contribute to the force acting on the bunch; the field shape is of the form 1/r like for the long-range encounters;

Feedback possible (e.g. 3-turn delay); Can be mismatched at BBLR and matched at power generator; Circulator is a complex device but seems feasible; Reduced relative bandwidth (approximately 1 octave); thus better control of cable

dispersion and matching. The coupling strength defines the resonator gain. There is a trade-off between power needed during ramp and power on the flat top. The phase slip during the ramp must be compensated for as for any other, accelerating rf system.

The soundness of the RF-BBLR concept was demonstrated by a low-budget proof-of-principle experiment in the laboratory, yielding a clean RF-BBLR ramp.

The critical phase noise can be measured with the help of a one-turn delay, which allows detecting exactly the noise experienced by a bunch. The delay is accomplished by quartz filters or via surface acoustic waves (electro-optical converters to noisy, lumped RC circuits would be too complicated).

The field produced by the RF-BBLR is a combination of the field excited by the generator and the field induced by the beam. The beam-induced signal has been measured in the SPS for one of the existing (non-RF) wire installations. The frequency Fourier spectrum shows a number of broad peaks, some of which correspond to resonances of the wire, and others to ones of the connecting cables.

Other notes – longitudinal effects might be an issue (the longitudinal kicks at the two ends may differ and not completely cancel each other). Alternatively, one could use a radiation-hard rf generator with vacuum tube in the tunnel, which would imply short cables and avoid the need for a circulator. The timing noise requires further studies, taking into account transit-time effects and longitudinal effects.

Summarizing, PACMAN effects in the LHC could be compensated through an RF wire. A conceptional design, advantages and open questions were reviewed.

SUMMARY OF SESSION 2SIMULATIONS & CONCEPTS FOR UPGRADE SCENARIOS

Jean-Pierre Koutchouk, CERN, Geneva Switzerland

CONTRIBUTIONS A simplified 2D model for tracking the BB effect in the D0 scheme by Guido Sterbini,

CERN & EPFL Large Piwinski angle and crab-waist scheme for the LHC by Kazuhito Ohmi, KEK

A SIMPLIFIED 2D MODEL FOR TRACKING THE BB EFFECT IN THE D0 SCHEME The early separation scheme is in essence a very simple dipole scheme. It allows a

significant reduction of the geometrical luminosity loss factor by reducing the crossing angle at the IP and offers luminosity levelling capabilities. The necessary dipoles can however not be placed in their ideal positions where they would interfere in an unacceptable way with the inner experimental detectors. In practice, a few long-range encounters (4, 8 or 12 for the latest ATLAS proposal) take place at a reduced beam separation. A minimum separation of 5 sigma has been conjectured. Experimental results and simulations have not given so far a clear answer as to whether this reduced separation is acceptable. The author shows:- The 9.5 sig beam separation in the nominal LHC is only an average. Actually, the

nominal LHC includes 11 encounters at 7 sig or less (Fig. 3). Simulations by several authors show it is acceptable.

- The Sp pS collider ran for years with 7 encounters at 6 sig and one at 3.5 sig at the ultimate LHC bunch charge.

Figure 3: Distribution of normalized bema-beam distances for the nominal LHC.

Hence dramatic effects of one or a few encounters at 7 sig predicted by some simulations do not seem to hold.

Given the large parameter space of this upgrade scenario, a tracking campaign using the simplest possible model of the dynamics (2D with tune modulation and tune averaging) has been launched. To partly overcome its simplifications, its results are calibrated with the most established experimental results of Sp pS operations. The results show qualitative agreement with former LHC tracking results and show indeed that the SppbarS scenario is acceptable. For the LHC upgrade, the simulations show that 8 encounters at 5 sig appear acceptable if the beam separation at all other encounters is increased from 9.5 sig to 13 sig. The limitations of 2D tracking are underlined during the discussion. It had been agreed with W. Herr and D. Kalchev to investigate the most significant findings with full 6D tracking. This has been under way and the results will become available later.

LARGE PIWINSKI ANGLE AND CRAB-WAIST SCHEME FOR THE LHC

Crab waistA crab waist scheme requires two sextupoles on either side of the IP. For proper phase

shifts between IP and sextupoles and a proper sextupole strength depending on the collision angle, the particles with various x positions collide with the other beam always at a beta waist. This complex optics becomes clear when, as shown in this presentation, the beam cross-section in the physical space is displayed versus the longitudinal coordinate.

For Super B, the luminosity scan versus tunes show a much more regular behaviour with constant high luminosity over large tune ranges when the crab waist scheme is switched on.

Taylor-map analysis shows the variation of the resonance excitation terms versus the crossing angle. The third order (12) term as well as the fourth order terms are very sensitive, with an optimum giving the optimal sextupole strength.

For the same beta function as in the arcs, the crab waist sextupoles would require a strength equivalent to 5 lattice sextupoles in the LHC, i.e. relatively weak. A significant improvement in luminosity (factor 2.6) however requires the transverse beam size normalized by the Piwinski angle to be small compared to the bunch length, requiring typically a beta function less than 10 cm.

Another use of waist control, this time for long-range beam beam interactions, is explored but rejected.

While the tracking shows no side effects of the crab waist scheme on the LHC intensity for nominal intensity, a possible effect on halo formation and lifetime is presently out of reach of simulations. The effect of the higher intensity of the LPA scheme is under investigation.

LPA and crab cavitiesFor the expected LHC situation where the beta function is larger than the bunch length, a

combination of LPA and crab crossing appears promising. For “LPA II” (25 ns scheme with increased bunch current, i.e. the parameters of the early separation scheme scenario), a significant luminosity improvement is shown as a function of the crab voltage.

.SUMMARY OF SESSION 3

EXPERIMENTS ON BEAM-BEAM EFFECT

G. Sterbini, CERN, Geneva, and EPFL, Lausanne

CONTRIBUTIONSIn the third session of the workshop, experiments on beam-beam were discussed with particular emphasis on the results for hadron colliders and their numerical simulation. In the LHC upgrade perspective the simulations' results of weak-strong and strong-strong Large Piwinski Angle Scheme (LPA), Early Separation Scheme (ES) and Crab Cavities (CC) were discussed by K. Ohmi (KEK). K. Cornelis (CERN), W. Fischer (BNL) and A. Valishev (FNAL) reported experiences with operation in S p pS, RHIC and Tevatron. Finally the U.S. LHC Accelerator Research Program (LARP) beam-beam studies and related simulation works were presented by T. Sen (FNAL).

BEAM--BEAM EFFECT FOR COLLISION WITH LARGE PIWINSKI ANGLE SCHEME AND HIGH FREQUENCY CRAB CAVITY IN LHC (KAZUHITO OHMI)

In this presentation tracking results of weak-strong and strong-strong simulations on the LPA and the crab cavities scheme were illustrated.

Large Piwinski angle - In the simulation 15 slices were considered for each bunch. The calculation time linearly increases with the number of slices in the weak-strong (WS) simulation, while it goes with the square of the number of slices in the strong-strong (SS) case. For the WS simulations the tracking lasts 4x105 turns, while for the SS case it lasts 5000 (for 15 slices). For the LPA two scenarios were considered: 50-ns bunch separation, LPA1, and 25-ns bunch separation, LPA2. In LPA1, for 4.9x1011 ppb population, there was no evident problem emerging from the simulations (WS and SS plus long range beam-beam): while with 6x1011 ppb a clear emittance growth was visible. In LPA2 the long range encounters were not included in the simulations, and the head-on contribution alone seemed not to have any significant impact on the luminosity lifetime. Studies on the high frequency transversal position noise were performed: the tolerance is within the nominal LHC one, which means 0.1%x.

Crab cavities - For the crab cavities scheme two choices of the cavity frequency (800 MHz or 400 MHz) were analyzed. In those conditions the voltage sinusoidal shape may not be negligible so beams distribute with snake shape. From the weak-strong simulation no problem emerged: the studies included the nominal LHC with one or two crab cavities and the early separation scheme with one or two cavities. Some plots on the relation between the crab kick angle and the luminosity were showed. In the strong-strong simulations 30000 turns were considered (10 slices per bunch): even if in this case the overall effect is stronger than in the weak-strong case, the luminosity lifetime is still acceptable. From the studies on the high frequency transversal position noise, the tolerance is more severe than in LPA: considering the higher beam-beam parameter it is reasonable.

The final remarks were: no problem was found in both LPA and crab cavity schemes even with high crab

cavity frequency (800 MHz) only geometric effects are seen in these simulations for the design bunch

population tolerances for fast noise are in a range similar to that of nominal LHC (~0.1%

x).

Sp pS EXPERIENCE WITH REDUCED DISTANCE ENCOUNTERS (KAREL CORNELIS)

The Sp pS operated in daily routine with a pretzel scheme: the beam experienced beam-beam interactions (Fig. 4).

The machine had an injection energy of 26 GeV and a coast energy of 315 GeV. In the SPS ring there were 6 proton bunches (1.7x1011 ppb) and 6 antiproton bunches (0.8x1011 ppb). The emittances ranged between 15 to 20 mm mrad. The total tune shift was in the range 0.015 - 0.020 with a chromaticity (Q/Q)/(p/p)=0.005.

Figure 4: The half-separation (in ) in the Sp pS at injection and in collision.

At the injection the mean distance of the beams was 3. At coast there were 3 encounters HOs, one at 3.5, 7 at 6 and one at 9. Additional sextupoles were installed on the separated orbits in order to tweak the tunes of p and p differently. In the presentation two tune scans were discussed: with and without scrapers (one p bunch and one p bunch: two HOs) and with full and half separation (one p bunch and two p bunches: two HOs and two LRs). In the first scan (in particular with scraping) the tune dependence is weak (less than a factor two of difference), while in the second scan the tune dependence is stronger (more than a factor 6 of difference).The tunes at injection and extraction were shown: the tune footprint at 26 GeV is given by a combination of beam-beam and space charge tune shifts. Moreover, the space charge is modulated with the longitudinal motion. The background in the experiments goes up reducing the separation (“small separation is not good”) and for the beam-beam effect, in addition to the tunes, the chromaticity does play a role in the beam lifetime. The general recommendation during the conclusions were:

beam separation reduces the tune spread, but it also creates new resonances; separation should be big enough (6 proved sufficient in the SPS); and avoid noise and shocks.

AN OVERVIEW OF RHIC BEAM--BEAM EXPERIMENTS (WOLFRAM FISCHER)At RHIC, in the present configuration there are no parasitic encounters, no crossing angle

and three head-on collisions. The following data and discussion concern the proton runs. The beam lifetime was dominated by two different time constants (1=0.35 hours and 2=50 hours in the 2008 run); beam losses are dominated by beam-beam in conjunction with other effects: initial beam lifetime from non-beam-beam effects is ~700 hours (calculated, including residual gas interactions and IBS). For the luminosity lifetime the conclusion is similar: including beam-beam effects it goes down from 40 hours to 12 hours (this is true

both for the 2006 and for the 2008 run). The goal for the luminosity upgrade (after 2011) is to reach the average luminosity of 60x1030 cm-2 s-1 reducing the * from 1 to 0.7 m and increasing the bunch population from 1.5 to 2x1011. In the meantime, to optimize the luminosity lifetime, several working points were tested: the near integer tunes are very promising in view of the large space between limiting resonance, but their practical implementation is still very problematic due to the 10 Hz oscillations of the triplets (with consequent significant excursion of the closed orbit (±5 mm)).

In the framework of the RHIC upgrade studies and in that of the LARP collaboration, two wires were installed in RHIC (one per ring, 125 A-m of integrated field per wire). Beam-beam experiments with and without the wire were done at RHIC starting in 2005 (long-range with p-beam at injection), in 2006 (long-range with p-beam at store), in 2007 (long-range with Au-beam and wire at store) and in 2008 (only parasitic test d-beam and wire). For the 2009 run, 3 times 3-hours of MD time has been requested. Measurements are beam lifetime observations with variations in separation, strength (wire), and other parameters (tune, chromaticity). The experiments in 2007 (Au-ions) were heavily studied by different tracking codes (by H.J. Kim, T. Sen, U. Dorda, A. Kabel,…): in the simulations the diffusive aperture threshold is within 1 to the observed one. Onset of losses from long-range interaction seen in experiments are [1]

at 4 for a single beam-LR interaction, and between 5-9 with wire (strong dependence on working point and

chromaticity).For the 2009 the plan is to use the wire with head on collision at the mirrored LHC working point, to make background studies and to try to compensate with the wire one long range interaction.

Other beam-beam experiments which are considered or which were already performed in the previous years at RHIC include:

coherent -mode generation and suppression; resonance driving terms with beam-beam (planned with AC dipole, not yet carried

out); lifetime and background as function of (Qx, Qy); tunes scans at injection and new working points (R. Tomas); near-integer working point (C. Montag); BTF measurements and simulation (T. Pieloni).

An extensive tracking campaign is ongoing for predicting the electron lens benefits at RHIC (expertise from FNAL, BNL, SBNL and SLAC are collaborating). The preliminary conclusion from these studies is that established methods used to evaluate magnet errors fail for electron lenses (all particles are chaotic and DA is not a good measure for BB since BB force becomes small at large amplitudes). Moreover, none of the short-term evaluations gives a reliable answer for long-term behaviour (tune footprints, tune diffusion maps and Lyapunov exponent maps). Simulations generally show improvements in particle behaviour below 3 and deterioration above 4. Emittance growth is too noisy in simulations and not useful as a figure of merit: electron lens benefits should be evaluated with beam lifetime simulations (using the SixTrack code on the BOINC platform, or LIFETRAC, together with diffusion models). These beam lifetime simulations must be benchmarked with measurements and tested for robustness of results against small parameter changes (for example phase advance between IPs and e-lens).

TEVATRON EXPERIENCE WITH REDUCED DISTANCE ENCOUNTERS (ALEXANDER VALISHEV)Tevatron experience with reduced distance encounters - The filling scheme in Tevatron is the following:

36 bunches in each beam (3 trains of 12 bunches); 39 ns bunch spacing; 2.6 ms abort gap; 72 parasitic encounters per bunch.

During the injection the beam lifetime decreases while the rings are filled. The vertical chromaticity plays an important role and passing from 6 units to two units it was possible to reduce by about a factor three the losses.

Another crucial impact of the beam-beam is during the squeeze phase: it can induce, together with the orbit errors, quenches. From a total of 140 quenches in Tevatron during 2008 (until the end of August), 15% occurred during the squeeze: the squeeze is the most frequent cause of beam induced quench. The minimum separation during the squeeze is between 3 and 5. The proton loss rate in the squeeze increases with the antiproton beam brightness. The coupling of the beams in the transversal plane can further enhance the BB effect (reducing the effective beam separation).

At store the helix size has an impact on the beam losses (Fig. 5).

Figure 5: The antiproton losses with respect to the helix size.

After an optimization of the helix the evolution of the beam lifetime was much better understood: this is due to the reduction of the beam-beam effect. The second order chromaticity correction does play an important role in both simulations and experiments.

The head-on BB tune shift reached in operation is ~2x10-2 for both beams. Head-on tune shift as high as 0.03 has been observed.

In the 2007-2008 run, adding up all these recipes, a gain of 10% in the daily integrated luminosity has been achieved with respect to the 2006-2007 run.

In conclusion we can summarize the following points: Long-range BB effects in the Tevatron are pronounced when the separation is

below 6 Separation is not the sole important parameter;

o RDTs of particular resonances;o First order chromaticity proved to be very important at injection;o Need to control betatron coupling;

Head-on BB effects are most pronounced in protons; Head-on tune shift as high as 0.03 has been observed; Second order chromaticity correction is important;

Unequal beam emittances cause lifetime degradation in the larger beam.For further reading we refer to [2-4].

Benefits of E-Lens in LHC - One possible application of the E-Lens (EL) in LHC can be the tune spread compensation by means of the footprint compression. The EL should match the beam profile at the IP (presumably Gaussian) and should have enough current: the total number of needed electrons (Ne) is

N e=N IP N p

1+βe .

where NIP is the number of IPs to compensate, Np the bunch current and e the electron relativistic . In the LHC nominal condition NIP=3 and Np=1.15x1011, that yields (assuming 10-keV electrons or e~0.2, Ne=3.45x1011, e.g. Je=1.4 A for a 2-m long EL. To the first order the location of the EL is not important (x=y and Dx=0 is desired). A weak-strong simulation has been carried out using LIFETRAC in the following hypotheses and parameters:

3D Gaussian bunch; LR and HO; 6D tracking. Weak bunch sliced at IPs; 104 particles for 107 turns; 6D maps, collision points and beam separations from MADX (ideal linear optics V. 6.5), 1st and 2nd order chromaticity included,

diffusion and aperture limit at 6 E-Lens with Gaussian e-beam profile, positioned at BB compensation slot (bbc)

near IP1: size and position are matched to the proton beam.The conclusions can be summarized as follows:

BB effects cause lifetime degradation in the present configuration at twice the nominal beam intensity;

Losses are induced by the combination of LR and HO collisions; EL acting as tune spread compensator can improve the beam lifetime.

Some aspects still to be further investigated are the effect of the electron-beam size, the stability and alignment of the EL as RDT compensator and the machine imperfections.

LARP BEAM-BEAM STUDIES (TANAJI SEN)LARP's mission is to help the LHC to achieve higher luminosity quicker and to develop

expertise in the US. In a pure beam-beam perspective, this means to investigate compensation schemes to mitigate the effects of long-range and head-on interactions with emphasis on the IR upgrade designs. All this is done through the development and analysis of software tools to better understand beam-beam phenomena.

The main topics addressed in the LARP framework are: Wire compensation experiments and simulations (RHIC, LHC); Electron lens compensation experiments and simulations (Tevatron, RHIC, LHC); Crab cavity simulations (LHC).

Beam-wire compensation - In that respect, beam losses with changing beam-wire separation (at injection and collision) and beam transfer function (BTF) simulations with and without wire were performed. Tracking and diffusion model for long term simulations were compared with the RHIC store data (emittance and lifetime). With the BBSIM simulations, the onset of sharp losses is well reproduced both at injection energy and at collision energy.

The BTF simulations without the wire agree well with the experimental data; issues with BTF measurements and simulations with the wire still need to be resolved.

The use of an independent diffusion equation solver is another study ongoing: the goal is to describe the emittance growth and lifetime over the length of the store, ~10-24 hours. This

is not feasible with direct tracking. Initial results are encouraging and the model is still under development (3D,…).

E-Lens simulations for LHC - Simulations for a possible application of the E-Lens (EL) in LHC were performed in the LARP framework: a Gaussian profile for the EL and the beam-beam compensator section near IP1 were assumed. The results show that 10 A-m is the optimal integrated current for the EL. The expected gain is 40% in extracted lifetime at nominal intensity with EL and larger improvement at higher intensity seems reasonable. From the experimental point of view, a Gaussian gun has been built and might be installed in October 2008 or next spring. The beam-beam compensation studies in the Tevatron will address the following topics:

quantification of the improvement in beam lifetime with respect to the EL current; quantification of low-frequency EL current jitter on proton lifetime; demonstration of the antiproton tune spread reduction.

Ongoing studies - Many other studies are ongoing: EL simulations and design at RHIC (W. Fischer); Impact of long-range interactions on emittance growth in the LHC with strong-

strong for head-on, soft-Gaussian for long-range (J. Qiang); Crab cavity simulations (preliminary results from J. Qiang); FNAL collaboration with TechX (Boulder, CO) on validation of diffusion model,

benchmark simulations with Tevatron and RHIC data, strong-strong models.Upcoming studies -

RHIC wire compensation experiments and comparisons with simulations; Benefit to LHC luminosity from wire compensation (simulations); Electron lens beam studies in the Tevatron; Impact of electron lens on beam behaviour in RHIC, LHC (simulations); Impact of crab cavity with BB interactions in the LHC (simulations).

Concluding, wire experimental data in RHIC and simulations show good agreement overall. We are looking forward to compensation experiment in 2009. The diffusion model for long-term beam evolution seems promising. Preliminary simulations show that LHC can benefit from an EL, especially at higher beam intensities. In RHIC the EL does not yet show a clear benefit.

References [1] N. Abreu, “Beam-beam with a few long-range encounters at long distance”, BEAM07, CERN[2] Yu. Alexahin, “Optimization of the Helical Orbits in the Tevatron”, PAC07[3] A. Valishev, “Simulation of Beam-Beam Effects and Tevatron Experience”, EPAC08[4] V. Shiltsev et al., “Beam-Beam Effects in the Tevatron”, Phys. Rev. ST AB 8 101001, 2005