P. Baudrenghien

Heavy Ions in 2011 and beyond

The LHCs first heavy ion run set and tested the operational pattern for 2011 and later years: a rapid commissioning strategy intended to ensure delivery of integrated luminosity despite the risks associated with the short time-frame. It also gave us hard data to test our understanding of the beam physics that will limit performance. The 2010 experience is fed into the commissioning plan, parameter choices and projected performance for 2011. The prospects for future stages of the LHC ion program, Pb-Pb collisions at higher energy and luminosity, hybrid collisions and other species, depend critically on the scheduling of certain hardware upgrades.

Synchronous Phase Shift at LHC

The electron cloud in vacuum pipes of accelerators of positively charged particle beams causes a beam energy loss which could be estimated from the synchronous phase. Measurements done with beams of 75 ns, 50 ns, and 25 ns bunch spacing in the LHC for some fills in 2010 and 2011 show that the average energy loss depends on the total beam intensity in the ring. Later measurements during the scrubbing run with 50 ns beams show the reduction of the electron cloud due to scrubbing. Finally, measurements of the individual bunch phase give us information about the electron cloud build-up inside the batch and from batch to batch.

Nominal longitudinal parameters for the LHC beam in the CERN SPS

A proton beam with the basic structure defined by the LHC requirements was first available for injection into the SPS in 1998. At the end of 2002, following a significant beam-studies and RF hardware upgrade programme, a beam having both the nominal LHC intensity and the correct longitudinal parameters was obtained at top energy for the first time. This beam, characterized by high local density, must satisfy strict requirements on bunch length, longitudinal emittance and bunch to bunch phase modulation for extraction to the LHC, where only very limited particle losses are acceptable. The problems to be solved came mainly from the high beam loading and microwave and coupled bunch instabilities which led both to beam losses and to unacceptably large longitudinal emittance on the flat top. In this paper the steps taken to arrive at these nominal beam parameters are presented.

Jitter Impact on Clock Distribution in LHC Experiments

The LHC Bunch Clock is one of the most important accelerator signals delivered to the experiments. Being directly derived from the Radio Frequency driving the beams in the accelerator by a simple division of its frequency by a factor of 10, the Bunch Clock signal represents the frequency at which the bunches are crossing each other at each experiment. It is thus used to synchronize all the electronics systems in charge of event detection. Its frequency is around 40.079 MHz, but varies with beam parameters (energy, particle type, etc) by a few hundreds of Hz. The present paper discusses the quality of this Bunch Clock signal in terms of jitter. It is in particular compared to typical requirements of electronic components of the LHC detectors and put in perspective with the intrinsic jitter of the beam itself, to which this signal is related.

RF And ADT after LS1

During LS1 a number of consolidations and upgrades have been undertaken in the LHC RF, including replacement of a cryomodule (four cavities, beam 2), upgrade of klystron collectors and new solid state crowbar systems. The RF parameters will be outlined in view of the consequences of the increased beam current and energy, and the exotic bunch spacing for the scrubbing beams. The LHC Transverse feedback system (ADT) is also undergoing a major upgrade during LS1, with double the total number of pickups to reduce the noise floor of the system, new beam position electronics and an upgraded digital signal processing system to accommodate all of the extra functionality that had been introduced during LHC Run I, and more sophisticated signal processing algorithms to be deployed for Run II. An external “observation box” to record transverse and longitudinal data from the RF and ADT systems is being implemented.

Validation of Off-momentum Cleaning Performance of the LHC Collimation System

The LHC collimation system is designed to provide effective cleaning against losses coming from off-momentum particles, either due to un-captured beam or to an unexpected RF frequency change. For this reason the LHC is equipped with a hierarchy of collimators in IR3: primary, secondary and absorber collimators. After every collimator alignment or change of machine configuration, the off-momentum cleaning efficiency is validated with loss maps at low intensity. We describe here the improved technique used in 2015 to generate such loss maps without completely dumping the beam into the collimators. The achieved performance of the collimation system for momentum cleaning is reviewed.

Low-level RF - Part I: Longitudinal dynamics and beam-based loops in synchrotrons

The low-level RF system (LLRF) generates the drive sent to the high-power equipment. In synchrotrons, it uses signals from beam pick-ups (radial and longitudinal) to minimize the beam losses and provide a beam with reproducible parameters (intensity, bunch length, average momentum and momentum spread) for either the next accelerator or the physicists. This presentation is the first of three: it considers synchrotrons in the lowintensity regime where the voltage in the RF cavity is not influenced by the beam. As the author is in charge of the LHC LLRF and currently commissioning it, much material is particularly relevant to hadron machines. A section is concerned with radiation damping in lepton machines.

Running the RF at higher energy and intensity

The improvements done to the RF parameters and hardware in 2011 are reviewed. Then the upgrades planned for 2012 are presented: further reduction of capture losses with the longitudinal damper, batch by batch blow-up at injection and modification of the controlled blow-up to preserve bunch profile. Operation at higher energy is readily possible with the present RF power, and does not degrade longitudinal stability thanks to the controlled longitudinal emittance growth during the ramp. For operation with higher beam current, the observations in 2011 indicate that there is no single bunch instability issue with up to 3 10 11 p per bunch. With the large gain of the RF feedback and One-Turn feedback, the cavity impedance at the fundamental will not be a limitation for ultimate intensity (1.7 10 11 p per bunch) with 25 ns spacing. The klystron power (300 kW RF at saturation) is sufficient for 25 ns operation with nominal intensity (2808 bunches per beam, 1.1 10 11 p per bunch). An RF roadmap for going beyond will be outlined: it calls for an upgrade of the LLRF only and should allow for operation with ultimate beam intensity (25 ns spacing, 2808 bunches, 1.7 10 11 p per bunch) after Long Shutdown one. NEW FEATURES 2011 VERSUS 2010 The statistics on RF faults has been presented at the Evian workshop [1] and will not be repeated here. Increased capture voltage At extraction, the SPS RF is 7 MV (200 MHz). The bunch has 1.5 ns length * (4t), and 4.5 10 -4 energy spread E/E (2 E), resulting in a 4tE emittance of 0.5 eVs † . In 2011 the LHC capture voltage was increased from 3.5 MV (2010) to 6 MV. The bucket area was increased from 0.9 eVs in 2010, to 1.2 eVs. The bucket half height is now 9.6 10 -4 E/E. With 7 MV at 200 MHz, the SPS bucket area at extraction is 3 eVs but the longitudinal distribution is limited to a much smaller region: controlled longitudinal blow-up is applied during the SPS ramp to keep the beam stable [2]. The blow-up is turned off near the moment when the voltage program corresponds to a bucket area of 1.05 eVs only. The voltage is then raised adiabatically to 7 MV for bunch shortening before transfer to the LHC. We can therefore * We quote the 4length of a Gaussian bunch having the same Full Width at Half Maximum as the measured bunch. † At CERN it is customary to quote the longitudinal emittance as † At CERN it is customary to quote the longitudinal emittance as 4tE. Note that, for a Gaussian distribution, and small filling factor, 95% of the particles are within a 6tE area. The 4tE area contains 86.5% of the particles. limit the SPS bunch to a 1.05 eVs contour in a stationary 7 MV bucket (at 200 MHz). Figure 1 shows the situation: the 1.05 eVs contour falls almost entirely within the LHC bucket. Assuming a Gaussian distribution for the SPS bunch, truncated at the 1.05 eVs contour, the calculated loss is 0.02%. Figure 1: Longitudinal phase space at injection: We assume a Gaussian distribution for the SPS bunch and display contours corresponding to steps of 5% in integrated intensity. The Gaussian is truncated at the 1.05 eVs contour (yellow). In figure 2 we introduce a small injection error (100 ps and 10 -4 p/p). This results in a small portion of the bunch falling outside the LHC bucket (calculated 0.4 % loss with the truncated Gaussian model). In 2011 we have observed 0.5% loss from injection to start ramp. Figure 2: Longitudinal phase space at injection with a small error: 100 ps and 10 -4 p/p. A consequence of the voltage mismatch (matched voltage is around 2.5 MV) is the bunch length reduction after capture (from 1.5 ns to 1.1 ns). We could take advantage of the large available bucket to blow up the longitudinal emittance after each injection and restore the 1.5 ns length (batch-by-batch blowup). With 1.5 ns and 6 MV, we get 0.83 eVs (4Et) emittance. We could increase it further by capturing with 8 MV as for the Lead ions, leading to 0.97 eVs. This is planned for 2012. Larger voltage in physics In 2011, controlled longitudinal emittance blow-up was applied in the eleven minutes long ramp, keeping the bunch length around 1.2 ns, while the RF voltage was increased linearly from 6 MV to 12 MV. (In 2010 we used 8 MV only in physics). The 12 MV provide a larger longitudinal emittance, thereby reducing the transverse emittance growth due to Intra Beam Scattering. At the beginning of the 3.5 TeV flat top we now have 2 eVs longitudinal emittance in a 4.7 eVs bucket (1.5 eVs in a 3.8 eVs bucket in 2010).