S. Myers

Effects of Space Charge and Reactive Wall Impedance on Bunched Beams

Space charge and reactive wall impedance create longitudinal forces inside the bunch which change the incoherent phase oscillation frequency, the bunch length and the size of the RF-bucket. These effects have been investigated with bunched beams in the ISR. By measuring the shift of the quadrupole mode phase oscillation frequency, the strength of the self-forces was determined. The inductive wall is dominant and its impedance (divided by the mode number) was measured to be |Z|/n z 26 Ohms. An increase of bunch length with current was measured. It can be explained by the inductive impedance up to a certain current; beyond that an excessive, unexplained bunch lengthening occurs. The reduction of the bucket size affects the stacking process. By correcting for it, an increased density of the stacked beam was achieved. Ez = -e az [4 g0 dz (1)

Change of the Energy Distribution in an Electron Storage Ring by a Dipole-Octupole Wiggler

A dipole-octupole wiggler is proposed to change the energy distribution of the electrons in a storage ring so that the peak current and the related collective effects are reduced. With a dipole-quadrupole wiggler the radiation damping partition can be changed and with a dipole-octupole wiggler this partition becomes dependent on the phase oscillation amplitude. These wigglers can be operated so that the longitudinal damping is negative for small amplitudes but increases for larger amplitudes. The particle distribution will peak around the amplitude which gives neither damping nor antidamping. This leads to a longitudinal blow-up of the bunch core without affecting very much the tails or the quantum lifetime. The distribution has been calculated analytically with an appropriate Fokker-Planck equation and by computer simulation. The results allow the optimization of the wiggler parameters. Due to the sum rule for the damping partitions this wiggler also affects the distribution in horizontal betatron amplitudes.

Acceleration and Stacking of Deuterons in the CERN PS and ISR

Deuteron acceleration in the CERN 50 MeV Linac has been tried out already 13 years ago followed by programmed acceleration in tne CPS up to about 100 MeV.

Phase Displacement Acceleration of High Intensity Stacks in the CERN ISR

In the ISR high intensity stacks of more than 25 A are accelerated by phase displacement from 26.6 to 31.4 GeV/c. Phase displacement is the only known means of accelerating stacks of such large momentum spread (¿p/p = 3%) with the existing low power RF system. Acceleration in this way may produce loss of intensity due to RF and power supply magnet noise, momentum blowup of the stack, closed orbit and working line variations, and changes in the RF bucket size while traversing the stack. The existing instrumentation allows close control of all relevant parameters during acceleration and has resulted in reducing the intensity losses to as little as 10%. In this way, luminosities significantly in excess of the ISR design luminosity are achieved in an operational way, making 31.4 GeV/c one of the standard ISR momenta for physics data taking and giving an equivalent nomentum of greater than 2,000 GeV/c when related to stationary target machines.

The variation of γ t with Δp/p in the CERN ISR

The variation of the transition energy γt m_oc² across the momentum aperture in the ISR has been determined by measuring the non-linear change of the revolution frequency as a function of the radial displacement and the momentum deviation and by measuring the phase oscillation frequency variation across the aperture while operating close to transition energy. The ratio of the relative slopes of γ_t and γ was found to be dγ_ttc ≈ -0.75. This value could be changed to nearly zero by strongly exciting the sextupole magnets. All measurements are in good agreement with each other and with computations carried out with a modified version of the program AGS. The change of γ_t across the aperture causes an area change of the empty buckets while traversing the beam during phase displacement acceleration. This leads to a momentum blow-up.

The ISR Impedance between 40 KHz and 40 GHz

Different current dependent effects have been measured in the ISR over the years and used to obtain information on the impedance of the beam surroundings over a large frequency range. The transverse impedance at low frequencies f < 20 MHz was obtained from the response of the beam to an excitation (beam transfer function) and at medium frequencies 30 < f < 200 MHz from the growth rate of the head tail instability. Potential well bunch lengthening and synchrotron frequency shifts gave the longitudinal reactive impedance over low and medium frequencies while the observation of the microwave instability (turbulent bunch lengthening) gave its resistive part at high frequencies f ~ 1 GHz. The measured energy loss of an unbunched beam gave an integral over the resistive impedance up to frequencies of 7 to 70 GHz depending on the beam energy. To analyse all the measured data the approximate relation between transverse and longitudinal impedance was used and a model impedance was fitted which has the expected behavior at low, intermediate and very high frequencies where the impedance is dominated by the smooth resistive wall, the effect of higher frequency resonances and by diffraction respectively.

Investigation of the Coherent Beam-Beam Effects in the ISR

Although the beam-beam tune shift is relatively small in the ISR (0.001 per intersection) and below the value where stochastic effects might be expected, coherent beam-beam effects can be important. Each high-intensity ISR beam is close to the transverse single beam stability limit. The addition of the beam-beam force has led to the loss of one of the beams accompanied by coherent oscillations. The change of the betatron frequency distribution due to the other beam has been investigated. The resonance excitation by the non-linear beam-beam force has been measured and seems to influence the transverse stability. Quantitative information about the beam-beam force is obtained by exciting a coherent betatron oscillation in one beam and observing the response of the other beam. This beam-beam transfer function depends on the beam separation in the interaction point and can be used to centre the two beams. At the same time the effective beam height is obtained, which together with the beam currents determines the luminosity.

Performance of the CERN ISR at 31.4 GeV

Due to the recent improvements in phase displacement acceleration, operating techniques and beam diagnostics, considerable progress has been achieved in operating the ISR at the maximum energy of 31.4 GeV (this centre of mass energy corresponds to a 2 TeV fixed target machine). High intensity stacks are stored at 26.6 GeV before acceleration by phase displacement to 31.4 GeV. At this energy up to 34 Amps are obtained with corresponding initial luminosities of 2.1031cm-2s-1 per intersection and 4.3 1031cm-2s-1 in a low-ß intersection. Due to the long beam lifetime, physics data taking may be performed over periods of 60 hours. The principle of phase displacement and the associated beam phenomena are discussed together with the control of the magnetic machine and the mutual interaction of the two beams. The operational technique is described and it is shown that the present day performance of the ISR at 31.4 GeV is comparable to the previous performance (1977) at 26.6 GeV. Consequently, in 1978 about 90% of the time requested for physics data collection was at 31.4 GeV.

Large Hadron Collider commissioning and first operation

A history of the commissioning and the very successful early operation of the Large Hadron Collider (LHC) is described. The accident that interrupted the first commissioning, its repair and the enhanced protection system put in place are fully described. The LHC beam commissioning and operational performance are reviewed for the period from 2010 to mid-2011. Preliminary plans for operation and future upgrades for the LHC are given for the short and medium term.

The CERN ISR Control Scheme for Acceleration by Phase Displacement

In the CERN ISR high-intensity coasting beams are accelerated routinely from 26.6 to 31.4 GeV/c using the technique of phase displacement. The constraints imposed by this technique dictate a rather unusual control scheme. The total acceleration is performed in around two hundred steps; each incremental step in momentum is produced by sweeping empty RF buckets through the beam. The RF parameters are calculated and set by the control computer for each sweep. During each sweep the computer controls all magnetic elements synchronously so as to compensate for normal variations in the working line, the closed orbits and the beam position. This control is based on linear interpolation on the values held in a number of intermediate files between 26.6 and 31.4 GeV/c. The differences in the power supply settings contained in these files produce the magnetic changes required to maintain the correct space charge tune shift and to compensate for magnetic saturation of the poles of the main bending magnets. The intermediate files are generated from the results of measurements made during the acceleration of a bunched beam using a different mode of the acceleration program.