HB2014 - Table of Session: THO4LR (Working Group A)

Paper

Title

Page

Longitudinal Microwave Instability in a Multi-RF System

404

T. Argyropoulos
CERN, Geneva, Switzerland

The longitudinal microwave instability is observed as a fast increase of the bunch length above some threshold intensity. Recently, this type of instability was seen for a single proton bunch at high energies in the CERN SPS and is proven to be one of the limitations for an intensity increase required by the HL-LHC project. In this paper a theoretical approach to the analysis of the microwave instability is verified by particle simulations. The study is applied to the SPS and is based on the current SPS impedance model. Finally, the effect of the 4th harmonic RF system on the microwave instability threshold is investigated as well.

Slides THO4LR01 [0.855 MB]

THO4LR02

Synchrotron Frequency Shift as a Probe of the CERN SPS Reactive Impedance

409

A. Lasheen, T. Argyropoulos, J.V. Campelo, J.F. Esteban Müller, D. Quartullo, E.N. Shaposhnikova, H. Timko
CERN, Geneva, Switzerland

Longitudinal instability in the CERN SPS is a serious limitation for the future increase of bunch intensity required by HiLumi LHC project. The impedance driving this instability is not known precisely and a lot of effort goes into creating an accurate impedance model. The reactive impedance of the machine can be probed by measuring the bunch length oscillations of a mismatched bunch at injection. The frequency of these oscillations as a function of intensity has a slope that depends on the reactive impedance and the emittance. Measurements were done for three values of longitudinal emittances and then compared with particle simulations based on the impedance model using particle distribution close to the measured one. Comparison of measured and calculated frequency shifts gives an estimation of the missing impedance in the model. In addition, scanning of initial emittance for diverse particle distributions in simulations shows that the frequency shift greatly depends on emittance and initial distribution. Small variations of these parameters can lead to very different results and explain partially the discrepancy between measured and calculated values of frequency shifts.

Slides THO4LR02 [1.499 MB]

THO4LR03

Studies on Controlled RF Noise for the LHC

414

H. Timko, P. Baudrenghien, E.N. Shaposhnikova
CERN, Geneva, Switzerland
T. Mastoridis
CalPoly, San Luis Obispo, California, USA

RF phase noise is purposely injected into the LHC 400 MHz RF system during the acceleration ramp for controlled longitudinal emittance blow-up, in order to maintain longitudinal beam stability. Although the operational blow-up works reliably, studies of the injected RF noise are desirable not only to allow for a better-controlled, more flexible blow-up, but also for other applications such as the mitigation of machine-component heating through appropriate bunch shaping. Concerning the noise injection, an alternative algorithm was developed and implemented in the hardware, but first tests revealed unexpected modulation of the achieved bunch length along the ring, and subsequently, theoretical studies have been launched. In this paper, we present a summary of ongoing measurement analysis and simulation studies that shall explain previous observations, predict what can be expected in different cases, and thus help to optimise the RF noise in general.

Slides THO4LR03 [1.440 MB]

THO4LR04

Fast Transverse Instability and Electron Cloud Measurements in Fermilab Recycler

419

J.S. Eldred, P. Adamson, D. Capista, N. Eddy, I. Kourbanis, D.K. Morris, J.C.T. Thangaraj, M.-J. Yang, R.M. Zwaska
Fermilab, Batavia, Illinois, USA
J.S. Eldred
Indiana University, Bloomington, Indiana, USA
Y. Ji
IIT, Chicago, Illinois, USA

A new transverse instability is observed that may limit the proton intensity in the Fermilab Recycler. The instability is fast, leading to a beam-abort loss within two hundred turns. The instability primarily affects the first high-intensity batch from the Fermilab Booster in each Recycler cycle. This paper analyzes the dynamical features of the destabilized beam. The instability excites a horizontal betatron oscillation which couples into the vertical motion and also causes transverse emittance growth. This paper describes the feasibility of electron cloud as the mechanism for this instability and presents the first measurements of the electron cloud in the Fermilab Recycler. Direct measurements of the electron cloud are made using a retarding field analyzer (RFA) newly installed in the Fermilab Recycler. Indirect measurements of the electron cloud are made by propagating a microwave carrier signal through the beampipe and analyzing the phase modulation of the signal. The maximum betatron amplitude growth and the maximum electron cloud signal occur during minimums of the bunch length oscillation.

Slides THO4LR04 [1.608 MB]

THO4LR05

Transverse Emittance Preservation Studies for the CERN PS Booster Upgrade

428

E. Benedetto, C. Bracco, V. Forte, B. Mikulec, V. Raginel, G. Rumolo
CERN, Geneva, Switzerland
V. Forte
Université Blaise Pascal, Clermont-Ferrand, France

As part of the LHC Injectors Upgrade Project, the CERN PS Booster will undergo an ambitious upgrade program which includes the increase of injection energy from 50 MeV to 160 MeV and the implementation of an H⁻ charge-exchange injection from the new Linac4. Compared to rings characterized by similar space-charge tune spreads (about 0.5 at low energy), the peculiarity of the PSB is the small transverse emittance that needs to be preserved in order to provide high brightness beams to the LHC. We here try to identify what is the minimum emittance that can be achieved for a given intensity, via measurements, scaling estimates and simulation studies. The latest are based on our best knowledge of the optics model and take into account known perturbations such as the one induced by the short and fast ramping chicane injection magnets.

Slides THO4LR05 [1.122 MB]

Paper	Title	Page
THO4LR01	Longitudinal Microwave Instability in a Multi-RF System	404
	T. Argyropoulos CERN, Geneva, Switzerland
	The longitudinal microwave instability is observed as a fast increase of the bunch length above some threshold intensity. Recently, this type of instability was seen for a single proton bunch at high energies in the CERN SPS and is proven to be one of the limitations for an intensity increase required by the HL-LHC project. In this paper a theoretical approach to the analysis of the microwave instability is verified by particle simulations. The study is applied to the SPS and is based on the current SPS impedance model. Finally, the effect of the 4th harmonic RF system on the microwave instability threshold is investigated as well.
	Slides THO4LR01 [0.855 MB]

THO4LR02	Synchrotron Frequency Shift as a Probe of the CERN SPS Reactive Impedance	409
	A. Lasheen, T. Argyropoulos, J.V. Campelo, J.F. Esteban Müller, D. Quartullo, E.N. Shaposhnikova, H. Timko CERN, Geneva, Switzerland
	Longitudinal instability in the CERN SPS is a serious limitation for the future increase of bunch intensity required by HiLumi LHC project. The impedance driving this instability is not known precisely and a lot of effort goes into creating an accurate impedance model. The reactive impedance of the machine can be probed by measuring the bunch length oscillations of a mismatched bunch at injection. The frequency of these oscillations as a function of intensity has a slope that depends on the reactive impedance and the emittance. Measurements were done for three values of longitudinal emittances and then compared with particle simulations based on the impedance model using particle distribution close to the measured one. Comparison of measured and calculated frequency shifts gives an estimation of the missing impedance in the model. In addition, scanning of initial emittance for diverse particle distributions in simulations shows that the frequency shift greatly depends on emittance and initial distribution. Small variations of these parameters can lead to very different results and explain partially the discrepancy between measured and calculated values of frequency shifts.
	Slides THO4LR02 [1.499 MB]

THO4LR03	Studies on Controlled RF Noise for the LHC	414
	H. Timko, P. Baudrenghien, E.N. Shaposhnikova CERN, Geneva, Switzerland T. Mastoridis CalPoly, San Luis Obispo, California, USA
	RF phase noise is purposely injected into the LHC 400 MHz RF system during the acceleration ramp for controlled longitudinal emittance blow-up, in order to maintain longitudinal beam stability. Although the operational blow-up works reliably, studies of the injected RF noise are desirable not only to allow for a better-controlled, more flexible blow-up, but also for other applications such as the mitigation of machine-component heating through appropriate bunch shaping. Concerning the noise injection, an alternative algorithm was developed and implemented in the hardware, but first tests revealed unexpected modulation of the achieved bunch length along the ring, and subsequently, theoretical studies have been launched. In this paper, we present a summary of ongoing measurement analysis and simulation studies that shall explain previous observations, predict what can be expected in different cases, and thus help to optimise the RF noise in general.
	Slides THO4LR03 [1.440 MB]

THO4LR04	Fast Transverse Instability and Electron Cloud Measurements in Fermilab Recycler	419
	J.S. Eldred, P. Adamson, D. Capista, N. Eddy, I. Kourbanis, D.K. Morris, J.C.T. Thangaraj, M.-J. Yang, R.M. Zwaska Fermilab, Batavia, Illinois, USA J.S. Eldred Indiana University, Bloomington, Indiana, USA Y. Ji IIT, Chicago, Illinois, USA
	A new transverse instability is observed that may limit the proton intensity in the Fermilab Recycler. The instability is fast, leading to a beam-abort loss within two hundred turns. The instability primarily affects the first high-intensity batch from the Fermilab Booster in each Recycler cycle. This paper analyzes the dynamical features of the destabilized beam. The instability excites a horizontal betatron oscillation which couples into the vertical motion and also causes transverse emittance growth. This paper describes the feasibility of electron cloud as the mechanism for this instability and presents the first measurements of the electron cloud in the Fermilab Recycler. Direct measurements of the electron cloud are made using a retarding field analyzer (RFA) newly installed in the Fermilab Recycler. Indirect measurements of the electron cloud are made by propagating a microwave carrier signal through the beampipe and analyzing the phase modulation of the signal. The maximum betatron amplitude growth and the maximum electron cloud signal occur during minimums of the bunch length oscillation.
	Slides THO4LR04 [1.608 MB]

THO4LR05	Transverse Emittance Preservation Studies for the CERN PS Booster Upgrade	428
	E. Benedetto, C. Bracco, V. Forte, B. Mikulec, V. Raginel, G. Rumolo CERN, Geneva, Switzerland V. Forte Université Blaise Pascal, Clermont-Ferrand, France
	As part of the LHC Injectors Upgrade Project, the CERN PS Booster will undergo an ambitious upgrade program which includes the increase of injection energy from 50 MeV to 160 MeV and the implementation of an H⁻ charge-exchange injection from the new Linac4. Compared to rings characterized by similar space-charge tune spreads (about 0.5 at low energy), the peculiarity of the PSB is the small transverse emittance that needs to be preserved in order to provide high brightness beams to the LHC. We here try to identify what is the minimum emittance that can be achieved for a given intensity, via measurements, scaling estimates and simulation studies. The latest are based on our best knowledge of the optics model and take into account known perturbations such as the one induced by the short and fast ramping chicane injection magnets.
	Slides THO4LR05 [1.122 MB]