SRF2011 - List of Authors (Valles, N.R.A.)

Paper

Title

Page

Superconducting RF for the Cornell Energy-Recovery Linac Main Linac

90

M. Liepe, Y. He, G.H. Hoffstaetter, S. Posen, J. Sears, V.D. Shemelin, M. Tigner, N.R.A. Valles, V. Veshcherevich
CLASSE, Ithaca, New York, USA

Funding: Supported by NSF award DMR-0807731.
Cornell University is developing the superconducting RF technology required for the construction of a 100 mA hard X-ray light source driven by an Energy-Recovery Linac. Prototype components of the 5 GeV cw SRF main linac cryomodule are under development, fabrication and testing. This work includes an optimized 7-cell SRF cavity, a broadband HOM beamline absorber, and a 5 kW cw RF input coupler. In this paper we give an overview of these activities at Cornell.

MOPO056

Beam Break Up Studies for Cornell's Energy Recovery Linac

229

N.R.A. Valles, D.S. Klein, M. Liepe
CLASSE, Ithaca, New York, USA

New results are presented of beam break-up (BBU) studies for the Cornell ERL main linac. Previously, a 1.3 GHz main linac 7-cell cavity was optimized to maximize the BBU current through the accelerator. This work realistically models the ERL main linac cavity shapes by taking into account small machining variations in ellipse dimensions. Cavity shapes were simulated with uniformly distributed errors, and their higher-order mode spectrum computed. The strongest higher-order modes can cause resonant excitations in the beam which can lead to beam loss. The threshold current through the accelerator is determined resulting from a linac comprised of cavities with machining variations using particle tracking and demonstrates that the threshold current is well above the 100 mA design goal for the Cornell's Energy Recovery Linac.

MOPO057

Coupler Kick Studies in Cornell's 7-Cell Superconducting Cavities

232

N.R.A. Valles, M. Liepe, V.D. Shemelin
CLASSE, Ithaca, New York, USA

Cornell is developing a 5 GeV Energy Recovery Linac operating at 100 mA with very small emittances (~30 pm-rad at 77 pC bunch charge) in the horizontal and vertical directions. We investigate the effect of the fundamental RF power couplers of the main linac SRF cavities on the beam using the ACE3P software package. The cavities in the ERL main linac will be operated at very high loaded quality factors of up to 6.5·10⁷, corresponding to a full bandwidth of only 20 Hz. Cavity microphonics will detune the cavities by more than one bandwidth during operation, thereby causing a time dependent change of the coupler kick in addition to its fast oscillation at the RF frequency. In order to investigate the dependence of the coupler kick on the cavity frequency, we calculate the coupler kick given to the beam for the case of a detuned RF cavity. We show that a compensation stub geometry located opposite to the input coupler port can be optimized to reduce the overall kick given to the beam and the emittace growth caused by its time dependence.

MOPO061

Effects of Elliptically Deformed Cell Shape in the Cornell ERL Cavity

244

L. Xiao, K. Ko, K.H. Lee, C.-K. Ng
SLAC, Menlo Park, California, USA
M. Liepe, N.R.A. Valles
CLASSE, Ithaca, New York, USA

The Cornell ERL cavity is optimized to minimize the dipole mode BBU parameters to achieve the required high beam current (100mA). Deformations due to errors in fabrication and tuning of the accelerating mode can result in a cavity shape different from the ideal. In elliptically deformed cells, this can cause dipole mode frequency spread and splitting of the mode polarizations leading to an x-y mode coupling. To investigate these effects, we use a mesh distortion technique to generate an elliptically deformed cell cavity model as a base for studying random imperfections. Simulation results from the eigensolver Omega3P of one hundred randomly elliptically deformed cell cavities covering the first three dipole bands will be presented. The results will be used as input to the beam tracking code BMAD to calculate the impact of such imperfections on the dipole mode BBU parameters.

TUIOA05

The Superheating Field of Niobium: Theory and Experiment

293

N.R.A. Valles, M. Liepe
CLASSE, Ithaca, New York, USA

This study discusses the superheating field of Niobium, a metastable state, which sets the upper limit of sustainable magnetic fields on the surface of a superconductors before it transitions into the normal conducting state. Current models for the superheating field are discussed, and experimental results are presented for niobium obtained through pulsed, high power measurements performed at Cornell. Material preparation is also shown to be an important parameter in exploring other regions of the superheating field, and fundamental limits are presented based upon these experimental and theoretical results.

Slides TUIOA05 [2.247 MB]

THPO009

Quench Studies in Large and Fine Grain Nb Cavities

714

S. Posen, M. Liepe, N.R.A. Valles
CLASSE, Ithaca, New York, USA

Quenches without radiation are sometimes observed at accelerating fields between 25 and 40 MV/m in niobium SRF cavities. The cause for this limitation is not well understood. This work presents results from vertical tests of seven 1.3 GHz single-cell cavities performing above 25 MV/m. Studies were carried out on both fine grain and large grain cavities in ILC and Cornell Reentrant shape geometries. The quenches were located by triangulation using Cornell oscillating superleak transducers and then cavities were optically inspected to determine the surface conditions of the cavity at the quench location. Optical inspection images are presented as well as 3D recreations of quench spots generated using a surface mold and a confocal microscope.

Poster THPO009 [2.013 MB]

Paper	Title	Page
MOPO016	Superconducting RF for the Cornell Energy-Recovery Linac Main Linac	90
	M. Liepe, Y. He, G.H. Hoffstaetter, S. Posen, J. Sears, V.D. Shemelin, M. Tigner, N.R.A. Valles, V. Veshcherevich CLASSE, Ithaca, New York, USA
	Funding: Supported by NSF award DMR-0807731. Cornell University is developing the superconducting RF technology required for the construction of a 100 mA hard X-ray light source driven by an Energy-Recovery Linac. Prototype components of the 5 GeV cw SRF main linac cryomodule are under development, fabrication and testing. This work includes an optimized 7-cell SRF cavity, a broadband HOM beamline absorber, and a 5 kW cw RF input coupler. In this paper we give an overview of these activities at Cornell.

MOPO056	Beam Break Up Studies for Cornell's Energy Recovery Linac	229
	N.R.A. Valles, D.S. Klein, M. Liepe CLASSE, Ithaca, New York, USA
	New results are presented of beam break-up (BBU) studies for the Cornell ERL main linac. Previously, a 1.3 GHz main linac 7-cell cavity was optimized to maximize the BBU current through the accelerator. This work realistically models the ERL main linac cavity shapes by taking into account small machining variations in ellipse dimensions. Cavity shapes were simulated with uniformly distributed errors, and their higher-order mode spectrum computed. The strongest higher-order modes can cause resonant excitations in the beam which can lead to beam loss. The threshold current through the accelerator is determined resulting from a linac comprised of cavities with machining variations using particle tracking and demonstrates that the threshold current is well above the 100 mA design goal for the Cornell's Energy Recovery Linac.

MOPO057	Coupler Kick Studies in Cornell's 7-Cell Superconducting Cavities	232
	N.R.A. Valles, M. Liepe, V.D. Shemelin CLASSE, Ithaca, New York, USA
	Cornell is developing a 5 GeV Energy Recovery Linac operating at 100 mA with very small emittances (~30 pm-rad at 77 pC bunch charge) in the horizontal and vertical directions. We investigate the effect of the fundamental RF power couplers of the main linac SRF cavities on the beam using the ACE3P software package. The cavities in the ERL main linac will be operated at very high loaded quality factors of up to 6.5·10⁷, corresponding to a full bandwidth of only 20 Hz. Cavity microphonics will detune the cavities by more than one bandwidth during operation, thereby causing a time dependent change of the coupler kick in addition to its fast oscillation at the RF frequency. In order to investigate the dependence of the coupler kick on the cavity frequency, we calculate the coupler kick given to the beam for the case of a detuned RF cavity. We show that a compensation stub geometry located opposite to the input coupler port can be optimized to reduce the overall kick given to the beam and the emittace growth caused by its time dependence.

MOPO061	Effects of Elliptically Deformed Cell Shape in the Cornell ERL Cavity	244
	L. Xiao, K. Ko, K.H. Lee, C.-K. Ng SLAC, Menlo Park, California, USA M. Liepe, N.R.A. Valles CLASSE, Ithaca, New York, USA
	The Cornell ERL cavity is optimized to minimize the dipole mode BBU parameters to achieve the required high beam current (100mA). Deformations due to errors in fabrication and tuning of the accelerating mode can result in a cavity shape different from the ideal. In elliptically deformed cells, this can cause dipole mode frequency spread and splitting of the mode polarizations leading to an x-y mode coupling. To investigate these effects, we use a mesh distortion technique to generate an elliptically deformed cell cavity model as a base for studying random imperfections. Simulation results from the eigensolver Omega3P of one hundred randomly elliptically deformed cell cavities covering the first three dipole bands will be presented. The results will be used as input to the beam tracking code BMAD to calculate the impact of such imperfections on the dipole mode BBU parameters.

TUIOA05	The Superheating Field of Niobium: Theory and Experiment	293
	N.R.A. Valles, M. Liepe CLASSE, Ithaca, New York, USA
	This study discusses the superheating field of Niobium, a metastable state, which sets the upper limit of sustainable magnetic fields on the surface of a superconductors before it transitions into the normal conducting state. Current models for the superheating field are discussed, and experimental results are presented for niobium obtained through pulsed, high power measurements performed at Cornell. Material preparation is also shown to be an important parameter in exploring other regions of the superheating field, and fundamental limits are presented based upon these experimental and theoretical results.
	Slides TUIOA05 [2.247 MB]

THPO009	Quench Studies in Large and Fine Grain Nb Cavities	714
	S. Posen, M. Liepe, N.R.A. Valles CLASSE, Ithaca, New York, USA
	Quenches without radiation are sometimes observed at accelerating fields between 25 and 40 MV/m in niobium SRF cavities. The cause for this limitation is not well understood. This work presents results from vertical tests of seven 1.3 GHz single-cell cavities performing above 25 MV/m. Studies were carried out on both fine grain and large grain cavities in ILC and Cornell Reentrant shape geometries. The quenches were located by triangulation using Cornell oscillating superleak transducers and then cavities were optically inspected to determine the surface conditions of the cavity at the quench location. Optical inspection images are presented as well as 3D recreations of quench spots generated using a surface mold and a confocal microscope.
	Poster THPO009 [2.013 MB]