In order to study dynamic effects in accelerator magnets, such as the decay of the magnetic field during the dwell at injection and the rapid so-called "snapback" during the first few seconds of the resumption of the energy ramp, a fast continuous harmonics measurement system was required. A new magnetic field measurement system, based on the use of digital signal processors (DSP) and Analog to Digital (A/D) converters, was developed and prototyped at Fermilab. This system uses Pentek 6102 16 bit A/D converters and the Pentek 4288 DSP board with the SHARC ADSP-2106 family digital signal processor. It was designed to acquire multiple channels of data with a wide dynamic range of input signals, which are typically generated by a rotating coil probe. Data acquisition is performed under a RTOS, whereas processing and visualization are performed under a host computer. Firmware code was developed for the DSP to perform fast continuous readout of the A/D FIFO memory and integration over specified intervals, synchronized to the probe's rotation in the magnetic field. C, C++ and Java code was written to control the data acquisition devices and to process a continuous stream of data. The paper summarizes the characteristics of the system and presents the results of initial tests and measurements

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A Fast Continuous Magnetic Field Measurement System Based on Digital Signal Processors

A systematic study of the persistent current decay and snapback effect in the fields of Tevatron accelerator dipoles was performed at the Fermilab Magnet Test Facility (MTF). The decay and snapback were measured under a range of conditions including variations of the current ramp parameters and magnet operational history. The study has mostly focused on the dynamic behavior of the normal sextupole component. In addition, the paper presents the persistent current effects observed in the other allowed field harmonics as well. The results provide new information about the previously observed "excess" decay during the first several seconds of the sextupole decay during injection and the correlation between the snapback amplitude and its duration.

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Measurements of the Persistent Current Decay and Snapback Effect in Tevatron Dipole Magnets

Since the beginning of 2002 an intensive measurement program has been performed at the Fermilab Magnet Test Facility (MTF) to understand dynamic effects in Tevatron magnets. Based on the results of this program a new correction algorithm was proposed to compensate for the decay of the sextupole field during the dwell at injection and for the subsequent field "snapback" during the first few seconds of the energy ramp. Beam studies showed that the new correction algorithm works better than the original one, and improves the Tevatron efficiency by at least 3%. The beam studies also indicated insufficient correction during the first 6s of the injection plateau where an unexpected discrepancy of 0.15 sextupole units of extra drift was observed. This paper reports on the most recent measurements of the Tevatron dipoles field at the beginning of the injection plateau. Results on the field decay and snapback in the Tevatron quadrupoles are also presented.

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Measurements of Field Decay and Snapback Effect on Tevatron Dipole and Quadrupole Magnets

Fermilab, as part of the US-LHC Accelerator Project, is producing superconducting low-beta quadrupole magnets for the large hadron collider (LHC). These 5.5 m long magnets are designed to operate in superfluid helium at 1.9 K with a nominal gradient of 205 T/m in the 70 mm bore. Two quadrupoles separated by a dipole orbit corrector in a single cryogenic assembly comprise the Q2 optical elements of the final focus triplets in the LHC interaction regions. The field quality of the quadrupoles is measured at room temperature during construction of the cold masses as well as during cold testing of the cryogenic assembly. We summarize data from the series measurements of the magnets and discuss various topics of interest.

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Magnetic Field Measurements of LHC Inner Triplet Quadrupoles Fabricated at Fermilab

Beam loss and emittance dilution during ramping from injection to collision energy is observed in the Tevatron, now in its collider run-II stage. Imperfect control of the machine chromaticity during the injection porch and the ensuing ramp to collision could contribute to the beam loss. Dipole magnets are the main source of chromaticity in the Tevatron. Sextupole correctors are distributed around the ring to compensate for the sextupole from the dipole magnets. Furthermore, the sextupole component in the superconducting Tevatron dipole magnets decays during the injection plateau and snaps back to the value before the start of the decay at the start of the ramp. The chromaticity correctors also correct for this so-called dynamic sextupole behavior. To determine if the sextupole compensation is working successfully in the Tevatron a thorough investigation of the sextupole (b2) in Tevatron dipole magnets was recently conducted. This included an extensive series of off-line magnetic measurements on several Tevatron dipole magnets from the spare pool. Traditionally one differentiates between three different b2 components, geometric, hysteretic and dynamic. This note discusses the results of these measurements. The experiments were also complemented with magnetic field calculations and analysis of data from the Tevatron magnetic measurement archive. On the basis of this document it is hoped that the chromaticity correction scheme in the Tevatron can be improved. Such improvements, however, are discussed in a separate note on the Tevatron chromaticity compensation (TD-03-008 – Revision I). This work is essentially a continuation of a similar study conducted in 1996 [1], on the basis of which the chromaticity correction scheme for the Tevatron run II was devised.

Measurements of geometric, hysteretic and dynamic sextupole in tevatron dipoles

To help optimize the performance of the Fermilab Tevatron accelerator in Collider Run II, we have undertaken a systematic study of the drift and subsequent snapback of dipole magnet harmonics. The study has mostly focused on the dynamic behavior of the normal sextupole component, b2, as measured on a sample of spare Tevatron dipoles at the Fermilab Magnet Test Facility. We measured the dependence of the decay amplitude and the snapback time on Tevatron ramp parameters and magnet operational history. A series of beam studies was also performed [1]. This paper summarizes the magnetic measurement results and concludes with proposals for an optimization of the b2 correction scheme which is derived from these measurements.

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Measurements of sextupole decay and snapback in tevatron dipole magnets

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Tevatron chromaticity and tune drift and snapback studies report

porch can be correlated with a total of ~100 units (of 10 -4 times the dipole field at injection) of quadrupolar field over the entire ring. This indicates a small effect, which can be produced by one (normal -b1- or skew -a1-) corrector quad running at ~7 A. The time-dependence of the tune and coupling drift is logarithmic, similar to that of the sextupole (b2) drift in the main dipoles. It is therefore suspected that the cause of the problem lies within the main superconducting magnets of the Tevatron. The following note discusses the tune drift issue in the Tevatron, and explores possible avenues of explanation. These are a) feed-down effects from the sextupole fields in the main dipoles and sextupole correctors due to beam-offset and/or systematic magnet misalignment and geometric errors; b) possible wiring errors in the corrector circuits, c) possible decaying main fields in quadrupoles and d) decaying skew and normal quadrupole components in the main dipoles.

G. Annala

Papers

Measurements of Field Decay and Snapback Effect on Tevatron Dipole and Quadrupole Magnets

Measurements of geometric, hysteretic and dynamic sextupole in tevatron dipoles

Measurements of sextupole decay and snapback in tevatron dipole magnets

Tevatron chromaticity and tune drift and snapback studies report

Analysis of possible magnet related causes of the tevatron tune and coupling drift and snapback during injection