Layouts of Zero-Crossing Extraction, Feed-Forward Correction,

and Fast Feedback Stabilisation at the ATF2

A. Kalinin

Accelerator Science and Technology Centre, Daresbury Laboratory, UK

ATF2 Weekly Meeting

KEK, July 11, 2007

Three systems are supposed to be useful in maintaining the beam quality at ATF2. If to follow the beam from DR to EXT, they are:

• The Zero-Crossing Extraction (ZC) in DR corrects for center-of-mass energy variation of the extracted bunch(es). variation residue << energy spread

• The bunch-by-bunch Feed-Forward Correction (FF) done from DR to EXT is intended for transverse jitter originated in DR.correction residue << beam size

• The Fast Feedback Stabilisation (FB) done in EXT corrects for transverse jitter of each subsequent bunch with reference to previous bunch(es). This system is intended mainly for cancellation of a jitter produced by the extraction kicker.residue << beam size

FF and FB is most required in the vertical plane.

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• ZC gives permission to extract the beam at that turn where the energy oscillation crosses zero. The oscillation is measured with a DR BPM.

• In FF, at start of the last turn, the bunch position and angle are measured with a pair of DR BPMs, and correction is applied to the same bunch in EXT, using a pair of kickers. The propagation time (feed-forward time ) is about 0.4μs.

• In FB, the position and angle of each bunch in a train are measured with a pair of EXT BPMs, and correction is applied to each trailing bunch witn an upstream pair of kickers. The feedback time is about 300(150)ns.

Objective: discussing optimal layouts of the systems, decide the places of additional BPMs in DR and EXT and additional kickers in EXT.

1. FF and FB elements in EXT 2. FF DR BPMs 3. ZC DR BPMs.4. Layouts of some FF and FB prototypes at the present EXT

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A historical slide:

from Tauchi's talk 050225 4

1. FF and FB elements in ATF2 EXT The following requirements and factors are taken into consideration:

• Feed-Forward Correction (FF) and Feedback Stabilisation (FB) should be accomplished upstream of the Diagnostic Section.

• FF and FB are done for both (y, y') and (x, x'). The (y, y') is most required.

• A kicker in each pair has an adjacent pickup that is necessary for FF/FB matrix measurements.

• The FB downstream pickup pair is also used for FF residue measurement.

• The kickers are common for FF and FB. Combining the kick signals is done on the kicker amplifier input.

• A kicker is a single plane strip line kicker. The length is supposed to be about 30cm.

• A pickup is a standard EXT (x, y) strip line pickup. The pickups are shared by FF and FB and some of them by the EXT Orbit as well. 5

(continued)

• A preference grade of the places for a kicker pair or a pickup pair: a drift space, one quad in-between, two quads…

• If a quad in-between is unavoidable, the case a quad between kickers is preferable to the case a quad between pickups.

• Preferable places for BPMs and kickers are where the beta function comes to its maximums.

• The optimal phase advance in a kicker pair or a pickup pair is π/2.

• The middle point between two BPMs of a FB pair should have the phase advance about π to the Extraction Kicker2. Alternatively, one of the BPMs should have the advance about π/2 to the Extraction Kicker 2.

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Vertical FF and FB: • Kicker Ky1 is downstream of QF9, kicker Ky2 is upstream of

QD10 (light green lines). The matrix pickups are adjacent to the kickers. The phase advance is about π/2. A drift space.

• The FF residue/FB pickups (dark green lines): P5y is up-stream of QD10, P7y is upstream of QD12. The phase advance is about π/2. Two quads QD10 and QF11 is in-between.

• No quad is there between the kicker pair and the pickup pair.

Horizontal FF and FB: • Kicker Kx1 is downstream of QF7, Kicker Kx2 is upstream of

EXT K2 (light blue lines). The matrix pickups are adjacent to the kickers. The phase advance is about π/2. One quad QD8 is in-between.

• The FF residue/FB pickups (dark blue lines): P3 is down-stream of QF9, P6 is upstream of QF11. The phase advance is about π/2. One quad QD10 is in-between.

• One quad QF9 is between the kicker pair and the pickup pair.

List of the functions of the BPMs:M is for matrix measurement, R is for residue measurement

• P1x (adjacent to Kx1): FFM+FBM• P2x (adjacent to Kx2): FFM+FBM• P3y (adjacent to Ky1): FFM+FBM• P3x FFR+FBM+FB(+FBR)• P4y (adjacent to Ky2): FFM+FBM • P5y: FFR+FBM+FB(+FBR)• P6x: FFR+FBM+FB(+FBR)• P7y: FFR+FBM+FB(+FBR)

The kickers and pickups can be placed in the following parts of the EXT structure at the downstream distances:

• Kx1+P1: L017A 21.50m• Kx2+P2: L018B 23.88m• Ky1+P3: L102C 25.30m• Ky2+P4: L102C 27.06m• P5: L102D 27.35m• P6: L103C 28.97m• P7: L104C 30.25m

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Some conclusions and observations

• The sqrt beta values at the pickups and kickers are typically quite low, somewhat between 1.1 and 1.5. This makes the requirement to the BPM resolution as tight as 1μm.

• The optimal position of a kicker or a pickup may fall in the place of other device. Combining is possible, as a steering magnet on a kicker/pickup, etc.

• A single plane strip line kicker can be similar to the KEK ILC prototype fitted to the EXT aperture. The length with flanges is taken as 35cm. The pickup length is taken as 15cm. Total length of a kicker+pickup unit comes to 50cm.

• It is supposed that we know how to split the pickup signals precisely.

• Attention should be taken to imbalance of the pair of pickup strip lines. Imbalance may deteriorate performance of the dif/sum BPM. In advance to installation, the pickups should be tested on a coaxial wire bench.

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2. FF DR Pickups

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• Pickup 1 is downstream of Q6 in LQ6C (93.05m), pickup 2 is downstream of Q17 in LZV43 (115.63m).

• Vertical phase advance in the pair is π/2, horizontal one is about π/4.

• Vertical phase advance from pickup 1 to the extraction point is about (-kπ - π/8), horizontal advance is about (-kπ+π/3).

• These FF pickups are supposed to be special DR pickups which are identical to the present EXT x,y strip line pickups.

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3. ZC DR Pickups

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• Two BPMs are used. The readings are:

and

For known, the first term in , that is a betatron jitter can be eliminated to have a pure energy deviation signal.

• The phase advance is about π/6. • Orbit DR pickups are used: LRBPM13.9 and LRBPM2.28.

The signals are combined as (t1+b2)-(t2+b1).

• It is supposed that we know how to split precisely the pickup signals to DR Orbit and ZC.

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sinAx 11

pp

sinAx 22

and , , 211 2x

4. Layouts of some FF and FB prototypes at the present EXT

• While investigation of a FF/FB prototype, the extraction kicker jitter may obscure the results.

• Assume one uses a FB short-cut prototype that stabilises y only but not (y,y'). To neutralise a jitter produced by a spurious vertical component of the extraction kick, a pickup should be used that is distanced by kπ from the kicker.

• The FB kicker should be distanced (-π/2) from the pickup.

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• The FB pickup is ML13. A witness pickup is ML14. • The upstream kicker is placed downstream of QD8 at L5320

(31.0m). The adjacent pickup ML11 can be used to measure the extraction kicker jitter.

The same ML11, ML13 and kicker can be used in a short-cut FF prototype. It can correct for position deviation at a point between the FF DR pickups, that is advanced by (-kπ) to the FF pickup ML13.

List of the functions of the prototypes' BPMs:

W is for witness

• ML11: extr kicker jitter +FBM+FFM• ML13: FB(+FBR)+FFR• ML14: FBW

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