progress in barrier stacking

MAP Meeting, IUCF March 12-13, 2007 1

Progress in Barrier Stacking

W. Chou, J.Griffin, K.Y. Ng, D. WildmanFermilab

Presented to MAP MeetingIUCF, Indiana

March 12-13, 2007


Content of Talk

• Motivation• Method• Simulation• Experiment


Fermilab Accelerator Complex


Booster – the Bottleneck

• The Booster is a 30 years old machine has never been upgraded.

• The 400-MeV Linac can deliver 25 x 1012 particles perBooster cycle.

• The 120-GeV Main Injector can accept 25 x 1012 particlesper Booster cycle.

• However, the 8-GeV Booster can only deliver 5 x 1012

particles per cycle.


Solution — Stacking

• A solution is to stack two Booster bunches into one Main Injector RF bucket.

• This is possible because the much larger momentum acceptance of the Main Injector.

(bucket width = 18.9 ns) Booster MI

Mom. Acceptance

0.13 eV-s (±11 MeV)

0.70 eV-s (±58 MeV)


Stacking Goal

• Goal for Run II – To increase protons per second (pps)on the pbar target by 50%– Baseline: 5 x 1012 every 1.467 sec– Goal: 2 x 5 x 1012 every 2 sec

• Goal for NuMi – To increase pps on NuMi target by 60%– Baseline: 3 x 1013 every 1.867 sec– Goal: 2 x 3 x 1013 every 2.333 sec

• Slip stacking can raise proton intensity from 5.0 x 1012 per batch to 7.0 x 1012. (K. Seiya, et al., PAC’05)

• We are going to study barrier stacking here.


Barrier Stacking by J. Griffin

Booster batch injected off-energy so that top of batch slips 42 bkts per booster cycle.

Barrier moves to left at 42 bkts per booster cycle.

After 1 booster cycle, first batch passes front of barrier.

2nd batch is injected 42 bkts from 1st batch.

Strength of barrier is determined by δf1 = δf2

.This is the only parameter in the model.No solution if energy spread is too large.


• For the injection of Booster batch into MI, allowablemaximum energy spread is ΔE = ±4.90 MeV.Corr. integrated barrier strength VT1 = 3.142 kV-µs.

• Booster bunch area: ~0.10 eV-s, bucket width: 18.9 ns.

• If completely debunched, ΔE = ±2.64 MeV.• For the bunch filling whole bucket,. ΔE = ±4.15 MeV• If a harmonic cavity is installed, booster bunch can be

lengthened with ΔE reduced.

• E.g., bunch at Vrf = 5.0 kV and a 3rd harmonic cavity reduces ΔE to ±5.18 MeV.

• E.g., bunch at Vrf = 4.8 kV and a 2nd harmonic cavity reduces ΔE to ±4.56 MeV.

• If ΔE can’t be reduced, method still works if barrier is allowed to move faster. However, this will reduce the number of batches to be injected.


Simulation of Stacked Injection

1st batch injection 2nd batch injection


3rd batch injection 4th batch injection


6th batch injection5th batch injection


8th batch injection7th batch injection


10th batch injection

9th batch injection


After 12th Booster cycle After 13th Booster cycle

Best time to re-capture


Hardwares

• Task: To build two ±8 kV wideband RF cavities. (i.e., the barrier RF)

• There is no low-level RF. The on-and-off of the RF voltage is handled by a high voltage solid-state fast switches made by Behlke Co. (German).

• These fast switches have been applied to the design of an RF chopper built at Chiba by a KEK-Fermilab team.

W. Chou, et al., Design and Measurements of a Pulsed Beam Transformer as a Chopper, KEK Report 98-10 (Sep. 1998).


Finemet Cavity as a Chopper(installed on the linac of HIMAC in Chiba)


Finemet Core(a nanocrystal magnetic alloy patented by Hitachi)


High-Voltage Fast Switch(MOSFET Switches made by Behlke Co.)


The Broad-Band Barrier RF Cavity

W.Chou, et al., Barrier RF System and Application in MI, PAC’05


Building of the Barrier RF System

Switch


Building the Barrier RF Cavity


Testing a RF Cavity

One barrier Two barriers per MI period


Barrier Stacking Experiment

Normal Inj. from Booster to MI at frf = 52,811,400 Hz.Injection is on-energy. No drift at all. Barrier is off.2nd batch injected 84 bkts from first batch.Mountain-view is 256 MI turns per trace.


Off-Energy Injection with Barrier Off

Inject at frf = 52,812,014 Hz (614 Hz > nominal).

Booster above transition, so beam energy <

nominal. 2nd batch injected 42 bkts from the first.


Computation of ΔE offset

• (Δfrf/frf)B = 1.163x10-5

• Booster slip factor ηB= 0.022436

• Mom offset Δp/p = -ηB-1(Δfrf/frf)B= 5.136x10-4

• Energy offset ΔE = -4.54 MeV

• However, once inside MI, which is at ηMI= -0.008888,

beam revolves at a lower frequency than nominal:


clock

beam

11.56 MeV

4.54 MeV

Because there is no low-level RF, the barrier and the mountain-view will be at the locked RF frequency.

The movement of the barrier can be accomplished by adding a delay.

frf = 52,812,014 Hz

frf = 52,811,400 Hz normal


Turning on Barrier on the Right Side

The beam is seen reflected from barrier on the right.


Stationary barrier One barrier moving

Barrier trigger = Mountain view = 52,812,014 Hz

Adjusting Barrier Position and Speed


Moving Barrier, 4 Pulses, No Bunch Rotation

Mountain view = 52,812,014 Hz, frf = 52,812,014 Hz

Consecutive batch spacing 42 buckets

Final beam width of 4 pulses only ~3.5 μs, half of that w/o barrier

3.5

μs (unstacked 4 batches: 6.36 μs)






(unstacked 6 batches: 9.54 μs)

5.5 μm

Some reflected beam catches up with moving barrier.





Moving Barrier, 6 Pulses, with Bunch Rotation

(unstacked 6 batches: 9.54 μs)

5.5 μs

Some reflected beam catches up with moving barrier.





(The 8 injections were lousy but no time to improve it)


Recapture (no bunch rotation)

Mountain view = 52,812,016 Hz,

frf = 52,812,016 Hz

2nd batch 42 bkts from 1st injection

Capture Vrf = 850 kV in ~45 ms


First Few Turns of the First Batch


No bunch rotation


First Few Turns of the First Batch


With bunch rotation, B:BRLVL = +8.7

Not as dramatic as expected.

Capture result almost the same.


beam

11.59 MeV

4.55 MeV

Barrier width is fixed at T1 = 0.3 μs, height is reduced gradually from V = 12 kV until beam leaks out.

is confined,

from which beam’s energy spread ΔE can be inferred.

frf = 52,812,014 Hz clock

frf = 52,811,400 Hz normal

beam

Beam’s Energy Spread

ΔE

ΔEtotal

barr

ier V

T1


With BR, 9.5 kV —> ΔE = 6.42 MeV With BR, 9.0 kV —> ΔE = 5.77 MeV

Thus half energy spread is 5.77 MeV < ΔE ≤ 6.42 MeV But with BR off, need 11 kV to avoid leakage, ΔE ≤ 8.14 MeV. Thus bunch rotation works, although not dramatically.


It is hard to imagine ΔE > 13.15 MeV for 6-turn beam.

We are told that it should be from 8 to 12 MeV.

With BR, 2-turns injection11 kV —> ΔE > 8.14 MeV

With BR, 6-turns16 kV —> ΔE > 13.15 MeV


Re-capture Results

First 3 turns After capture

Vrf 786 kV 926 kV

Full Bunch Length 5.10 ns 14.0 ns

Half Bunch Height 0.025 V 0.0111 V

Bunch Area (xconstant)

0.099 eV-s 0.807 eV-s

Height x Width 0.128 V-ns 0.156 V-ns

Bunch area increases 8.15-fold.

Amount of charge captured proportional to Height x Width, or 0.156/0.128/2 = 61%.


Recapture

Vrf= 850 kV, frf= 52,811,400 Hz

bunch rotation (yes or no?), 6 Booster turns

Large beam loss. Maybe ΔE is much larger and cannotpenetrate the moving barrier. Beam passes through reflecting barrier on the right; strength of that barrier is not large enough.


Moving barrier: 6 kV, 0.3 µs, integrated strength 1.8 kV-µs. Barrier is not strong enough to accel. top of beam to +ve energy. Final energy spread is large —> beam loss in recapture. At this moving rate, barrier V can increase up to V = 8.69 kV.

frf = 52,811,400 Hz nominal

frf = 52,812,014 Hz clock

4.6 MeV

11.6 MeV

6.4 MeV

17.5 MeV

17.5 MeV

beam

0.15 MeV

Then final half spread is ΔE = 15.0 MeV. This can be further reduced by increasing V and let barrier move faster.

beam


• We have been successful in– injecting into MI off-energy,– setting a barrier moving at a prescribed rate,– stacking so far up to 8 booster batches into a width of

~ 4 batches,– re-capturing the stacked beam, although with large

increase in bunch area and large beam loss.

• Future improvement:– Better understanding of the beam and RF maneuvering.– Improvement in bunch rotation in Booster so as to reduce

ΔE, which is the source of beam loss in recapturing.– To built a low-level RF, if possible, so that barrier and

mountain view can be referenced to MI nominal frequency.– Study with more intense beam and more batches.

Summary

progress in barrier stacking

Documents

injection of booster

batch slips

griffinbooster batch

gev booster

booster bunches

bottleneckthe booster

booster cycleafter

booster bunch area