Laser Notcher Pulse Energy Requirements & Demonstration Experiment

David Johnson, Todd Johnson, Vic Scarpine

Linac Beam Bunch Structure

24.2 us (~ 11 Booster turns)

2.2 us

67 ms

~22-25 us

~5 ns (bunch spacing)~bunch length 200 MHz bunch spacing

450 kHz “notch” spacing

15 Hz macro pulse rep rate

MACRO

MINI (11 turns)

MICRO

2.2 us

10-12*5ns = 50 -60 ns

2

Cross Section

From: Broad and Reinhardt “One-and two electron photoejection from H-: A multichannel J-matrix calculation”. Phys. Rev. A14(1976 2159-2173

02004006008001000120014001600180020000

0.5

1

1.5

2

2.5

3

3.5

4

4.5

x 10-17 H- Photoneutralization Cross Section

Photon wavelength in CM frame [nm]

Cro

ss s

ectio

n [c

m2 ]

Laser wavelength 1064nm

Photoneutralization (1)

)1( crosssing)*(*

01

EfCMeNNF

H

gcros v

Wx 1

sinsin

)cos1( LABcm ff

)1( sin)(

)cos1(2

cWx

EWxWyrhc

E

neutlaser

LABlaser

eF

)1

)((WxWyhc

Ef

laser

LABlaserLAB

The fraction of electrons that are detached from the moving H- ions is:

The photon flux (generated by the laser) in the lab frame [photons/cm2/sec]

The photon flux in the lab frame is transformed into the rest frame of moving ion as:The interaction (crossing) time is just the ion path length/ ion velocity

The neutralization factor for an ion crossing on axis of the laser beam may be written in terms of lab frame parameters

4

Photoneutralization (2)

Rest frame energy 1.16 eV for 1064 nm laser

Cross section 3.66x10-17 cm2

3

2/30

2/30

max

)(8)(

E

EEEE

To maximize the neutralization probability->maximize the product eractionCM Ef int)( fixed

5

To maximize increase pulse energy -> minimize reduce pulse length -> bunch length reduce laser spot size -> ion size

CMf

To maximize t for a given H- ion energy increase horizontal laser beam size or increase the number of laser interactions with the ion bunch

20-30 mJ

Assume:Laser size 0.6x6mmAssume:

Laser size 0.6x6mm

~1J

1.5 ns 60 ns

To maximize increase pulse energy -> minimize reduce pulse length -> bunch length reduce laser spot size -> ion size

Reduction of Pulse Energy• To reduce the required pulse energy we can effectively increase the

interaction time by utilizing an optical cavity such that the laser interacts with the ion beam multiple times.

6

H-

Linear cavity (zig-zag) Laser follows ion to interact many times

(increase t) Cavity length proportional to number of

interactions Cavity dimensions determined by ion

velocity and physical space Reduces required laser pulse energy by ~

number of interactions

NN FF )1(1 1

)1( **1

fluxeF R. Shafer , 1998 BIW

2 mJ for 99.92%

Assume mirror reflectivity 99.95%

20 reflections

PREVIOUS WORK

• In 2001, Ray Tomlin reported on a successful experiment to create notches in the CW linac beam in the H- 750 keV beam line and have this notch survive at Booster injection.

• He created a 25 ns notch utilizing a 200 mJ 5 ns laser pulse.

• To create multiple notches in the linac pulse he proposed a

– “4 pass bow-tie cavity some 665 meters long (with a storage time of 2.22 m s)”. Large Pockels cells would shuttle the laser to and from the delay line.

• A disk gain section would restore optical losses. • Average laser pulse energy of 103 mJ • Peak power of ~20 MW to strip 99.9% of the ions.• This was not implemented.

8

First Laser Notching Experiment

New Demonstration Experiment

Demonstration Experiment

• Purpose: To demonstrate the creation of a single bunch notch in the H- pulse train and measure the efficiency as a function of pulse energy.

• Equipment:– Quantel Q-switched pulse laser (1064 nm) 100 mJ in 8 ns with 4mm beam diameter.

Repetition rate 20 Hz– Variable attenuator 0-100% attenuation– Optical telescope to match laser to beam shaper– Beam shaper– Transport optics– Steering transport (piezo-mirrors)– Laser mounting system (need to get floor fixed & 3 point adjusters for optical table– Timing. We need to be able to adjust timing of laser flash lamp & q-switch to hit a bunch– Diagnostics (WVM/BPM) to measure single bunch extinction– ES&H - requirements for operation in Linac

What do we expect?

Final design

Demonstration

2 mJ

Wall Current Monitor in 400 MeV lineButton BPM (one plate ) LPM

Can We see only 1 bunch Neutralized?

64 averages