BOOSTER OPTICAL NOTCHER

David Johnson –APC -

INTRODUCTION (1)

The Proton Improvement Plan is tasked with Booster upgrades which will increase the Booster throughput required for future operation while maintaining acceptable losses.

Ultimate goal: 2.2E17 protons/hr @ 15 hz by 2016 Breaks down as: 3.7E15/min -> 6.1E13/sec -> 4.1E12/pulse -> assuming 10 turns (22 us

linac pulse length) -> 4400 linac bunches per cycle -> 9.3E8 per bunch. For numbers sake let’s just use 1E9/200MHz

Notching in the Booster ring deposits the removed bunches in the gradient magnets. This represents on the order of 30% of Booster losses ( few % of beam intensity).

Ultimately one needs about 30 -40 ns notch in the beam at extraction time for the extraction kicker rise time.

In the 200 MHz beam structure this would correspond to ~8 200 MHz bunches every 2.2 us and for 10 turns this would be a total of about 60 to 80 bunches or about 1-2% of injected beam.

Assuming 1E9/bunch this would be 6 to 8E10 /pulse but for simplicity lets assume 1E11 H- need to be neutralized per pulse. At 15 Hz -> 1.5E12/sec -> 100 watts of beam power to be removed.

INTRODUCTION (2)

QUESTIONS: Can we move the notching function out of the ring and provide a

shielded dump for the H0 beam removed? Early notching experiments by Ray (circa 2000 and before)) were

done at 750 keV at the 90 deg bend. They demonstrated a 25-30 ns notch as seen at Tank 2 BPM. With the addition of the RFQ the interaction region would have to be just after the RFQ or at the entrance to tank 1, where there are vacuum windows.

There is another location, just upstream MV2 and MW06, that contains a dipole (MV2) to remove the H0 into a beam bump.

Can we utilize an optical notching system similar to that proposed for Project X?

See Project X docs 909 and 911 on Laser Chopper for Project X.

What are the requirements for this notching system? Laser pulse energy (peak power) Laser system (how can the required laser pulses be generated?) Optical system / delivery system Controls

H- NEUTRALIZATION

Assume the laser beam has a uniform irradiance and fully encompass’ the H- beam.

When the probability of interaction is high, and the interaction mechanism is not dependent on intensity (i.e cross section is not dependent on laser flux) , the single interaction detachment rate, F1, can be expressed as

where fluence is the number of photons per cm2 at the interaction point, tlaser is the laser pulse length, s is the photoneutralization cross section [cm2] and tcrossing is the transit time of the H- across the laser beam [sec].

The photon flux is transformed like the photon frequency (energy) which, which like the energy is ~ the same in the lab and CM.

)1( crosssing**1

fluxeF laserfluenceflux /where

H-NEUTRALIZATION

Assume a laser pulse length , tlaser , of 500 ps, the single pass neutralization fraction as a function of laser pulse energy for several laser beam radii may be calculated.

For ~100% neutralization need to have ~10 to 30 mJ per laser pulse

At 325 MHz rep rate this corresponds to a 3 to 10 MW laser !

At this point everyone says it can’t be done.But can these numbers be beat down ?

)1( **1

fluxeF

Laser Beam Radius

Q-switch10’s to 100’s mJ10-100’s Hz

LOCATION FOR BOOSTER NOTCHER

launch box

optics box

laser transport

vacuum chamber

chute

Q8

Mirror boxes

H0 wastedump

H- to Booster

Current laser profile monitor insert is 1.14 m

*MAX 10 bunches/notch *10 notch/inj*1E9/bunch = 1E11/pulse*15 Hz = 1.5E12 particles/sec-

~ 100 W*

400 MEV LINE BEAM SIZE

MW06

Laser beam size needs to enclose the smaller of the Booster beam dimensions i.e 6*1.3 ~ 7.8mm

Can the linac vertical beam size at MW06 be reduced?

Is this optical model of the lattice accurate? Data: March 2011

PHOTONEUTRALIZATION

To maximize neutralization and minimize the required laser power we want to make sure that the C.M photon energy is near the peak of the cross section.

The lab laser wavelength is dependent on the geometry between the laser and the H- beam.

3

2/30

2/30

max

)(8)(

E

EEEE

Photoneutralization cross section of H- by incident photons of energy E is given by:

where smax is 4.2E-17 cm2 and E0 is 0.7543 eV.

At 400 MeV , b = 0.7128 and g = 1.4258, with ion velocity of 2.138E8 m/s.

LASER AND H- GEOMETRY

Assume the following geometry of the H- and laser beam.

[From: Shafer LA-UR 98-2643]

The photon energy seen by the H- electron in it’s rest frame is

)cos1( Lcm EE Q = 180 : head on interaction

TWO POTENTIAL GEOMETRIES

For multiple interactions Assume 99.95%reflectivity Total neutralization~ N*single

interaction Laser beam size constrained by

smallest H- beam size (i.e. Vertical)

Requires 1 um laser

Single Interaction

Multiple Interactions

For single interaction Interaction length(time) ~x_ laser size/sin(180 - Q) Interaction length must be

smaller than bunch separation Requires 2 um lab frame laser

)cos1( Lcm EE

BOOSTER BEAM STRUCTURE

11

24.2 us (~ 11 Booster turns)

2.2 us

67 ms

~22-25 us

5 ns (bunch spacing)~500 ps 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

If each notch created by a single 30-40 nslaser pulse with a pulse energy of 65 mJ.Implies 10 lasers timed to generate 10 notches each Booster cycle (15 Hz reprate) or a cavity with 2.2 us delay

SEED MODULATOR PULSE STRUCTURE

12

2.2us

50-60ns

~22 us

~500 ps

5 ns

~1 ns

~500-600 ps

~100-200 ps

Laser temporal profile for a single bunch

Burst of 6 to 8 laser pulses at 200 MHzThis provides about 4 ns between laser pulses (or reduces requirements on rise/fall time of laser pulse)

CW 100 mWNarrow line width Seed laser

Pulse patterngenerator

Fiber Amp

Pre-AmpGain ~ ?

10GHz modulator

Amplifier Gain ~ ?

Coupling OpticsZig-zag cavity w/40 reflections

Input pulse to cavity:~1 -3 mJ, 0.5ns, burst mode

100 pJ (0.5 ns) 200 MHz

10-100 nJ (0.5ns) 200 MHz

Couple to free spaceoptics

1-3 mJ (0.5 ns) 200MHz/450 kHz burst

BOOSTER NOTCHER CONCEPTUAL SYSTEM

TIMING Card /lock

Approx 1 m length

450 kHz bursts

<P200MHz> 2 to 20W<P450kHz> 46 to 460 mW

Top Hat Converter

r=5mm 1/e Ppeak=3mJ/.78 cm2 in 500 ps = 3.8 mJ/cm2 in 500 ps MAX

H- PARAMETERS

Energy 400.000000 [MeV] Linac current 30 [mA] Linac bunch intensity 9.355704e+008 [ppb] Beta 0.712832 Gamma 1.425852 H- velocity 2.138495e+008 [m/s] RFQ frequency 200.000000 [Mhz] Bunch spacing 5.000000 [ns] Bunch length +/-19.500000 [degrees] Bunch length (full) 11.583514 [cm] Bunch length (full) 0.541667 [ns] RMS beam size: H 0.13 [cm] V 0.13 [cm] 100pct beam size: H 0.78 [cm] V 0.78 [cm]

LASER PARAMETERS Laser Energy 1.000000e-003 [Joules] Pulse length 5.958333e+002 [ps] Laser beam area 7.361640e-001 [cm^2] Spot size: H 0.86 [cm] V 0.86 [cm] Photon wavelength 1030 nm Lab Frame Photon Energy 1.931068e-019 [Joulse]

1.204433e+000 [eV] Photons per pulse 5.178482e+015 Photon rate 8.691158e+024 [photons/sec] Photon fluence 7.034413e+015 [photons/cm^2] Photon flux 1.180601e+025 [photons/cm^2/sec] Laser Peak pulse power 1.678322e+000 [MW]

LASER REQUIREMENT FOR EACH NOTCH Distance between crossings 2.000000e+000 [cm] Distance between reflections on mirror 4.000000e+000 [cm] distance laser travel between crossing 1.402856e+000 cavity radius 5.000000e+000 [cm] Incident angle 1.130993e+001 [degrees] Crossing angle 78.69 [degrees] Lorentz Factor 1.226521e+000 Rest frame photon energy 1.477263e+000 [eV] Rest frame wavelength 8.397737e+002 crossing time per interaction 2.045812e-010 Number of passes 40.000000 total crossing time ~ 8 ns Cross section in rest frame 4.197165e-017 [cm2] Photon flux in H- rest frame 1.448031e+025 [photons/cm^2/sec] Mirror Reflectivity 9.995000e-001 Average Neutralization factor per interaction 1.147403e-001 Total Neutralization 9.923647e-001 Multipass insertion length 7.800000e+001 [cm] Total laser path length 1.094227e+000 [m] Peak laser pulse power 1mJ/600ps : 1.6 MW Average 200MHz power of laser 8*1mJ pulses per 40 ns 200.000000 [kW] Average power of laser for each injection (10 notches in 22 us) 3.6 kW Over all average power of laser system at 15Hz ~ 1.2 W

EXAMPLE Q-SWITCH LASER (LPM)

50 mJ “10 ns pulse”

EXISTING LASER WIRE INSERTION

MV2 SEPARATION MAGNET

DOWNSTREAM QUADS

DOWNSTREAM END OF MV2 DIPOLE

ISSUES/COMPONENTS

Beamline Will both H0 and H- fit through the downstream end of MV2

or do we need larger aperture? initial look seems OK Will H0 beam miss the first quad downstream of MV2 or is a

lattice modification necessary? To be determined Need to support downstream quads with new stands. Just

engineering, no show stopper Size of H0 dump (similar to MTA absorber) maybe 12”

diameter and 30 “ long. Should fit under downstream quads. Notcher

Laser system (seed + amplifier) Zig-zag optics (in vacuum) Controls