Linac Laser Notcher David Johnson, Todd Johnson, Matt Gardner, Kevin Duel April PMG Meeting April 2, 2015

Topics Motivation Concept Laser Requirements System Design Demonstration Experiment Status 2

Motivation Reduce losses associated with the “Notching” process – Current Notching process takes place in the ring – Notching occurs 400 MeV for non-cogged cycles 700 MeV for MI injection Move losses & notching out of Booster tunnel  Move to 750 KeV where activation is not an issue  Reduce uncontrolled loss power from 300W to 17 W 300W is calculated from 15 Hz at 700 MeV notch 17 W is calculated from a 15 Hz at 400 MeV cleanup, assuming: – 90% Linac Notch – 10% Booster Beam notch cleanup Where to install this device in the 750 keV MEBT ? What technique do we use? 3

Linac 750 keV Medium Energy Beam Transport TANK 1 RFQ Test laser ~2 ½ “ ~30 “ 4

Move Notching to Linac Beam Requires creating N +/- 1 notches in each 15 Hz Linac pulse – N is the number of turns injected into Booster Each notch ultimately needs to be ~ 60 ns in length (determined by extraction kicker rise time) Separation of notches is determined by Booster injection revolution frequency (~450 kHz) 67 ms 2.2 us*N ~60-90 ns 2.2 us 15 Hz Linac pulse structure Single Linac pulse 5

Utilize Laser to Photoneutralize* the H- Rest frame energy 1.16 eV for 1064 nm laser Cross section 3.66x cm 2 To maximize the neutralization probability->maximize the product fixed 6 To maximize  for a given H- ion energy  increase the number of laser interactions with the ion bunch *Proposed & demonstrated by Ray Tomlin, circa 2000  Laser pulse energy required to neutralize a single 60 ns section of linac pulse  Must be repeated N times within a single linac pulse Requires ~1J/notch

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. 7 H- Linear cavity (zig-zag)  Laser follows ion to interact many times (increase  )  Cavity length proportional to number of interactions  Cavity dimensions determined by ion velocity and bunch spacing R. Shafer, 1998 BIW Assume mirror reflectivity 99.95% 20 reflections  This still requires ~ 100 mJ laser at 450 kHz  (Not a small laser system)

Further Reduction of Laser Energy  Laser energy illuminating the space between H- bunches is wasted and only adds to laser average power  To further reduce the required laser energy -> match the laser temporal pulses to linac bunch structure 5 turn Booster injection 2 mJ Energy required per bunch for a 21 interaction cavity  Where do we put this interaction cavity? 8

Linac Beam Direction Laser IN Laser OUT RFQ Linac Beam Direction 9 1 st MEBT Quad

10

H-H- 11

Laser Requirements  Laser pulses much match bunch frequency  MHz  Laser pulse length (duration) = or > bunch length  Laser pulses must be phased with RFQ  All ions in bunch should see same photon density  Uniform temporal profile  Uniform spatial profile  The burst of laser pulses must match the Booster injection revolution frequency (~450 kHz)  The 450 kHz burst must have appropriate timing within the linac pulse  do we want to clean up head/tail  where within the pulse do we want notches (beginning or end of each turn ?)  The pulse energy should neutralize > 99% ( my GOAL) of ions in each bunch 12

System Design Technique (M aster O scillator P ower A mplifier design)  Utilize a CW seed laser and wave-guide modulator to create required laser pulse pattern (both 200 MHz and 450 kHz) at low pulse energies (pJ)  Amplify pulse pattern using a three-stage fiber amplifier (nJ to uJ)  Further amplify using two free-space solid state amplifier modules (mJ)  Create a spatially uniform photon beam  Insert laser pulse into a linear zig-zag interaction cavity where the laser reflections inside the cavity match the ion velocity 13

Fiber Amp 2 MDC Bias Control DA12000 AWG RF Amp Seed Modulator Fiber Pre-Amp Fiber Amp 1 Ext. clock sync to RFQ Pump VDRIVE supply RBA Controller REA Controller Fiber Port Pump REA Amp Pump RBA Amp Fermi OPG PriTel Yb FA Optical Engines PCF FA Free Space Osc. Keep Alive Trigger sync to RFQ Matching Optics Beam Stacker Transport Optics 15 Hz CW E=50 pJ P=24mW =300 uW E= 200 nJ P= 100W = 1.5W E= 3 nJ P= 1.5 W = 21 mW E= 6.5 uJ P= 3 kW = 46 W E= 2 mJ P= 1 MW 450 = 14 KW 15 = 7.7 W ^ ^ ^ ^ ^ 14

Demonstration Experiment (1) 1.Demonstrate the neutralization of a single bunch at the exit of the RFQ using the installed optical cavity. 2.Measure the neutralization fraction as a function of laser pulse energy to compare with the predicted values. 3.Measure the vertical ion beam size to determine the optimal vertical size of the laser pulse to maximize neutralization. 4.Refine installation details of the transport system and dump system. 5.Get practice in and understand the tuning process for launching the laser into the cavity and the transport to the dump for energy and profile monitoring. Todd Johnson Matt Gardner Vic Scarpine Rick Tesarek Andrea Saewert Mike Kucera Kevin Duel Jamie Santucci Fred Mach High Rise & PPD Machine shops Matt Quinn Dave Baird Ray Lewis Fernanda Garcia, Bob Zwaska, Bill Pellico Since laser system still under design -> substitute laser (Quantel 100 mJ Nd:YAG Qsw.) -> laser spatial parameters VERY diff. from diode -> build optics to match laser  Proposed-mid Nov cm100 cm 214 cm TM11 15

D.E. Layout (space challenge) 16

Demonstration Experiment (2) PD signal Zoom Notch Errors are in rms Efficiency error sources Baseline uncertainty (~5%) Pulse amplitude uncertainty (~5%) Effective laser pulse energy error sources Temporal and geometric matching to beam pulse  Laser system produced Notch in linac beam – Jan 8, 2015 Increase Vertical laser dimension from ~6 mm -> 10mm Consistent with later simulation and measurement 17

Status We are back to where we were this fall. We learned a lot from the Demonstration experiment – Measured extinction agrees with expectations – Space is tight (which we already knew) re-thinking cart orientation – Vertical H- beam size larger than simulations and first reported, which we can accommodate – Learned a lot about technique for getting laser into/out of cavity – Allowed us to firm up what we want to use for Laser Dump diagnostics/monitoring – Got experience working with ES&H and what will be required to install class 4 laser system in a public area Certifying operation of fiber amplifiers Developing instrumentation for laser beam monitoring We have all major amplifiers and optical components Installation planned this summer shutdown 18