Activities at the PITZ facility

Igor IsaevLaPlas 2017NRNU MEPhI, 2017.01.25

> The PITZ is a dedicated site to develop and test RF guns for high brightness beams including all subsystems:gun cavity, cathodes, interlock system, water cooling system, RF feed system, photocathode laser, electron beam diagnostics, ...

> PITZ goals: Development of an electron source for the European XFEL:

- very small transverse emittance (

PITZ facility

7MeV/c25MeV/c

Facility parameters> RF photoelectron gun > Booster> Diagnostics:

slit scan (transverse phase space) streak camera, TDS, dipole (longitudinal

phase space) screen stations (beam shape) tomography (transverse phase space)

Parameter ValueBeam bunch charge, nC 0.001 .. 4Beam momentum after gun / booster, MeV/c 7 / 25

Number of pulses in a train ≤800

Repetition rate, Hz 10

Maximum average beam current, µA ≤32Optimized emittance (1nC ), mm mrad New developments (e.g. plasma acceleration)

Photoelectron gun setupPITZ photoelectron gun setup consists of:> RF cavity

L-band 1.6-cell copper (OFHC) cavity Dry-ice cleaning → low dark current ( Solenoids Dedicated for emittance compensation Max. on-axis field ~0.3 T (500 A in the main solenoid) Bucking solenoid for compensation of field at cathode

> Photocathode laser Pulse train structure Micropulses temporally and spatially shaped

Gun parameter Value

Max. accelerating gradient at the cathode, MV/m 60

Frequency, MHz 1300

Unloaded quality factor ~20000

Beam momentum after gun, MeV/c 7

RF peak power, MW 6.5

RF pulse duration, µs ≤650

Repetition rate, Hz 10

RF cavity water cooling channels(Gun 4 type)

Three designs of the cathode area

Watchband spring design

Mo plug

CuBe spring

Cs2Te cathode

Watchband Reloaded

spring design

Mo plug

CuBe spring

Cs2Te cathode

Contact stripe design

Mo plug

CuBe spring

Cs2Te cathode

RF gun

Gun 4.6

Ramp-up procedure:• RF power increase by steps of max 0.2 MW every 15 min• vacuum pressure < 10-7 mbar• In case of significant vacuum or other trips:

• re-ramp RF power from 0 with short pulses (10 µs)• Initially, the rf gun solenoid is off (then sweep)

Conditioning of the Gun 4.6

PMT

e-det

PG

pump

dir.cpl.

valvestemp.

PMT PMT

PMT

PMTse-det e-det

IR IR

PMT

IGP

IGP

IGP IGP

dir.cpl.

dir.cpl. dir.cpl.

dir.cpl.

temp.

temp.

temp.

temp.

New developments and technologies3D Ellipsoidal Laser (ELLA) (collaboration with IAP)

Plasma acceleration

THz radiation generation based on PITZ facility

> Fiber-based laser oscillator and amplifier> Multi-pass diode pumped disk amplifier with Yb:KGW crystals> SLM-based spatio-temporal shapers in 4f zero-dispersion layout> Second and fourth harmonic converters> Scanning auto/cross-correlators (diagnostic)

Status:> Laser is installed > First photoelectrons are produced> Drift correction to be implemented> Adjustments to be done

The plasma cell is designed as a heat pipe with metal grooves inside the oven acting like a wick.Results of 1st experimental run:•transverse defocusing was observed•time resolved measurement show transverse oscillations•energy spectrum undergo widening •energy modulations via longitudinal phase space measurements were observed•plasma cell still needs further improvements

Motivation: To be a prototype facility for a proposal to use IR/THz radiation

generated by a “PITZ-like” accelerator together with X-rays from the European XFEL for pump-probe experiments.

Studies of radiation-based electron beam diagnostics.

Outlook for the first half of 2017:•CTR station design and installation•First experimental generation of THz CTR at PITZ

> Optimum machine parameters: experiment ≠ simulations

> The electron beam asymmetry was observed during emittance measurements at PITZ

Electron beam asymmetries

E-beam x-y asymmetry (tails)

Emittance vs. Imain Bunch charge vs. laser energy

Qsimul.≠Qmeas.

Emittance vs. laser XYrms

?

Phase space of the electron beam

X phase space

Y phase space

Optimum: Imain≠MaxB

Coupler kick

> RF field simulations (CST MWS): The full model of the gun:

− the gun cavity− the coupler − simplified cathode

Frequency domain solver (F-solver) Tetrahedral mesh (~106 elements) with 2nd order curved elements Half structure symmetry

E-field distribution: H-field distribution:

Larmor angle experiment

Measurements (29.09.2015M-A):

• Pgun=5MW (6.1MeV/c max)

• Launch phase: MMMG• Cathode laser:

Gaussian 11.5 ps FWHM (expected)

BSA=1.2mm (VC2)• Charge 0.5 nC

Beam at High1.Scr1 Main solenoid current is 361 A, opposite polarity, bucking current is 0

angle1angle2

I main = - 361 A, I bucking = 0 A

I main = + 361 A, I bucking = 0 A

“Tracking back” towards cathode (M.Krasilnikov)Cathode Z=0.18m EMSY

I mai

n =

+361

AI m

ain

= -3

61A

The kick at z=0.18m is oriented by 45°.

Could it be described by a skew quadrupole?

Quads like field from coupler kicker or/and solenoid

#1 David Dowell, Analysis and Cancellation of RF Coupler- induced Emittance Due to Astigmatism. LCLS-2 TN-15-05 3/23/2015.#2 John Schmerge, LCLS Gun Solenoid Design Considerations. SLAC-TN-10-084.

Main idea:1. the kick optics can be modeled as a rotated quadrupole with focal length and rotation angle

given in terms of the (complex) Voltage kicks. 2. a rotated quadrupole near the coupler is effective at compensating for the kick, cancelling both

the coupler emittance and the astigmatic focusing.

Simulations: with assumed skew quadrupoles fields at z= 0.18m. These assumptions are also based on other beam asymmetry studies (Larmor angle experiment).All ASTRA simulation set up are same with experiment set up, beam momentum and solenoid current.

Experimental setup:Pgun=5MW , 6.178 MeV/c, gradient is 54.2 MeV/c, 500 pC no booster 05.09A-06.09N.2015.

/skew quadrupoleQ_type(1) = 'skew',Q_length(1)=0.01,Q_K(1)= XXX,Q_pos(1)=0.18,

Simulations with rotation quads model (Q.Zhao)

Q_length(1)=0.01,Q_K(1)= +-0.6,Q_pos(1)= x.xx,Q_zrot(1)= y.yy

Use rotation quads model in ASTRA simulation by scanning the rotation angle and z position. Find the parameters for beam images at High1.Scr1 to fit the experiment images, the direction of the

beam wings for both solenoid polarity. 2D-3D space charge used in ASTRA simulation, z_trans=0.12m.

I main = +361A

~13 degree ~78 degree

Images table from simulation analysisI main = -361A

Summary of the simulations: Position: around z=0.18m Rotation angle: Skew quads: 45 degree( negative polarity) / 135 degree( positive polarity). Polarity: same, not effected by solenoid field polarity.

Position: around z=0.34m Rotation angle: Normal quads. Polarity: when change the solenoid polarity, the quads polarity also changed.

First design of the Gun Quad (4 coils)Parameters:• Aluminum frame• 0.56 mm copper cable• 180 windings per coil• 2 thermal switchers (80 degC max)• Non-magnetic screws• Fixed by radiation-hard cable tie• Q_grad = 0.0207 T/m @ 1A

Influence on the transverse beam shapeFocused Beam @ High1.Scr1Imain = 335A, Positive polarityGun.Quad is OFF

Rounded beam @ High1.Scr1Imain = 335A, Normal polarityGun.Quad 0.5A, Normal oriented

Influence on emittance

• One quad was able to compensate beam asymmetry, but only for one solenoid polarity• The compensation of the beam asymmetry for both solenoid polarities two quads are needed• Emittance does not get smaller but more equal

Beam size

Emittance

Red- X value, Blue - Y value, Green - XY value

Second design of the Gun Quad (8 coils)Parameters:• Combination of a normal and a skew quads• Aluminum frame• 0.56 mm copper cable• 140 windings per coil• 2 thermal switchers (80 degC max)• Non-magnetic screws• Fixed by radiation-hard cable tie• Q_grad = 0.0117 T/m @ 1A

Influence on emittanceBeam @ LEDA Beam @ High1.Scr1without Gun.Quads

with Gun.Quads

Beam is tilted

No beam tilt

Sol.pol.=Positive Sol.pol.=Negative

Sol.pol.=NegativeSol.pol.=Positive

Xemit=1.60mm mrad

Xemit=1.34mm mrad

Yemit=1.23mm mrad

Yemit=1.47mm mrad

XYemit=1.41mm mrad

XYemit=1.41mm mrad

• Possible to compensate beam asymmetry for all solenoid settings• LEDA tilt can be compensated• Emittance more symmetric, but not smaller

Current & future PITZ activities

A brighter & more robust electron source for FLASH & XFEL➢Gun 4.6 conditioned with higher gradient & stability (2 DESY RF windows)➢Generation of 3D ellipsoidal electron bunches➢Measurements and beam dynamics (e.g. slice energy spread or➢projected/slice emittance vs gun gradient and vs laser shape)➢Studies of beam imperfections and charge stability in bunch train➢Gun 5 development

New accelerator technology and its application➢Self modulation➢Transformer ratio➢Lab Astrophysics➢Experiments with dielectrics➢THz studies

Thank you for your attention.

Slide 1Slide 2PITZ facilityPhotoelectron gun setupSlide 5Slide 6MotivationCoupler kick studiesLarmor angle experimentQuads like field from coupler kicker or/and solenoidSimulations with rotation quads model (Q.Zhao)First design of the Gun Quad (4 coils)Second design of the Gun Quad (8 coils)Summary and OutlookSlide 15