A Polarized Electron PWT Photoinjector

David Yu

DULY Research Inc.

California, USA

SPIN2004, Trieste, Italy 10/14/04

The DULY Team

David Yu

Al Baxter

Marty Lundquist

Yan Luo

Chen Ping

Alexei Smirnov

*Work supported by US DOE Phase II SBIR

DULY Plane-Wave-Transformer (PWT) Photoinjectors

• S band (2.856 GHz) PWT fabricated and installed at UCLA

• X band (8.65 GHz) PWT design• S band (2.856 GHz) polarized electron PWT in progress• S band (2.998 GHz) high rep rate PWT preliminary design• Scaleable to L band (1.3 GHz) for ILC

US patent numbers 6744226, 6448722, 6025681, others pending.

PWT Photoinjector Applications

• High quality (polarized) electron source for nuclear physics

• Low emittance (polarized) electron source for future linear colliders

• High brightness, short electron bunches for advanced light sources (FEL, XFEL, synchrotrons)

• Exotic applications (e.g. Compton x-ray source)

Linear Collider Requirements for a Polarized Electron Source

• See J. Clendenin talk (SLAC) (Friday pm)• High polarization (Strained GaAs photocathode)• High peak brightness (Laser intensity, pulse

shape and wavelength; emittance compensation)• Multi-bunch operation (Low dark current– ultra

high vacuum, surface preparation; low gradient)• High rf pulse and/or beam bunch rep rate

(Effective cooling required to remove heat)• L-band SC linacs recommended for ILC

Parameters of an S-Band PWT Polarized Electron Gun

Charge per Bunch (nC) 0.1 1.0 2.0

Frequency (MHz) 2856

Energy (MeV) 9.5 17.4 18.3

Normalized RMS Emittance

(mm-mrad, no thermal emit.)

0.34 0.5 1.5

Energy Spread (%) 1.1 0.6 0.95

Bunch Length (rms, ps) 2.3 2.8 2.3

Peak Current (A) 12.5 103 251

Linac Length (cm) 58

Beam Size (rms, mm) 1.59 2.1 2.0

Peak Magnetic Field (Gauss) 778 1376 1358

Peak Electric Field (MV/m) 30 55 60

Peak Brightness (1014 A/m2-rad2) 2.1 8.2 2.2

PWT RF Design

SW 10+2/2 cell structure 11 rod-supported disks TEM-like mode outside

TM01-like -mode on axis

PWT Magnetic Design

Principle of Emittance Compensation (a la B. Carlsten, J. Rosenzweig and L. Serafini)

Focusing solenoids and magnetic flux

Longitudinal field vs axial distance

Parmela Beam Dynamics Simulations with and without Matching (Q = 1 nC)

Matching TW linac (red bar) Accelerating field = 25 MV/mInjection phase = 0 degree

PWT (blue bar) onlyPeak field=55 MV/m

Rmax=1.8 mmBunch length=10 ps

Parmela Simulations for Ultra-Short Bunch (Q = 10 pC)

Initial beam spot size = 2 mm, bunch length = 30 fsAt beam waist, spot size=0.4 mm, bunch length=50 fs

Cathode

Cathode

1st Iris 1st Iris

1st Iris1st Iris

from first iris to cathoderf peak field=55 MV/m, 90 deg

from cathode holder to 1st irisrf peak field=22.5 MV/m, 90 deg

Energy Gain

Secondary Electrons Backstreaming

Operating the PWT at a low field gradient helps prevent backstreaming electrons emitted from the first PWT iris from reaching the photocathode. Inset shows half iris geometry.

Dark Current Suppression at Low Field Gradient

Thermal Hydraulic Design (S-band PWT)

• NLC parameters: 120-Hz, 3 microsecond-long rf pulses, 50-MW peak power at S band ⇒ 18 kW total.

• In a PWT structure with copper disks and rods and SS tank, 29.9% goes into the 11 disks (489 W per disk) and 11.4% into the 4 rods (186 W per section between disks).

• Heat in disk and rod (0.18” ID) is removed by water flow through hollow rods and disk internal channels. Required flow in each disk is 5.2 lpm at T=9°C.

• Use 2 parallel cooling circuits (for 5 and 6 disks respectively), with a variable orifice size (0.06” to 0.18”) for each disk to account for line pressure drop.

• Required external pressure head is 323 psi for a total flow of 30.9 lpm through the inlet/outlet pipes.

Flow rate vs pressure drop

0

0.5

1

1.5

2

2.5

3

3.5

0 20 40 60 80 100 120

Pressure drop(psi)

Flo

w r

ate

(lit

er/

m)

do=0.02 inchdo=0.04 inchdo=0.06 inchdo=0.10 inch

Hydraulic test for a single disk

PWT Disk Thermal Study

• Microcomputer measurements• Cooling fluid flow rate• Temperature monitoring • Closed loop temperature generation

Ultra High Vacuum Design

Block diagram of major components of

the PWT load lock

Isometric view of polarized electron PWT gun with load lock

Activated GaAs photocathode vacuum

< 10-10–10-11 Torrrequires ultra high

Pumping box with Conflat design (left) and Helicoflex design (right)

Close-up of the PWT tank section inside the

pumping box

• Achievable vacuum is better than 10-10 Torr at the cathode. • 2 NEG pumps and 1 ion pump in the PWT provide a total pumping capacity > 1200 liters/sec. • Large vacuum conductance is only limited by the perforated

section of PWT tank inside the pumping box.

Load-Lock System forActivated GaAs Photocathode

• Ultra high vacuum transport

• Ultra high vacuum processing

• Proprietary puck handling

An L-Band PWT Gun as an ILC Polarized e– Injector?

PWT L-band TESLA designs 2001 Tech

Report N cells 5+2/2 5+5 17 Length, m 0.692 1.15 1.96 Aperture radius a, cm 2.3 2.6 Shunt impedance r, M/m 20.23 31.9 35.4 Eacc., MV/m, (P=10MW) 17 16.6 13.4 Q-factor ~19590 Cell coupl. coeff. kc, % 13.7 Cooling capability, kW/m >100 ~30 Heat, kW (5Hz, 1.37ms, 10MW) 68.5

Beam parameters for q=3.7nC bunch Energy, MeV ~11 11.3 Laser pulse length, ns 0.03 2 Phase extension (rms), deg 5.3 Bunch length (rms), mm 3.4 Energy spread (rms), keV 45 Norm. emittance, mmmrad 42.5 Beam size, mm 2.6 Beam angular spread, mrad 0.7

L-band PWT polarized injector

to the SC linac

Can be replaced by:

Thermal Hydraulic Design (L-band PWT)

• ILC parameters: 5-Hz, 1370 microsecond-long rf pulses, 10-MW peak power at L band ⇒ 68.5 kW total.

• In a PWT structure with copper disks and rods and SS tank, 30% goes into the 6 disks (3.4 kW per disk) and 11% into the 4 rods (1.3 kW per section between disks).

• Heat in disk and rod (0.43” ID) is removed by water flow through hollow rods and disk internal channels. Required flow in each disk is 48.8 lpm at T=9°C.

• Use 2 parallel cooling circuits (3 disks each), with a variable orifice size (0.28” to 0.40”) for each disk to account for line pressure drop.

• Required external pressure head is 106 psi for a total flow of 146 lpm through the inlet/outlet pipes.

Conclusions• Electromagnetic, thermal hydraulic, vacuum and

mechanical designs have been performed for an S-band polarized electron PWT photoinjector.

• Polarization > 85%, charge/bunch up to 2 nC, energy gain ≈ 18 MeV, normalized transverse emittance ≈1 mm-mrad. An S-band PWT is suitable for nuclear physics and HEP experiments.

• Ultra short bunches are available for FEL and other applications.

• An L-band polarized electron PWT photoinjector offers low transverse emittance and excellent cooling for the International Linear Collider (ILC).