FEA status
M. Paraliev, S. Tsujino, E. Kirk, S. Ivkovic, C. Gough
Hollow cathod
e Anode e-
beam
Where did we stop? MOSFET driver - 4ns pulse
Gated FEAs tested in OBLA 500kV pulser FE Array diameter 2mm
Number of tips ~40 000
Gate pulse width 15ns FWHM Current pulse width 4ns FWHM
Voltage over FEA 1nF (150V charge)
-200 -150 -100 -50 0 50
0 50 100 150 200
Tim e, ns
U, V
FEA emitted current
-250 -200 -150 -100 -50 0 50
0 10 20 30 40 50 60 70 80 90 100 Time, ns
Current, uA
Uch = 117V
Gate voltage dummy FEA chip Emitted current (conditioning chamber)
500kV pulser
Conditioning chamber
Pattern evolution with increasing gate voltage
Machine settings: 250kV @ 15mm, 1Hz
Gate voltage (Uch) increased from 116 to 130V Charge changed from 2.2 to 7.9pC
Uch = 116V Uch = 118V Uch = 120V Uch = 122V Uch = 124V Uch = 126V Uch = 128V Uch = 130V
1.3 nF FEA capacitance
~4 ns (FWHM) FEA pulse 30 MV/m at FEA surface 300 keV FEA beam energy
~10 pC emitted charge
Emittance - 3 to 6 mm mrad (solenoid scan)
Emission is not sensitive to inert (Ar) gas background
pressure (7.6e
-9to 2.2e
-5mbar) How to get faster emission?
(RF acceleration compatibility)
Direct driving scheme – sub-nanosecond pulse
~300 ps rise time
~1 ns FWHM pulse width 5 kV amplitude (positive) 0.5 MW peak power
> ±30 A injected in the gate
~ 1.5 ns gate pulse
Inversion / bipolar pulse DC bias
Monitor signal (jitter)
FID Pulser
FID pulser
-1 0 1 2 3 4 5 6
5.0 6.0 7.0 8.0 9.0 10.0 11.012.0 13.014.0 15.0
Time, ns
Voltage, kV
Normal structure ~1.3 nF :
Array:
0.2-2.2 mm Array:
0.2-2.2 mm
6.8 mm 6.8 mm GateGate
5.4 mm 5.4 mm
Low(er) capacitance structure ~ 0.6 nF:
Insulator-1: ~1 m Insulator-1: ~1 m
Insulator-2: ~1 m or thicker Insulator-2: ~1 m or thicker
New FEA generation with lower capacitance
FEA cross section
Top view Even for the larger FEA
diameter (2.2 mm) its capacitance
contribution is only ~17%
Direct driver in Low Gradient (LG) test stand (ODRA) First results (~1ns pulse)
Expected FEA pulse < 500 ps Could not go below 1ns!
It was not possible to synchronize the scope directly to the gate signal and due to averaging the jitter was contributing to the apparent pulse length.
System was carefully checked and improved but the particular limiting factor was found to be wrong
scope termination…
Direct driver in LG test stand Sub-nanosecond FEA emission
FEA pulse ~ 460 ps !
Long cable
-0.007 -0.006 -0.005 -0.004 -0.003 -0.002 -0.001 0 0.001 0.002
1.520E-07 1.540E-07 1.560E-07 1.580E-07 1.600E-07 1.620E-07 Time, s
U, V (Over 50 Ohm)
With long cable
connection (22 m) the FEA pulse was still ~700 ps
(RF cycle 666 ps)
700 ps
Direct driver in High Gradient test stand (OBLA) Sub-nanosecond FEA emission + RF
But how long is the bunch here?
200 keV beam (no RF) ~3.5 MeV beam ~5.6 MeV beam (+ dark current)
Sub-nanosecond FEA emission + RF RF Phase scan - ~550 ps FWHM
Normalized ratio @5.6 MeV
Phase scan
-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00
-1000.00 -500.00 0.00 500.00 1000.00
Time, ps
A, normalized
e- bunch cut function RF (1.5 GHz) collected beam
Phase scan
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
0 50 100 150 200 250 300 350 400
RF phase, deg
Charge signal, pVs
600 ps FWHM Measured 500 ps FWHM
Phase scan
0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30
0 50 100 150 200 250 300 350 400
RF phase, ged
Norm ratio
600 ps FWHM Measured 500 ps FWHM
Charge capturing model Cathode
Cavity
DS1
FC Screen
FC signal @5.6 MeV
550 ps
667 ps
400keV diode acceleration (12 MV/m)
What needs to be explored:
Maximum gradient with emission Maximum diode voltage (ion back bombardment - degradation)
Get higher emitted charge
Reproducibility / statistics – more tested samples (only ~ 4 samples were tested in OBLA up to now)
Faster emission + RF Jitter performance Emittance evaluation
400 keV beam (no RF)
Outlook and future
FEAs are compatible with high gradient (30 MV/m) Maximum FEA based beam energy 5.6 MeV
FEA emission is stable (over hours)
FEAs are resistant to ion back bombardment
FEA emission can be controlled quickly (460 ps FWHM) Setup can tolerate (some) small FEA breakdowns
FEA operation with RF acceleration is feasible (1.5 GHz)
To do next
Get better statistics (1 to 1.5 tested chips per week)
Explore FEA surface gradient limits (with emission)
Reduce further gate capacitance probably down to 400 pF
Optimize driver scheme – new FID pulser (negative, 100 ps rise time, 500 ps pulse length FWHM)
Emission homogenization using balancing resistive layer below the emitters
Double gate FEA chip
integration and driving scheme development
Pulse length
0.1 1 10 100 1000
Jun.08 Dec.08 Jul.09 Jan.10 Aug.10 Feb.11 Time
Pulse length FWHM, ns
Without RF With RF
100 ns
4 ns
460 ps 550 ps
Emittance in 4 MeV machine (OBLA)
Geom. emittance vs cathode spot size
0.020 0.030 0.040 0.050 0.060 0.070
0 0.2 0.4 0.6 0.8
Spot size RMS radius, mm
Emittance, mm mrad
Geom. emittance vs cathode spot size
0.020 0.030 0.040 0.050 0.060 0.070
0 0.2 0.4 0.6 0.8
Spot size RMS radius, mm
Emittance, mm mrad
Emittance vs. emission spot size (@10pC)
Machine setup
Pulser: 470kV @ 8mm PSL: 1000A
RF: 4.3MW (-20deg) Ek: 5.8MeV
DS1: 130A
Machine parameters were kept strictly identical except below mentioned.
Charge 10pC - kept adjusting laser energy.
Beam image size on emittance monitor screen was kept constant using DS2.
Laser spot diameter was variable.
For each measurement point 10 consecutive emittance images are recorded and analyzed.
Looks quite constant!
LaserE-beamEmittance
Geom. emittance vs charge
0.020 0.040 0.060 0.080 0.100 0.120
0 10 20 30 40 50
Charge, pC
Emittance, mm mrad
RMS radius 0.60 mm RMS radius 0.19 mm
Emittance vs. charge
Machine parameters were kept strictly identical except below mentioned.
Beam image size on emittance monitor screen was kept constant using DS2.
Charge was variable (through laser energy)
For each measurement point 10
consecutive emittance images are recorded No data
In only 2 out of 10 images emittance evaluation was possible
Emittance vs. pulser voltage (@10pC)
Machine parameters were kept strictly identical except below mentioned.
Pulser voltage is variable.
Beam image size on emittance monitor screen was kept constant using RF phase and PSL (phase changed according
calculated transit time and PSL adjusted for the same beam size on emittance monitor screen – unfortunately there is some inconsistency in RF and PSL setup ).
For each measurement point 10
consecutive emittance images are recorded and analyzed.
Emittance vs. pulser voltage
0.02 0.04 0.06 0.08
400 450 500 550
Voltage, kV Geom. emittance, mm mrad
0.2 0.4 0.6 0.8
Norm. emittance, mm mrad
529kV 473kV
418kV
Emittance (mm mrad) vs. machines
28.05.10 140pC 6.6MeV 0.044 [0.61] 100%
28.05.10 140pc 6.6MeV 0.039 [0.54] 100%
28.05.10 140pC 6.6MeV 0.034 [0.47] 100%
01.09.10 100pC 6.2MeV 31.08.10 111pC 8.3MeV 0.031 [0.53] 100%
05.07.10 180pC 5.7MeV 0.052 [0.63] 100%
0.034 [0.41] 90%
05.07.10 75pC 5.7MeV 0.034 [0.41] 100%
0.031 [0.38] 97%
0.029 [0.35] 94%
05.07.10 180pC 5.7MeV 0.044 [0.53] 100%
0.030 [0.36] 90%
OBLA
OBLA
OBLA
WLHA
WLHA
WLHA
WLHA
WLHA
Week 15 to
present
Two reasonable measurements
Date: 28.05.2010 Charge: 140pC Beam energy: 6.6MeV
Norm. emittance: 0.61mm mrad (100%) Date: 05.07.2010
Charge: 180pC Beam energy: 5.7MeV
Norm. Emittance: 0.63 mm mrad (100%)
OBLA WLHA
The two values are surprisingly similar!
For better comparison the scale factor of the left one is changed to match the one of the right one.
Thank you for your
attention!
Image:
Different PP hole sizes drilled in a ceramic substrate (20pC)
