Transcript High Gradient and High Q R&D Topics

High Gradient and High Q
R&D Topics
• 1- Explore Q > 1010 for TESLA parameter flexibility
– higher luminosity through higher rep rate, longer rf pulse…
• 2- Improve geometry of TESLA cavity to increase
accelerating field
– reduce peak magnetic field by 10%
– allow peak electric field to rise 20%
• 3- Understand gradient limitations related to dark current
and voltage breakdown in copper for NLC
Gradient Related R&D con’t
• 4 - Study gradient vs rf pulse length
– Can we get 45 MV/m for 100 msec instead of 30 MV/m for 1000 msec?
• 6- Streamline TESLA cavity production and preparation
– Heat treat stack of half-cells,electropolish half-cell (instead of 9-cell
structure)
– Spin entire 9-cell structure out of one tube (done for copper)
21,000, 9-cell cavities
Item 1: Potential Q improvements can lead to Q from 1010 to 1011
Achieved in single cells already,
Need to achieve in 9-cells
Explore Improved TESLA
Cavity Shape
Proposed New Geometry
10% higher Eacc for same Hpk
Simulations show that new
shape will not be prone to
multipacting
Better Surface Treatments Lead to High Gradients in TESLA Cavities
Module 5 cavities. Vertical test results.
10
Q0
10
C45
AC60
AC61
AC65
AC66
AC79
AC78
AC62
10
9
0
5
10
15
Eacc [MV/m]
20
25
30
Denis Kostin, DESY, MHF/SL
100 mm
More surface studies needed to show how to get to 40 + MV/m?
Dark Current and
Voltage Breakdown
Studies Important for
Warm NLC
J. Wang
1995
Eacc = 65-75 MV/m
R. Kirby/SLAC
Individual
Sparks
Craters
not
harmful
Reduce
dark
current
(condition
ing)
SLAC DS2 1.8 m long structure
- Cell 8, Downstream Side AFTER RF
But
Sustained
Sparks
Cause
Too many
craters=>
surface
erosion=>
cavity
detuning
Previous Studies Show Many Common
Features between RF and DC Breakdown
• DC single breakdown events are very fast
(few ns) like in cavities
• Cause few isolated craters (not harmful)
emission halted! (processing)
• But multiple breakdown events can be long
and cause severe surface damage (erosion)
Copper
Single Breakdown Event
1 mm
Digital Video of Spark
CERN Diamond Machined Copper cathode, HPR
Craters from
single breakdown
event
?
Ion bombardment
Starburst region
20 um
Sustained Spark at 170 MV/m
QuickTime™ and a
DV - NTSC decompressor
are needed to see this picture.
SEM
50 microns
20 microns
Contaminants are one source of field emission and breakdown
1) Diamond
Machined
Cu, as for cavities
2) Mn
Particles found
After furnace
brazing
3) Sparked
4) Mn particles
Were here Now craters
Mn particles
Were here Now empty
craters
Mn particles outside starburst survived the spark
Intentional V particle:
Strong emitter
On copper surface
SEM
70 MV/m
Spark
10 microns
20 microns
Starbursts present
on copper are
carbon depleted
areas due to ion
bombardment
Vanadium from Auger
Auger Detection
can find residue
of original
emitter
Simulation Results
• Modelling of breakdown has started
• But could benefit from more effort
More Needed
Gas Distribution
- starts by surface desorption
-or melting of emitter tips
- builds up by ion bombardment
- may be ultimately
dominated by cathode material
Electrons
Gas
Electrons
Ions
Ions
50 microns
Resources Needed for First Year
Existing RAs and SRA
3 - 4 Graduate Students - 130 K
2 Technician support 150 K
Equipment
150 K
Materials and Supplies 100 K
Outside Contracts
100 K
Total 630 K