Parameter sensitivity tests for the baseline variant Konstantin Lotov , Vladimir Minakov, Alexander Sosed Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Rus Novosibirsk State University, Novosibirsk, Russia AWAKE Collaboration
Dec 28, 2015
Parameter sensitivity tests for the baseline variant
Konstantin Lotov, Vladimir Minakov, Alexander Sosedkin
Budker Institute of Nuclear Physics SB RAS, Novosibirsk, Russia
Novosibirsk State University, Novosibirsk, Russia
AWAKE Collaboration
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
The baseline variant:
Infinite plasmaHalf-cut beam
discussed here discussed earlierno study or interest
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of plasma density:It is convenient to characterize the field by the noise-resistant wakefield potential and use max(z) as the integral measure of wakefield quality
max
2e14
3e14
5e14
7e14
1e151.5e15
2e15
3e15
5e15
High density plasmas support stronger fields, but hosing or filamentation may develop
The baseline is at the optimum excitation efficiency
2e14
3e14
5e14 7e14 1e15
1.5e152e15 3e15
5e15
n0*50/7, E0*7, E = const,: 0.1 -> 3/16,Em/E0: 0.37 -> 0.22
limit due to elongation of wave period:
not enough bunches to reach saturation field
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of ion mass:
1 (H)
6.9 (Li)
24.3 (Mg)
85.5 (Rb)
39.9 (K)
77 94 133 (Cs) 197 (Au)
Rubidium is heavy enough to have no problems with ion motion
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of driver population:
5e114e11
3.5e11
3e11
2.5e11
2e11
1.5e11
1.15e11
The baseline regime is close to the limit (~40% of wavebreaking field),
further increase of population does not result in proportional field growth
N1/3
The limit due to nonlinear elongation of the wakefield period: K.V.Lotov, Phys. Plasmas 20, 083119 (2013)
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of driver length and population (constant peak current):
8 cm
6 cm
12 cm
20 cm
Fewer bunches at shorter lengths produce proportionally weaker fields,
but more bunches make no sense, as the limit is reached
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of driver length (constant population):
z=6 cm
z=12 cm
z=24 cm Weak dependence,
fewer and denser macro-bunches are more efficient
In the linear model, the field is determined by the total driver charge and independent on how many bunches it is distributed over.
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of driver radius (constant emittance):
0.2 mm
0.25 mm
0.3 mm
0.5 mm
0.15 mm
0.1 mm
0.05 mm
Wide beams are not dense enough to drive the wave to the limiting fieldNarrow beams are quickly diverging due to the transverse emittance
Baseline radius is the optimum one for this emittance
Beam fragments at z=3.6 m:
rb=0.2 mm
rb=0.05 mm
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of the driver energy:
100
200
400
600 800 1000 2000 GeV
/Wb1/2
Weak dependence:
peak field is near saturation,
Wb1/2 scaling of the length
(at constant normalized emittance)
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of driver energy spread:
0.17%0.35%0.7%
No dependence
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of driver emittance (angular spread):
1e-5
2e-53e-5
4.5e-5
4e-5
5e-5
7e-5
1e-4
2e-4
Lower emittance: same fields, slower beam destruction
Higher emittance: lower fields
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Variations of cut location:
propagation
length fraction remained
20%
25%
35%
50%
65%
95%
80%
Late cut: no beam to excite high fields
Early cut: slow instability growth, beam has time to diverge
Optimum: z ahead of the center
0 12 24-12-24cm from the center
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Varying plasma radius:
E.Oz, P.Muggli, NIMA paper:
simulations
cone-shaped plasma
infinite plasma (baseline)
fears: detuning of the wave frequency,different wave structure,different trapping conditions
no more
Wakefield phase: regions of stable phase velocity are still there
has small effect on wakefields
Wakefield amplitude: changes are small
presented by K.Lotov at AWAKE collaboration meeting, Geneva 05.12.2013
Summary:
Infinite plasmaCut location
Sensitivity, S
max/max = S X/X
0.7
0 (extremum)
0.40
0.3
0.0700.3
0
150%
The way to higher fields: lower emittance, smaller beam radius, higher plasma density
Of less importance: increase of beam peak current
False target: increase of beam population at the expense of elongation