CN102496539B - Electronic pulse quasi-linear symmetric type pulse width compression device and method thereof - Google Patents

Abstract

The invention relates to an electronic pulse quasi-linear symmetric type pulse width compression device and a method thereof. A Z axis is established with a transmission direction of an electronic pulse. A transmission photoelectric cathode, an alternative electric field resonant cavity and a direct current power supply are arranged along a Z axis direction. One side of the alternative electric field resonant cavity is provided with an anode grid, and the other side is provided with a modulation grid. According to the device and the method of the invention, a restricted problem of a present 100fs magnitude ultra-short electronic pulse generation method and a related system at an aspect of engineering application is solved, the invention brings forward the electronic pulse quasi-linear symmetric type pulse width compression device and the method, and an alternative electric field is utilized to apply a differential energy modulation effect on the electronic pulse so as to achieve a purpose of compressing a pulse width of the electronic pulse.

Description

Electronic impulse almost linear symmetric type pulse width compression device and method

Technical field

The invention belongs to femtosecond Superfast time resolution technical field, relate in particular to a kind of electronic impulse almost linear symmetric type pulse width compression device and method.

Background technology

Time resolution ultrafast phenomena is just launched in many fields of basic research, research in new high-tech, the ultrashort electronic impulse of take is controlled fast as ultrafast property diagnostic tool, especially image converter tube streak camera and the ultrafast electric diffraction instrument on basis and bringing into play the effect that is difficult to substitute in the research of time resolution ultrafast phenomena.In order to improve its temporal resolution, produce 100fs left and right (1fs=10 ^-15s) even more short pulse duration and each packet of pulses contain 10 ³～10 ⁴the ultrashort electronic impulse technology of individual electronics is brought into schedule already.Yet, this type of technology still fails to enter in conceptual phase in engineering application at present, its bottleneck is mainly significant space charge effect in the first energy dispersion of photocathode utilizing emitted light electronics and high concentration electric subpulse, and these two factors can cause serious electronic impulse broadening.Also just therefore, existing relevant scholar has done a large amount of theories and experimental study to electron gun system electronic impulse broadening, and has proposed various electronic impulse Pulse Compression thinkings and produced the ultrashort electronic impulse pulsewidth of 100fs magnitude method.

Up to now, the ultrashort electronic impulse production method that is seen in newpapers and periodicals is roughly divided into two classes: a class is to adopt electrostatic field and (or) magnetostatic field to carry out the modulation in time-domain and (or) spatial domain to electronic impulse, as based on electronic impulse broadening effect analysis result, adopt the design of novel electron optical texture electron gun (as " S " shape and " returning " font) or introduce the methods such as electronic impulse broadening inhibition electrode to reach the object of compression of electronic pulse; Another kind of is to adopt transient state modulated electric fields, as based on electronic impulse broadening effect analysis result, utilize transient state modulated electric fields to modulate to reach the object that produces ultrashort electronic impulse to imposing distinctiveness along electronics before and after electronic impulse, and utilize Intense Laser Field from treat modulation electric subpulse, to isolate the electronic impulse of Ah's second-time to the matter kinetics function of electronic impulse.Although in theory even some has been proved and has had certain feasibility above method in engineering research, but regrettably, these methods or because the feasibility of its engineering construction is compared with little or take the related request that ultrashort electronic impulse controls the ultrafast diagnosis research application that is basis fast and always exist the limitation in application because it can not meet image converter tube streak camera and ultrafast electric diffraction instrument etc.Also just therefore, the exploration of the ultrashort electronic impulse production method of this type of 100fs magnitude and related system is still being continued.

Summary of the invention

Limitation for the ultrashort electronic impulse production method of current 100fs magnitude and related system in engineering application aspect, the present invention proposes electronic impulse almost linear symmetric type pulse width compression device and method, utilize alternating electric field to impose distinctiveness energy modulation to electronic impulse and do in order to reach the object of compression of electronic pulse.

Technical solution of the present invention is:

Electronic impulse almost linear symmetric type pulse width compression device, Z axis is set up in transmission direction with electronic impulse, its special character is: comprise along Z-direction and set gradually transmission-mode photocathode 1, alternating electric field resonant cavity 3 and DC power supply, one side of described alternating electric field resonant cavity 3 is provided with anodic grid mesh 4, opposite side is provided with modulation aperture plate 5

The positive pole of described DC power supply connects anodic grid mesh 4, the negative pole of described DC power supply connects transmission-mode photocathode 1, it between described anodic grid mesh 4 and transmission-mode photocathode 1, is static evenly axial accelerating field district, between described alternating electric field resonant cavity 3 Anodic aperture plates 4 and modulation aperture plate 5, it is electronic impulse energy modulation district, between described modulation aperture plate 5 and objective plane 7, be the field-free drift space of electronic impulse, along Z-direction, uniform magnetic field be set , described electronic impulse energy modulation district is provided with axial alternating electric field.

Above-mentioned triggering signal produces axial alternating electric field by signal trigger 6 in electronic impulse energy modulation district.

Electronic impulse almost linear symmetric type pulse width compression method, its special character is: comprise the following steps:

1] with electronic impulse transmission direction, set up Z axis and along Z-direction, uniform magnetic field is set, and lateral cross section in electronic impulse bundle transmitting procedure is remained unchanged;

2] in uniform magnetic field, along Z-direction, set up static evenly axial accelerating field district, electronic impulse energy modulation district and the field-free drift space of electronic impulse; Wherein: static evenly axially accelerating field district is by transmission-mode photocathode 1 and anodic grid mesh 4 structures, and electronic impulse energy modulation district is built with modulation aperture plate 5 by the anodic grid mesh 4 of alternating electric field resonant cavity 3; Electronic impulse is field-free drift space is by modulating aperture plate 5 and objective plane 7 builds;

3] almost linear symmetric form energy modulation and Pulse Compression:

3.1] electronic impulse is accelerated: by DC power supply for static state evenly axially accelerating field district accelerating voltage U is provided, according to the primary power disperse Δ ε of electronic impulse ₀, inceptive impulse width is τ ₀and the width d in static evenly axial accelerating field district ₁, electronic impulse primitive axis is to length l ₀and forefront and the last poor Δ v of initial velocity along electronics ₀, there is following relation:

$\mathrm{\Δ}{v}_0=\sqrt{\frac{2\mathrm{\Δ}{\mathrm{\ϵ}}_0}{m_e}}+\frac{\mathrm{\ηU}{\mathrm{\τ}}_0}{d_1}.---\left(1\right)$

η=e/m wherein _efor the charge-mass ratio of electronics,

Obtain: the axial energy of electronic impulse forefront electronics is electronic impulse is finally ε along the axial energy of electronics _last=0,

Known initial axial energy is ε _ielectronic impulse in static state, evenly axially the transit time in accelerating field district is:

$t_1\left({\mathrm{\ϵ}}_i\right)=\frac{m_e{d}_1}{\mathrm{eU}}[\sqrt{\frac{2({\mathrm{\ϵ}}_i+\mathrm{eU})}{m_e}}-\sqrt{\frac{2{\mathrm{\ϵ}}_i}{m_e}}],---\left(2\right)$

Can try to achieve accordingly the pulse duration τ=t of electronic impulse arrival anodic grid mesh ₁(ε _last)-t ₁(ε _first);

If establish electronic impulse forefront place electronics, get over that static evenly axially the time in accelerating field district is t _min, there is following relation:

$t_{\min }=t_{\mathrm{mid}}-\frac{\mathrm{\τ}}{2},---\left(3\right)$

In like manner, electronic impulse is last gets over the static even axially time t in accelerating field district along place's electronics _maxalso just like ShiShimonoseki, be tied to form vertical:

$t_{\max }=t_{\mathrm{mid}}+\frac{\mathrm{\τ}}{2}.---\left(4\right)$

Wherein the middle electronics of electronic impulse is getted over the time in static evenly axial accelerating field district:

t _mid=(t _min+t _max)/2；

3.2] modulation:

3.2.1] apply axial alternating electric field to electronic impulse energy modulation district, described axial alternating voltage is U _m(t)=-U ₀sin (2 π t/T _m), wherein the positive direction of alternating voltage is Z-direction, U ₀for alternating voltage peak value, T _mfor the alternating voltage cycle, t=0 is the moment that transmission-type photocathode produces photoelectron pulse;

Alternating voltage cycle T wherein _mmeet the following conditions:

$t_{\mathrm{mid}}=(n+\frac{1}{2})T_m,---\left(5\right)$

$t_{\min }\≥(n+\frac{5}{12})T_m.---\left(6\right)$

Wherein n is positive integer,

According to (5) and (6), obtain:

3.3] pulse compression amplitude is calculated:

Meeting modulation voltage cycle T _mcondition under, the electronic impulse forefront Pulse Compression amplitude of electronic impulse from exit, electronic impulse energy modulation district to objective plane is:

$\mathrm{\Δ}{t}_{\mathrm{first}}=\frac{{\mathrm{LU}}_m\left(t_{\min }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\ηU}}},---\left(7\right)$

The last suppressed range along electronics of the pulsed electron of electronic impulse from exit, electronic impulse energy modulation district to objective plane is:

$\mathrm{\Δ}{t}_{\mathrm{last}}=\frac{{\mathrm{LU}}_m\left(t_{\max }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\ηU}}},---\left(8\right)$

The reduced overall amplitude of electronic impulse from exit, electronic impulse energy modulation district to the pulsed electron of objective plane is:

Δt=Δt _last-Δt _first. （9）

Δt=2Δt _last. （10）。

Tool of the present invention has the following advantages:

1, the almost linear of pulse compression: be mainly reflected in the linear relationship of the inner each several part electronics suppressed range of electronic impulse and its energy modulation voltage, and electronic impulse compress for modulation voltage almost linear variation relation in time;

2, the symmetric form of pulse compression: the almost linear feature based on pulse compression, pulse compression amplitude presents the symmetry about its center time point;

3, compression set miniaturization: electronic impulse reduced overall amplitude is proportional to pulse compression modulation parameter, this makes can pass through to improve energy modulation voltage with the axial dimension in reduction pulse compression district, thereby finally realizes the miniaturization of this compression set.

Accompanying drawing explanation

Fig. 1 alternating electric field electronic impulse pulse width compression device;

Fig. 2 is the signal of alternating electric field pulse compression principle;

Fig. 3 is that axial magnetic field is to electron focusing effect of contraction;

Fig. 4 is alternating electric field electronic impulse Pulse Compression track;

Fig. 5 be pulse forefront with finally along electronic time～axial displacement relation;

Wherein Reference numeral is: 1-transmission-mode photocathode; 2-electronic impulse; 3-alternating electric field resonant cavity; 4-anodic grid mesh; 5-modulates aperture plate; 6-signal trigger; 7-objective plane.

Embodiment

Electronic impulse almost linear symmetric type pulse width compression device as shown in Figure 1, adopt cylindrical shaft symmetrical structure, comprise along Z-direction and set gradually transmission-mode photocathode 1, alternating electric field resonant cavity 3 and power supply, one end face of described alternating electric field resonant cavity 3 is that double aperture plate 4, other end of doing anode is modulation aperture plate 5, and triggering signal produces the axial alternating electric field that meets set pulse compression requirement by signal trigger 6 between resonant cavity anodic grid mesh and modulation aperture plate.Axially uniform magnetic field completes the transverse focusing of electronic impulse to limit the lateral cross section of electronic impulse.The positive pole of described power supply connects anodic grid mesh 4, and the negative pole of described power supply connects transmission-mode photocathode 1.It between described anodic grid mesh 4 and transmission-mode photocathode 1, is static evenly axial accelerating field district, between described anodic grid mesh 4 and modulation aperture plate 5, being electronic impulse energy modulation district, is the field-free drift space of electronic impulse between described modulation aperture plate 5 and objective plane 7.Alternating electric field district and field-free drift space form electronic impulse compression stroke jointly.

Electronic impulse almost linear symmetric type pulse width compression method, comprises the following steps:

1] with electronic impulse transmission direction, set up Z axis and along Z-direction, uniform magnetic field B is set ^v, described magnetic field B ^vdirection and Z axis in the same way, it act as: by the transverse focusing to electronic impulse, retrain, so that electronic impulse bundle lateral cross section remains unchanged;

2] in uniform magnetic field, set up electronic impulse almost linear symmetric type pulse width compression device, comprise along Z-direction and set gradually transmission-mode photocathode 1, alternating electric field resonant cavity 3 and power supply, one side of described alternating electric field resonant cavity 3 is provided with anodic grid mesh 4, opposite side is provided with modulation aperture plate 5, the positive pole of described power supply connects anodic grid mesh, the negative pole of described power supply connects transmission-mode photocathode 1, it between described anodic grid mesh 4 and transmission-mode photocathode 1, is static evenly axial accelerating field district, it between described anodic grid mesh 4 and negative electrode aperture plate 5, is electronic impulse energy modulation district, it between described modulation aperture plate 5 and objective plane 7, is the field-free drift space of electronic impulse,

3] almost linear symmetric form energy modulation and Pulse Compression: for photoelectron primary power, disperse is Δ ε ₀, inceptive impulse width is τ ₀rectangle electronic impulse, in the process of getting over accelerating field district, its primary power disperse and discrete space-charge effect will cause significant pulse-spreading phenomenon jointly.If electronic impulse width is τ when it arrives anodic grid mesh, the shortest the and the longest transit time of electronic impulse internal electron, is also pulse forefront and last edge t electron transit time of place _minwith t _maxbe respectively

$t_{\min }=t_{\mathrm{mid}}-\frac{\mathrm{\τ}}{2},---\left(1\right)$

$t_{\max }=t_{\mathrm{mid}}+\frac{\mathrm{\τ}}{2}.---\left(2\right)$

Wherein, t _mid=(t _min+ t _max)/2.If the axial alternating voltage producing in electronic impulse energy modulation district is U _m(t)=-U ₀sin (2 π t/T _m), voltage positive direction is propagated Z direction, U along electronic impulse here ₀for modulation voltage peak value, T _mfor the modulation voltage cycle, t=0 produces the moment of photoelectron pulse corresponding to photocathode.Selected almost linear change in voltage region is the region between positive and negative half peak value of alternating voltage, and the condition of electronic impulse almost linear symmetric form modulation Pulse Compression is

$t_{\mathrm{mid}}=(n+\frac{1}{2})T_m,---\left(3\right)$

$t_{\min }\≥(n+\frac{5}{12})T_m.---\left(6\right)$

Here n=0,1,2 ...Simultaneous formula (3) can obtain with (4)

$n\≤\frac{6t_{\min }-5t_{\mathrm{mid}}}{12(t_{\mathrm{mid}}-t_{\min })}---\left(5\right)$

In reality, often get n for meeting the maximum integer of formula (5).

Energy modulation voltage is U _gtime by anodic grid mesh, enter the electronics of modulator zone, the axial velocity variation Δ v in exit, Qi energy modulation district _zfor

$\mathrm{\Δ}{v}_z=\sqrt{2\mathrm{\ηU}}-\sqrt{2\mathrm{\η}(U+U_g)},---\left(6\right)$

The velocity variations of supposing to ignore the relative displacement of electronics between slot and only considering to be caused by energy modulation, when whole electronic impulse, passing through longitudinal length is the drift space of L while arriving objective plane, and its relative position having of the electronics of above-mentioned energy modulation is changed to

$\mathrm{\Δz}=\frac{\mathrm{L\Δ}{v}_z}{\sqrt{2\mathrm{\ηU}}}.---\left(7\right)$

Therefore, its relative time changes delta t having is

$\mathrm{\Δt}=-\frac{\mathrm{\Δz}}{\sqrt{2\mathrm{\η}(U+U_g)}},---\left(8\right)$

Utilize condition U _g<<U can obtain

$\mathrm{\&Delta;t}\&ap;-\frac{\mathrm{\&Delta;z}}{\sqrt{2\mathrm{\&eta;U}}}=\frac{L{U}_g}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}}.---\left(9\right)$

Formula (9) illustrates: at U _gunder positive condition, forward energy modulation makes electronics have forward direction speed modulation, and this has finally shortened electron transit time; And U _gunder negative condition, negative sense energy modulation makes electronics have negative sense velocity modulation, and this has finally extended electron transit time.This means: if impose negative sense energy modulation to extend its transit time to pulse forefront electronics, finally along electronics, impose forward energy to pulse simultaneously and modulate to shorten its transit time, electronic impulse width in anodic grid mesh place will be compressed after it gets over electronic impulse energy modulation district and field-free drift space in succession.In formula (9), LU _gcan be referred to as pulse compression modulation parameter, this formula also means that pulse compression is proportional to the two/cube of modulating parameter and being inversely proportional to accelerating voltage simultaneously.

4] pulse compression amplitude is calculated:

Meeting under the prerequisite of electronic impulse almost linear symmetric type pulse width contractive condition, pulse forefront is respectively with the last suppressed range along electronics and the whole Pulse Compression amplitude of electronic impulse

$\mathrm{\&Delta;}{t}_{\mathrm{first}}=\frac{{\mathrm{LU}}_m\left(t_{\min }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(10\right)$

$\mathrm{\&Delta;}{t}_{\mathrm{last}}=\frac{{\mathrm{LU}}_m\left(t_{\max }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(11\right)$

Δ t=Δ t _last-Δ t _first. (12) and symmetric form contractive condition means U _m(t _maxthe U of)=- _m(t _min), because of and have

Δt=2Δt _last. （13）

The specific embodiment of the invention mainly comprises: velocity modulation is selected with alternating electric field; Pulse Compression amplitude is calculated; The focusing effect of contraction of axial magnetic field.

1] energy modulation is selected with alternating electric field:

In electronic impulse transmitting procedure, cause the principal element of its pulse stretcher to have: the difference of electronic impulse internal electron initial velocity aspect; Before and after its inner space charge effect paired pulses, along the qualitative difference-frontier electron of electronic action power, be subject to acceleration and then along electronics, be subject to decelerating effect.This two species diversity makes electronic impulse internal electron occur the disperse on transit time after transmission equidistance, and this is electronic impulse broadening.About the calculating of space charge effect electronic impulse broadening, still there are not parsing or comparatively simple and clear computational methods at present, thereby for more clearly setting forth operation principle of the present invention, in this partial pulse broadening discussion below, will temporarily ignore.But known to the research of space charge effect according to existing theory and experiment aspect, its pulse stretching causing presents good symmetry, be also along thering is almost equal broadening effect before and after pulse.Even if this makes in the situation that counting due to space charge effect pulse stretching, below the discussion of relevant operation principle of the present invention still set up.

In axial magnetic field focusing effect of contraction under, electron motion can adopt holography.In system, electronical line shaft to the equation of motion is

$\frac{d^{2}z}{{\mathrm{dt}}^2}=\mathrm{\&eta;}{\mathrm{\&phi;}}^{\&prime;}\left(z\right),---\left(1\right)$

In formula, Z axis be take transmission-type photocathode center as initial point and is pointed to the direction that electronic impulse is advanced, and z represents that electronics is in t axial coordinate constantly, and φ (z) represents the current potential that on axle, z is ordered, η=e/m _echarge-mass ratio for electronics.Transform (1) integration can obtain

${\left(\frac{\mathrm{dz}}{\mathrm{dt}}\right)}^2=2\mathrm{\&eta;}[\mathrm{\&phi;}\left(z\right)-\mathrm{\&phi;}\left(0\right)]+{\left(\frac{\mathrm{dz}}{\mathrm{dt}}\right)}^2{|}_{t=0}.---\left(2\right)$

For primary power disperse, be Δ ε ₀, pulse original width is τ ₀rectangle electronic impulse, its pulse primitive axis is to length l ₀and forefront and the last poor Δ v of initial velocity along electronics ₀as follows respectively:

$l_0={\mathrm{\&tau;}}_0\sqrt{\frac{2\mathrm{\&Delta;}{\mathrm{\&epsiv;}}_0}{m_e}}+\frac{\mathrm{\&eta;U}{\mathrm{\&tau;}}_0^2}{2d_1},---\left(3\right)$

$\mathrm{\&Delta;}{v}_0=\sqrt{\frac{2\mathrm{\&Delta;}{\mathrm{\&epsiv;}}_0}{m_e}}+\frac{\mathrm{\&eta;U}{\mathrm{\&tau;}}_0}{d_1}.---\left(4\right)$

Consider that current potential and structure setting approach the situation of practical engineering application, micron dimension l ₀cause is much smaller than millimeter magnitude d ₁and can ignore, and the first energy of electronics is because thinking that much smaller than acceleration area voltage pulse is last along being respectively with the initial axial energy of forefront electronics , by (2) Shi Ke get, for initial axial energy, be ε _iphotocathode electron emission, it in the transit time of accelerating place is

$t_1\left({\mathrm{\&epsiv;}}_i\right)=\frac{m_e{d}_1}{\mathrm{eU}}[\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}{m_e}}-\sqrt{\frac{2{\mathrm{\&epsiv;}}_i}{m_e}}],---\left(5\right)$

Obviously, pulse front edge electronics is compared rear edge and is had the less flight time, is also that frontier electron first arrives anodic grid mesh.Pulsewidth τ in the time of can being obtained this electronic impulse and passed through axial accelerating field district by formula (5) is

τ=t ₁(ε _last)-t ₁(ε _first). （6）

Going up according to this analysis easily knows: if can impose distinctiveness velocity modulation-frontier electron negatively-modulated, rear along electronics positively-modulated to before and after electronic impulse along electronics at anodic grid mesh place, the electronic impulse of accelerating place broadening must, because the phenomenon that pulse narrows down appears in this distinctiveness velocity modulation, be also pulse compression after necessarily getting over space.

If the alternating voltage producing in resonant cavity is U _m(t)=-U ₀sin (2 π t/T _m), voltage positive direction is propagated Z direction, U along electronic impulse here ₀for modulation voltage peak value, T _mfor the modulation voltage cycle, t=0 produces the moment of photoelectron pulse corresponding to photocathode, and initial axial energy is ε _ithe transit time of electronics in system shown in Figure 1 be

t(ε _i)=t ₁(ε _i)+t _m(ε _i)， （7.1）

$t_1\left({\mathrm{\&epsiv;}}_i\right)=\frac{m_e{d}_1}{\mathrm{eU}}[\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}{m_e}}-\sqrt{\frac{2{\mathrm{\&epsiv;}}_i}{m_e}}],---\left(7.2\right)$

t _m(ε _i)=t ₂(ε _i)+t ₃(ε _i)， （7.3）

$t_2\left({\mathrm{\&epsiv;}}_i\right)=\frac{m_e{d}_2}{e{U}_m\left(t_1\left({\mathrm{\&epsiv;}}_i\right)\right)}[\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU}+e{U}_m\left(t_1\left({\mathrm{\&epsiv;}}_i\right)\right))}{m_e}}-\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}{m_e}}],---\left(7.4\right)$

$t_3\left({\mathrm{\&epsiv;}}_i\right)=\sqrt{\frac{m_e{L}^2}{2({\mathrm{\&epsiv;}}_i+\mathrm{eU}+e{U}_m\left(t_1\left({\mathrm{\&epsiv;}}_i\right)\right))}}.---\left(7.5\right)$

Obviously, t ₁(ε _i), t ₂(ε _i) and t ₃(ε _i) be respectively transit time of acceleration place, energy modulation district and the drift space of electronics, t _m(ε _i) be that electronics is in the transit time in pulse compression space.Considering that initial axial energy meets is related to ε ₁< ε ₂two electronics, if its corresponding speed modulation voltage is met

|U(ε ₁)|,|U(ε ₂)|>ε ₁,ε ₂， （8.1）

U(ε ₁)>0， （8.2）

U(ε ₂)<0， （8.3）

Known according to formula (7.1)～(7.4), perseverance has

t _m(ε _last)<t _m(ε _first)， （9）

Also there is pulse compression.

Known according to Fig. 2: for electronic impulse being carried out to the compression of almost linear symmetric modulation, must to make electronic impulse energy modulation voltage be positioned at almost linear change in voltage region and pulse forefront is equated with the energy modulation amplitude being finally subject to along electronics.Here selected almost linear change in voltage region is the region (having sin (x) ≈ x in this region) between positive and negative half peak value of alternating voltage.Electronic impulse being carried out to the compression of almost linear symmetric type pulse width needs satisfied condition to be

$t_{\mathrm{mid}}=(n+\frac{1}{2})T_m,---\left(10\right)$

$t_1\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)\&GreaterEqual;(n+\frac{5}{12})T_m.---\left(11\right)$

Here t _midfor accelerating the center flight time of electronic impulse in place, n=0,1,2 ...Simultaneous formula (10) can obtain with (11)

$n\&le;\frac{6t_1\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)-5t_{\mathrm{mid}}}{12[t_{\mathrm{mid}}-t_1\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)]}.---\left(12\right)$

In reality, often get n for meeting the maximum integer of formula (12).

By formula (5), known, electronic impulse is the shortest to be respectively with the longest transit time:

t _min=t ₁(ε _first)， （13）

t _max=t ₁(ε _last)， （14）

Its flight time of electronics and the initial axial energy with the center flight time are:

$t_{\mathrm{mid}}=\frac{t_{\min }+t_{\max }}{2},---\left(15\right)$

${\mathrm{\&epsiv;}}_{\mathrm{mid}}=\frac{m_e}{2}{(\frac{d_1}{t_{\mathrm{mid}}}-\frac{\mathrm{eU}{t}_{\mathrm{mid}}}{2m_e{d}_1})}^2.---\left(16\right)$

2] Pulse Compression amplitude is calculated

For the quantitatively variation of explanation electronic impulse pulsewidth, can try to achieve the transit time t without pulse compression space in velocity modulation situation ₀(ε _i) as follows,

$t_0\left({\mathrm{\&epsiv;}}_i\right)=\sqrt{\frac{m_e{(L+d_2)}^2}{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}},---\left(17\right)$

Meeting under almost linear symmetric modulation contractive condition, pulse forefront with finally along electron transit time, be changed to

$\begin{array}{c}\mathrm{\&Delta;t}\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)=t_0\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)-t_m\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)\\ =\sqrt{\frac{m_e{(L+d_2)}^2}{2({\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU})}}-\frac{m_e{d}_2}{e{U}_m\left(t_{\min }\right)}[\sqrt{\frac{2[{\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU}+e{U}_m\left(t_{\min }\right)]}{m_e}}-\sqrt{\frac{2({\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU})}{m_e}}]-,\\ \sqrt{\frac{m_e{L}^2}{2[{\mathrm{\&epsiv;}}_{\mathrm{first}}+\mathrm{eU}+e{U}_m\left(t_{\min }\right)]}}\end{array}---\left(18\right)$

$\begin{array}{c}\mathrm{\&Delta;t}\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)=t_0\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)-t_m\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)\\ =\sqrt{\frac{m_e{(L+d_2)}^2}{2({\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU})}}-\frac{m_e{d}_2}{e{U}_m\left(t_{\max }\right)}[\sqrt{\frac{2[{\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU}+e{U}_m\left(t_{\max }\right)]}{m_e}}-\sqrt{\frac{2({\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU})}{m_e}}]-.\\ \sqrt{\frac{m_e{L}^2}{2[{\mathrm{\&epsiv;}}_{\mathrm{last}}+\mathrm{eU}+e{U}_m\left(t_{\max }\right)]}}\end{array}---\left(19\right)$

The whole Pulse Compression amplitude of electronic impulse is

Δ t=Δ t (ε _last)-Δ t (ε _first) .(20) in actual applications, the relatively whole pulse compression of axial length district, velocity modulation district is very little, is also d ₂<<L; The same form of associating pulse compression condition (8.1) again, formula (18) can be approximately respectively with (19)

$\mathrm{\&Delta;}t\left({\mathrm{\&epsiv;}}_{\mathrm{first}}\right)=\frac{{\mathrm{LU}}_m\left(t_{\min }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(21\right)$

$\mathrm{\&Delta;}t\left({\mathrm{\&epsiv;}}_{\mathrm{last}}\right)=\frac{{\mathrm{LU}}_m\left(t_{\max }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(22\right)$

And symmetric form contractive condition means U _m(t _maxthe U of)=- _m(t _min), formula (20) is

Δ t=2 Δ t (ε _last). the width τ ' that (23) can obtain the electronic impulse of objective plane place is accordingly

τ′=τ-Δt. （24）

3] the focusing effect of contraction of axial magnetic field

This part analysis is benchmark with electronic coordinate: electronic coordinate is that initial point is positioned at the time pole-face launch point that powers on, and its reference axis X-axis, Y-axis and Z axis are parallel to respectively the corresponding reference axis of system shown in Figure 1.In system axial magnetic field constraint under, initial radial energy is ε _rthe radial motion of electronics in XY plane as shown in Figure 3.Wherein, (0,0) is electronic launching point, and (x, y) is the lateral attitude coordinate of electronics in electronic coordinate system.Cyclotron frequency, radius of gyration and the convolution cycle of electronics in this axial magnetic field is respectively

ω=ηB, （25）

$R_i=\frac{1}{\mathrm{\&eta;B}}\sqrt{\frac{2{\mathrm{\&epsiv;}}_r}{m_e}},---\left(26\right)$

$T=\frac{2\mathrm{\&pi;}}{\mathrm{\&eta;B}}.---\left(27\right)$

For primary power disperse, be Δ ε ₀, electron emission effective radius is r ₀initial cylindric electronic impulse, under the radial constraint effect in magnetic field, its lateral radius is

$r_c=r_0+\frac{2}{\mathrm{\&eta;B}}\sqrt{\frac{2\mathrm{\&Delta;}{\mathrm{\&epsiv;}}_0}{m_e}}.---\left(28\right)$

Known according to upper surface analysis, electron cyclotron cycle and electronics initial radial energy are irrelevant, and electronics has in the moment through convolution cycle integral multiple the radial distance equating with initial position again, and this focussing force defines the horizontal cross section of electron beam and remains unchanged

Example is as follows:

Consider that one close to the ultrafast diagnostic system of reality pulsed electron beam used: accelerate place accelerating voltage 30kV; Initiating electron pulse 150fs; Photoelectronic initial energy disperses Δ ε ₀=0.2eV; Electron beam initial radium 0.5mm; Anodic grid mesh and photocathode axial distance are 3mm; Energy modulation district axial length is 1mm; Electronic impulse drift space axial length is 10mm; Axially uniform magnetic field is 300Gauss.Under such structure and current potential arrange, expection pulse compression amplitude minimum is 200fs.

By parameter settings such as said structure, current potentials, can obtain electronic impulse in the parameter of accelerating place is: t _min=5.8107 * 10 ^-11s, t _max=5.8407 * 10 ^-11s, t _mid=5.8257 * 10 ^-11s.According to the satisfied condition of almost linear symmetric form pulse compression, be also that relational expression (10)～(15) in this " embodiment " part can obtain n=31, T _m=1.8494 * 10 ^-12s.According to the relational expression (18) in " embodiment " part, (19) and (20), the pulse compression amplitude of minimum 200fs means that device electric parameter must meet following relationship

Δt≥200fs. （29）

By above gained time parameter, can obtain t _max=31.5811T _m, t _max=31.4189T _m, thereby have U _m(t _maxthe U of)=- ₀sin (2 π t _max/ T _m)=0.50U ₀, U _m(t _minthe U of)=- ₀sin (2 π t _min/ T _mthe 0.50U of)=- ₀.Separating conditional relationship formula (29) can obtain

U ₀≥120V. （30）

By formula (28), can obtain the transverse focusing effect of contraction characterization parameter r of axial magnetic field to electronic impulse _c=0.6mm.

Above-mentioned current potential and structure arrange Fig. 1 shown device internal electron pulse under condition and compress track as shown in Figure 5.As seen from the figure: due to the first energy dispersion of photoelectron, its pulsewidth of 150fs initiating electron pulse at accelerating field offset from the about 300fs of broadening rapidly in the scope of the about 0.1mm of photocathode; The pulse compression space that Er velocity modulation district and drift region form, the modulated 100fs that is compressed to of electronic impulse, suppressed range is 200fs.Pulse forefront with as shown in Figure 5 along electronic time～axial displacement relation finally, wherein (a) and (b) accelerate place, velocity modulation district and drift space with (c) laying respectively at.Easily know by scheming: accelerating place, before and after pulse, along distance to axial, along with the increase of transit time, increasing, thereby occurred pulse stretching; In velocity modulation district, weak mudulation effect has caused pulse compression phenomenon, but not obvious with respect to accelerating place pulse width variation; At drift space, significant pulse compression effect occurs.

Claims (1)

1. electronic impulse almost linear symmetric type pulse width compression method, is characterized in that: comprise the following steps:

1] with electronic impulse transmission direction, set up Z axis and along Z-direction, uniform magnetic field is set, and lateral cross section in electronic impulse bundle transmitting procedure is remained unchanged;

2] in uniform magnetic field, along Z-direction, set up static evenly axial accelerating field district, electronic impulse energy modulation district and the field-free drift space of electronic impulse; Wherein: static evenly axially accelerating field district is by transmission-mode photocathode (1) and anodic grid mesh (4) structure, and electronic impulse energy modulation district is built with modulation aperture plate (5) by the anodic grid mesh (4) of alternating electric field resonant cavity (3); Electronic impulse is field-free, and drift space builds by modulating aperture plate (5) and objective plane (7);

3] almost linear symmetric form energy modulation and Pulse Compression:

3.1] electronic impulse is accelerated: by DC power supply for static state evenly axially accelerating field district accelerating voltage U is provided, according to the primary power disperse Δ ε of electronic impulse ₀, inceptive impulse width is τ ₀and the width d in static evenly axial accelerating field district ₁, electronic impulse primitive axis is to length l ₀and forefront and the last poor Δ v of initial velocity along electronics ₀, there is following relation:

$\mathrm{\&Delta;}{v}_0=\sqrt{\frac{2\mathrm{\&Delta;}{\mathrm{\&epsiv;}}_0}{m_e}}+\frac{\mathrm{\&eta;U}{\mathrm{\&tau;}}_0}{d_1}.---\left(1\right)$ η=e/m wherein _efor the charge-mass ratio of electronics,

Obtain: the axial energy of electronic impulse forefront electronics is electronic impulse is finally ε along the axial energy of electronics _last=0,

Known initial axial energy is ε _ielectronic impulse in static state, evenly axially the transit time in accelerating field district is:

$t_1\left({\mathrm{\&epsiv;}}_i\right)=\frac{m_e{d}_1}{\mathrm{eU}}[\sqrt{\frac{2({\mathrm{\&epsiv;}}_i+\mathrm{eU})}{m_e}}-\sqrt{\frac{2{\mathrm{\&epsiv;}}_i}{m_e}}],---\left(2\right)$

Can try to achieve accordingly the pulse duration τ=t of electronic impulse arrival anodic grid mesh ₁(ε _last)-t ₁(ε _first);

If establish electronic impulse forefront place electronics, get over that static evenly axially the time in accelerating field district is t _min, there is following relation:

$t_{\min }=t_{\mathrm{mid}}-\frac{\mathrm{\&tau;}}{2},---\left(3\right)$

In like manner, electronic impulse is last gets over the static even axially time t in accelerating field district along place's electronics _maxalso just like ShiShimonoseki, be tied to form vertical:

$t_{\max }=t_{\mathrm{mid}}+\frac{\mathrm{\&tau;}}{2}.---\left(4\right)$

Wherein the middle electronics of electronic impulse is getted over the time in static evenly axial accelerating field district:

t _mid=(t _min+t _max)/2；

3.2] modulation:

3.2.1] apply axial alternating electric field to electronic impulse energy modulation district, described axial alternating voltage is U _m(t)=-U ₀sin (2 π t/T _m), wherein the positive direction of alternating voltage is Z-direction, U ₀for alternating voltage peak value, T _mfor the alternating voltage cycle, t=0 is the moment that transmission-type photocathode produces photoelectron pulse;

Alternating voltage cycle T wherein _mmeet the following conditions:

$t_{\mathrm{mid}}=(n+\frac{1}{2})T_m,---\left(5\right)$

$t_{\min }\&GreaterEqual;(n+\frac{5}{12})T_m.---\left(6\right)$

Wherein n is positive integer,

According to (5) and (6), obtain:

3.3] pulse compression amplitude is calculated:

Meeting modulation voltage cycle T _mcondition under, the electronic impulse forefront Pulse Compression amplitude of electronic impulse from exit, electronic impulse energy modulation district to objective plane is:

$\mathrm{\&Delta;}{t}_{\mathrm{first}}=\frac{{\mathrm{LU}}_m\left(t_{\min }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(7\right)$

The last suppressed range along electronics of the pulsed electron of electronic impulse from exit, electronic impulse energy modulation district to objective plane is:

$\mathrm{\&Delta;}{t}_{\mathrm{last}}=\frac{{\mathrm{LU}}_m\left(t_{\max }\right)}{2U}\sqrt{\frac{1}{2\mathrm{\&eta;U}}},---\left(8\right)$

The reduced overall amplitude of electronic impulse from exit, electronic impulse energy modulation district to the pulsed electron of objective plane is:

Δt=Δt _last-Δt _first. （9）

Δt=2Δt _last. （10）